recursive + 900 elements + neighbor check = causes stackoverflow - android

I have a city simulation game and try to find a way to check the flow of our power system.
The basics:
The map for the city is based on tiles (30 by 30 tiles = 900 tiles).
Now i start at a power plant and do a recursive neighbor check (top, left, right, bottom) to check if there is something that will transport the power. If there is something, I start checking this tiles for neighbors, too.
To prevent double checks and/or infinite recursive calls, I fill a ArrayList with processed tiles and check if a new tile was already processed and added to the ArrayList...
Recursively started:
public void updatePowerEnvironment(int id, ArrayList<Integer> elements) {
Log.w("GT", "update env for id: " + id);
int newId = id - GameMap.mMapSize;
if (newId >= 0 && GameMap.mMapCells.get(newId).mPowerEnabled
&& !elements.contains(newId)) {
elements.add(newId);
updatePowerEnvironment(newId, elements);
}
newId = id + GameMap.mMapSize;
if (newId < GameMap.mMapCells.size() && GameMap.mMapCells.get(newId).mPowerEnabled
&& !elements.contains(newId)) {
elements.add(newId);
updatePowerEnvironment(newId, elements);
}
newId = id - 1;
if (newId >= 0 && GameMap.mMapCells.get(newId).mPowerEnabled
&& !elements.contains(newId)) {
elements.add(newId);
updatePowerEnvironment(newId, elements);
}
newId = id + 1;
if (newId < GameMap.mMapCells.size()
&& GameMap.mMapCells.get(newId).mPowerEnabled
&& !elements.contains(newId)) {
elements.add(newId);
updatePowerEnvironment(newId, elements);
}
}
If I can trust the log output, no tile was tried to processed twice. That means, that I have no errors in the recursive calls. Which also means, the stack is simply too small.
Does someone have an idea how to avoid the stack limit?
[Update and my code as a result of Erics answer]
public void updatePowerEnvironment(int id, ArrayList<Integer> elements) {
Stack<Integer> toProcess = new Stack<Integer>();
toProcess.push(id);
int mapSize = GameMap.mMapCells.size();
while (!toProcess.empty()) {
id = toProcess.pop();
Log.e("GT", "id to process: " + id);
if (elements.contains(id)) {
continue;
}
int[] neighborIds = computeNeighbors(id);
for (int neighbor : neighborIds) {
if (neighbor < 0 || neighbor >= mapSize) {
continue;
}
if (!GameMap.mMapCells.get(neighbor).mPowerEnabled) {
continue;
}
toProcess.push(neighbor);
}
elements.add(id);
}
}
private int[] computeNeighbors(int id) {
return new int[] {id + GameMap.mMapSize, id - GameMap.mMapSize, id + 1, id - 1};
}

If I understand your problem correctly you are attempting to compute the transitive closure of the "is powered by" relation between two tiles. It is certainly possible to compute a transitive closure non-recursively.
Here's a non-recursive algorithm that computes the transitive closure of a relation in C#. You should be able to adapt that to the language of your choice.
http://blogs.msdn.com/b/ericlippert/archive/2010/02/08/making-the-code-read-like-the-spec.aspx
Note that basically what I'm doing here is avoiding the stack limit by allocating my own stack on the heap. That thing can grow as big as you like. (If you run out of heap memory then you've got bigger problems!)
Note also that it would be wise to choose a data structure that makes the "is a member of?" predicate extremely cheap. An array list of size n is usually O(n) to answer the question "is this element a member of this collection?" which means your algorithm is O(n^2) overall. Can you use a collection like a set or a hash table that has O(1) containment testing?
Also, on a purely "code quality" level, this method could use some work. The fact that there is so much duplicated code in there is a red flag. I would be inclined to write this method like this sketch:
Set<int> PoweredTiles(int powersource)
{
Set<int> result = an empy set;
Stack<int> stack = an empty stack;
stack.Push(powersource);
while (stack is not empty)
{
int current = stack.Pop();
if (result.Contains(current)) continue;
result.Add(current);
int[] neighbours = { compute the neighbours }
foreach(int neighbour in neighbours)
{
if (neighbour is not in range of grid) continue;
if (neighbour is not a power carrier) continue;
stack.Push(neighbour);
}
}
return result;
}
Short, to the point, not recursive, no duplicated code, and O(n).

You just need to convert your recursive implementation into an iterative one (as theory tells us is always possible).
For example, you could:
maintain a queue of cells-to-be-checked
while this queue is not empty, process the front element
to process a cell, do whatever you have to do to the cell itself, then for each of its four nneighbours
if they are not already in the queue, add them to the queue
repeat until the queue is empty

An efficient, recursive algorithm should work provided you do clear the flags (I assume you're simply setting flags on whether a tile has power or not) before doing the recursion. Something like this:
void updateCell(position)
{
for each direction (north, south, east, west) do the following:
-- is there a cell there? (test for edges), if not, exit now;
-- can it be powered?
false: exit now;
true: set powered=true, call updateCell(this position);
}
void updatePowerGrid(start)
{
clearPowerFlags();
set powered=true for start;
updateCell(start);
}
This should work well enough until you use really huge grid sizes.

You can make it iterative. Have two lists, one that keeps track of where you have been, and one that keeps track of where you are currently checking.
Psuedo Code with your code:
While(ToBeChecked is not empty) {
//Note In python i'd be using a copy of the list so I could edit it without
//concequence during the iteration. ie for a in b[:]
for each element in ToBeChecked
updatePowerEnvironment(...);
//Remove element you are checking
removeElementFromToBeChecked(...);
}
public void updatePowerEnvironment(int id, ArrayList<Integer> elements) {
Log.w("GT", "update env for id: " + id);
int newId = id - GameMap.mMapSize;
if (newId >= 0 && GameMap.mMapCells.get(newId).mPowerEnabled
&& !elements.contains(newId)) {
elements.add(newId);
//call addElementToBeChecked instead and I beleive the above already
//makes sure it has not already been checked
addElementToBeChecked(newId, elements);
}
newId = id + GameMap.mMapSize;
if (newId < GameMap.mMapCells.size() && GameMap.mMapCells.get(newId).mPowerEnabled
&& !elements.contains(newId)) {
elements.add(newId);
addElementToBeChecked(newId, elements);
}
newId = id - 1;
if (newId >= 0 && GameMap.mMapCells.get(newId).mPowerEnabled
&& !elements.contains(newId)) {
elements.add(newId);
addElementToBeChecked(newId, elements);
}
newId = id + 1;
if (newId < GameMap.mMapCells.size()
&& GameMap.mMapCells.get(newId).mPowerEnabled
&& !elements.contains(newId)) {
elements.add(newId);
addElementToBeChecked(newId, elements);
}
}
addElementToBeChecked(...) {
ToBeChecked.add();
//Some other stuff if needed
}
removeElemenToBeChecked(...) {
ToBeChecked.remove();
//Some other stuff if needed
}

The very first thing I would try is just to change the search order from North-South-West-East to North-East-South-West. Like this:
public void updatePowerEnvironment(int id, ArrayList<Integer> elements) {
if (!GameMap.ValidCellId(id))
return;
if (!GameMap.mMapCells.get(id).mPowerEnabled)
return;
if (elements.Contains(id))
return;
elements.Add(id);
updatePowerEnvironment(id - GameMap.mMapSize, elements);
updatePowerEnvironment(id + 1, elements);
updatePowerEnvironment(id + GameMap.mMapSize, elements);
updatePowerEnvironment(id - 1, elements);
}
This might reduce the recursion depth, depeding on the maps involved.

Related

Android Studio - App crash when using intent parameter

I have a function that 'crafts' products using two String parameters.
This is working fine when I put in hard coded strings like 'Wheel' & 'Car'.
But it makes my application crash if I try to put in the exact same strings but then provided by an intent.
I already tried to give in variable into the intent instead of a hard coded string. That did not work either.
Here is some part of the code. EDIT: Error log now included
productLeft = getIntent().getStringExtra("PRODUCT LEFT");
productRight = getIntent().getStringExtra("PRODUCT RIGHT");
public void craft(String product1, String product2) {
String[][] Products = factory.getProductList();
int i = 0;
while (finalProduct == "") {
int j;
for(j = 0; j < 3; j++){
if (product1 == Products[i][0] || product2 == Products[i][0]) {
if (product1 == Products[i][1] || product2 == Products[i][1]){
finalProduct = Products[i][2];
}
}
i++;
}
}
}
Problem is with the array index obviously. The array has only four elements and you are fetching index 4, probably in for loop with i variable. But then again I also do not see the role of j in that loop, can't tell without other parts of code.

Comparison method violates its general contract! while sorting [duplicate]

I saw many questions about this, and tried to solve the problem, but after one hour of googling and a lots of trial & error, I still can't fix it. I hope some of you catch the problem.
This is what I get:
java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.util.ComparableTimSort.mergeHi(ComparableTimSort.java:835)
at java.util.ComparableTimSort.mergeAt(ComparableTimSort.java:453)
at java.util.ComparableTimSort.mergeForceCollapse(ComparableTimSort.java:392)
at java.util.ComparableTimSort.sort(ComparableTimSort.java:191)
at java.util.ComparableTimSort.sort(ComparableTimSort.java:146)
at java.util.Arrays.sort(Arrays.java:472)
at java.util.Collections.sort(Collections.java:155)
...
And this is my comparator:
#Override
public int compareTo(Object o) {
if(this == o){
return 0;
}
CollectionItem item = (CollectionItem) o;
Card card1 = CardCache.getInstance().getCard(cardId);
Card card2 = CardCache.getInstance().getCard(item.getCardId());
if (card1.getSet() < card2.getSet()) {
return -1;
} else {
if (card1.getSet() == card2.getSet()) {
if (card1.getRarity() < card2.getRarity()) {
return 1;
} else {
if (card1.getId() == card2.getId()) {
if (cardType > item.getCardType()) {
return 1;
} else {
if (cardType == item.getCardType()) {
return 0;
}
return -1;
}
}
return -1;
}
}
return 1;
}
}
Any idea?
The exception message is actually pretty descriptive. The contract it mentions is transitivity: if A > B and B > C then for any A, B and C: A > C. I checked it with paper and pencil and your code seems to have few holes:
if (card1.getRarity() < card2.getRarity()) {
return 1;
you do not return -1 if card1.getRarity() > card2.getRarity().
if (card1.getId() == card2.getId()) {
//...
}
return -1;
You return -1 if ids aren't equal. You should return -1 or 1 depending on which id was bigger.
Take a look at this. Apart from being much more readable, I think it should actually work:
if (card1.getSet() > card2.getSet()) {
return 1;
}
if (card1.getSet() < card2.getSet()) {
return -1;
};
if (card1.getRarity() < card2.getRarity()) {
return 1;
}
if (card1.getRarity() > card2.getRarity()) {
return -1;
}
if (card1.getId() > card2.getId()) {
return 1;
}
if (card1.getId() < card2.getId()) {
return -1;
}
return cardType - item.getCardType(); //watch out for overflow!
You can use the following class to pinpoint transitivity bugs in your Comparators:
/**
* #author Gili Tzabari
*/
public final class Comparators
{
/**
* Verify that a comparator is transitive.
*
* #param <T> the type being compared
* #param comparator the comparator to test
* #param elements the elements to test against
* #throws AssertionError if the comparator is not transitive
*/
public static <T> void verifyTransitivity(Comparator<T> comparator, Collection<T> elements)
{
for (T first: elements)
{
for (T second: elements)
{
int result1 = comparator.compare(first, second);
int result2 = comparator.compare(second, first);
if (result1 != -result2)
{
// Uncomment the following line to step through the failed case
//comparator.compare(first, second);
throw new AssertionError("compare(" + first + ", " + second + ") == " + result1 +
" but swapping the parameters returns " + result2);
}
}
}
for (T first: elements)
{
for (T second: elements)
{
int firstGreaterThanSecond = comparator.compare(first, second);
if (firstGreaterThanSecond <= 0)
continue;
for (T third: elements)
{
int secondGreaterThanThird = comparator.compare(second, third);
if (secondGreaterThanThird <= 0)
continue;
int firstGreaterThanThird = comparator.compare(first, third);
if (firstGreaterThanThird <= 0)
{
// Uncomment the following line to step through the failed case
//comparator.compare(first, third);
throw new AssertionError("compare(" + first + ", " + second + ") > 0, " +
"compare(" + second + ", " + third + ") > 0, but compare(" + first + ", " + third + ") == " +
firstGreaterThanThird);
}
}
}
}
}
/**
* Prevent construction.
*/
private Comparators()
{
}
}
Simply invoke Comparators.verifyTransitivity(myComparator, myCollection) in front of the code that fails.
It also has something to do with the version of JDK.
If it does well in JDK6, maybe it will have the problem in JDK 7 described by you, because the implementation method in jdk 7 has been changed.
Look at this:
Description: The sorting algorithm used by java.util.Arrays.sort and (indirectly) by java.util.Collections.sort has been replaced. The new sort implementation may throw an IllegalArgumentException if it detects a Comparable that violates the Comparable contract. The previous implementation silently ignored such a situation. If the previous behavior is desired, you can use the new system property, java.util.Arrays.useLegacyMergeSort, to restore previous mergesort behaviour.
I don't know the exact reason. However, if you add the code before you use sort. It will be OK.
System.setProperty("java.util.Arrays.useLegacyMergeSort", "true");
Consider the following case:
First, o1.compareTo(o2) is called. card1.getSet() == card2.getSet() happens to be true and so is card1.getRarity() < card2.getRarity(), so you return 1.
Then, o2.compareTo(o1) is called. Again, card1.getSet() == card2.getSet() is true. Then, you skip to the following else, then card1.getId() == card2.getId() happens to be true, and so is cardType > item.getCardType(). You return 1 again.
From that, o1 > o2, and o2 > o1. You broke the contract.
if (card1.getRarity() < card2.getRarity()) {
return 1;
However, if card2.getRarity() is less than card1.getRarity() you might not return -1.
You similarly miss other cases. I would do this, you can change around depending on your intent:
public int compareTo(Object o) {
if(this == o){
return 0;
}
CollectionItem item = (CollectionItem) o;
Card card1 = CardCache.getInstance().getCard(cardId);
Card card2 = CardCache.getInstance().getCard(item.getCardId());
int comp=card1.getSet() - card2.getSet();
if (comp!=0){
return comp;
}
comp=card1.getRarity() - card2.getRarity();
if (comp!=0){
return comp;
}
comp=card1.getSet() - card2.getSet();
if (comp!=0){
return comp;
}
comp=card1.getId() - card2.getId();
if (comp!=0){
return comp;
}
comp=card1.getCardType() - card2.getCardType();
return comp;
}
}
I had the same symptom. For me it turned out that another thread was modifying the compared objects while the sorting was happening in a Stream. To resolve the issue, I mapped the objects to immutable temporary objects, collected the Stream to a temporary Collection and did the sorting on that.
The origin of this exception is a wrong Comparator implementation. By checking the docs, we must implement the compare(o1, o2) method as an equivalence relation by following the rules:
if a.equals(b) is true then compare(a, b) is 0
if a.compare(b) > 0 then b.compare(a) < 0 is true
if a.compare(b) > 0 and b.compare(c) > 0 then a.compare(c) > 0 is true
You may check your code to realize where your implementation is offending one or more of Comparator contract rules. If it is hard to find it by a static analysis, you can use the data which cast the exception to check the rules.
If you try to run this code you will meet the kind this exception:
public static void main(String[] args) {
Random random = new Random();
List<Integer> list = new ArrayList<>();
for (int i = 0; i < 50000; i++) {
list.add(random.nextInt());
}
list.sort((x, y) -> {
int c = random.nextInt(3);
if (c == 0) {
return 0;
}
if (c == 1) {
return 1;
}
return -1;
});
}
Exception in thread "main" java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.util.TimSort.mergeLo(TimSort.java:777)
at java.util.TimSort.mergeAt(TimSort.java:514)
at java.util.TimSort.mergeCollapse(TimSort.java:441)
at java.util.TimSort.sort(TimSort.java:245)
at java.util.Arrays.sort(Arrays.java:1512)
at java.util.ArrayList.sort(ArrayList.java:1462)
at Test.main(Test.java:14)
The reason is when implementing the Comparator, it may meet the case of A > B and B > C and C > A and the sort method will be run around to be broken. Java prevent this case by throw exception this case:
class TimSort<T> {
.
.
.
else if (len1 == 0) {
throw new IllegalArgumentException(
"Comparison method violates its general contract!");
.
.
.
In conclusion, to handle this issue. You have to make sure the comparator will not meet the case of A > B and B > C and C > A.
I got the same error with a class like the following StockPickBean. Called from this code:
List<StockPickBean> beansListcatMap.getValue();
beansList.sort(StockPickBean.Comparators.VALUE);
public class StockPickBean implements Comparable<StockPickBean> {
private double value;
public double getValue() { return value; }
public void setValue(double value) { this.value = value; }
#Override
public int compareTo(StockPickBean view) {
return Comparators.VALUE.compare(this,view); //return
Comparators.SYMBOL.compare(this,view);
}
public static class Comparators {
public static Comparator<StockPickBean> VALUE = (val1, val2) ->
(int)
(val1.value - val2.value);
}
}
After getting the same error:
java.lang.IllegalArgumentException: Comparison method violates its general contract!
I changed this line:
public static Comparator<StockPickBean> VALUE = (val1, val2) -> (int)
(val1.value - val2.value);
to:
public static Comparator<StockPickBean> VALUE = (StockPickBean spb1,
StockPickBean spb2) -> Double.compare(spb2.value,spb1.value);
That fixes the error.
I ran into a similar problem where I was trying to sort a n x 2 2D array named contests which is a 2D array of simple integers. This was working for most of the times but threw a runtime error for one input:-
Arrays.sort(contests, (row1, row2) -> {
if (row1[0] < row2[0]) {
return 1;
} else return -1;
});
Error:-
Exception in thread "main" java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.base/java.util.TimSort.mergeHi(TimSort.java:903)
at java.base/java.util.TimSort.mergeAt(TimSort.java:520)
at java.base/java.util.TimSort.mergeForceCollapse(TimSort.java:461)
at java.base/java.util.TimSort.sort(TimSort.java:254)
at java.base/java.util.Arrays.sort(Arrays.java:1441)
at com.hackerrank.Solution.luckBalance(Solution.java:15)
at com.hackerrank.Solution.main(Solution.java:49)
Looking at the answers above I tried adding a condition for equals and I don't know why but it worked. Hopefully we must explicitly specify what should be returned for all cases (greater than, equals and less than):
Arrays.sort(contests, (row1, row2) -> {
if (row1[0] < row2[0]) {
return 1;
}
if(row1[0] == row2[0]) return 0;
return -1;
});
A variation of Gili's answer to check if the comparator satisfies the requirements described in the compare method's javadoc - with a focus on completeness and readability, e.g. by naming the variables the same as in the javadoc. Note that this is O(n^3), only use it when debugging, maybe just on a subset of your elements, in order to be fast enough to finish at all.
public static <T> void verifyComparator(Comparator<T> comparator, Collection<T> elements) {
for (T x : elements) {
for (T y : elements) {
for (T z : elements) {
int x_y = comparator.compare(x, y);
int y_x = comparator.compare(y, x);
int y_z = comparator.compare(y, z);
int x_z = comparator.compare(x, z);
// javadoc: The implementor must ensure that sgn(compare(x, y)) == -sgn(compare(y, x))
if (Math.signum(x_y) == -Math.signum(y_x)) { // ok
} else {
System.err.println("not holding: sgn(compare(x, y)) == -sgn(compare(y, x))" //
+ " | x_y: " + x_y + ", y_x: " + y_x + ", x: " + x + ", y: " + y);
}
// javadoc: The implementor must also ensure that the relation is transitive:
// ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.
if (x_y > 0 && y_z > 0) {
if (x_z > 0) { // ok
} else {
System.err.println("not holding: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0" //
+ " | x_y: " + x_y + ", y_z: " + y_z + ", x_z: " + x_z + ", x: " + x + ", y: " + y + ", z: " + z);
}
}
// javadoc: Finally, the implementor must ensure that:
// compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z.
if (x_y == 0) {
if (Math.signum(x_z) == Math.signum(y_z)) { // ok
} else {
System.err.println("not holding: compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z" //
+ " | x_y: " + x_y + ", x_z: " + x_z + ", y_z: " + y_z + ", x: " + x + ", y: " + y + ", z: " + z);
}
}
}
}
}
}
I had to sort on several criterion (date, and, if same date; other things...). What was working on Eclipse with an older version of Java, did not worked any more on Android : comparison method violates contract ...
After reading on StackOverflow, I wrote a separate function that I called from compare() if the dates are the same. This function calculates the priority, according to the criteria, and returns -1, 0, or 1 to compare(). It seems to work now.
What about doing something simpler like this:
int result = card1.getSet().compareTo(card2.getSet())
if (result == 0) {
result = card1.getRarity().compareTo(card2.getRarity())
}
if (result == 0) {
result = card1.getId().compareTo(card2.getId())
}
if (result == 0) {
result = card1.getCardType().compareTo(card2.getCardType())
}
return result;
You just need to order the comparisons in order of preference.

How to change edittext string sum to int [duplicate]

I'm trying to write a Java routine to evaluate math expressions from String values like:
"5+3"
"10-4*5"
"(1+10)*3"
I want to avoid a lot of if-then-else statements.
How can I do this?
With JDK1.6, you can use the built-in Javascript engine.
import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
import javax.script.ScriptException;
public class Test {
public static void main(String[] args) throws ScriptException {
ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine engine = mgr.getEngineByName("JavaScript");
String foo = "40+2";
System.out.println(engine.eval(foo));
}
}
I've written this eval method for arithmetic expressions to answer this question. It does addition, subtraction, multiplication, division, exponentiation (using the ^ symbol), and a few basic functions like sqrt. It supports grouping using (...), and it gets the operator precedence and associativity rules correct.
public static double eval(final String str) {
return new Object() {
int pos = -1, ch;
void nextChar() {
ch = (++pos < str.length()) ? str.charAt(pos) : -1;
}
boolean eat(int charToEat) {
while (ch == ' ') nextChar();
if (ch == charToEat) {
nextChar();
return true;
}
return false;
}
double parse() {
nextChar();
double x = parseExpression();
if (pos < str.length()) throw new RuntimeException("Unexpected: " + (char)ch);
return x;
}
// Grammar:
// expression = term | expression `+` term | expression `-` term
// term = factor | term `*` factor | term `/` factor
// factor = `+` factor | `-` factor | `(` expression `)` | number
// | functionName `(` expression `)` | functionName factor
// | factor `^` factor
double parseExpression() {
double x = parseTerm();
for (;;) {
if (eat('+')) x += parseTerm(); // addition
else if (eat('-')) x -= parseTerm(); // subtraction
else return x;
}
}
double parseTerm() {
double x = parseFactor();
for (;;) {
if (eat('*')) x *= parseFactor(); // multiplication
else if (eat('/')) x /= parseFactor(); // division
else return x;
}
}
double parseFactor() {
if (eat('+')) return +parseFactor(); // unary plus
if (eat('-')) return -parseFactor(); // unary minus
double x;
int startPos = this.pos;
if (eat('(')) { // parentheses
x = parseExpression();
if (!eat(')')) throw new RuntimeException("Missing ')'");
} else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers
while ((ch >= '0' && ch <= '9') || ch == '.') nextChar();
x = Double.parseDouble(str.substring(startPos, this.pos));
} else if (ch >= 'a' && ch <= 'z') { // functions
while (ch >= 'a' && ch <= 'z') nextChar();
String func = str.substring(startPos, this.pos);
if (eat('(')) {
x = parseExpression();
if (!eat(')')) throw new RuntimeException("Missing ')' after argument to " + func);
} else {
x = parseFactor();
}
if (func.equals("sqrt")) x = Math.sqrt(x);
else if (func.equals("sin")) x = Math.sin(Math.toRadians(x));
else if (func.equals("cos")) x = Math.cos(Math.toRadians(x));
else if (func.equals("tan")) x = Math.tan(Math.toRadians(x));
else throw new RuntimeException("Unknown function: " + func);
} else {
throw new RuntimeException("Unexpected: " + (char)ch);
}
if (eat('^')) x = Math.pow(x, parseFactor()); // exponentiation
return x;
}
}.parse();
}
Example:
System.out.println(eval("((4 - 2^3 + 1) * -sqrt(3*3+4*4)) / 2"));
Output: 7.5 (which is correct)
The parser is a recursive descent parser, so internally uses separate parse methods for each level of operator precedence in its grammar. I deliberately kept it short, but here are some ideas you might want to expand it with:
Variables:
The bit of the parser that reads the names for functions can easily be changed to handle custom variables too, by looking up names in a variable table passed to the eval method, such as a Map<String,Double> variables.
Separate compilation and evaluation:
What if, having added support for variables, you wanted to evaluate the same expression millions of times with changed variables, without parsing it every time? It's possible. First define an interface to use to evaluate the precompiled expression:
#FunctionalInterface
interface Expression {
double eval();
}
Now to rework the original "eval" function into a "parse" function, change all the methods that return doubles, so instead they return an instance of that interface. Java 8's lambda syntax works well for this. Example of one of the changed methods:
Expression parseExpression() {
Expression x = parseTerm();
for (;;) {
if (eat('+')) { // addition
Expression a = x, b = parseTerm();
x = (() -> a.eval() + b.eval());
} else if (eat('-')) { // subtraction
Expression a = x, b = parseTerm();
x = (() -> a.eval() - b.eval());
} else {
return x;
}
}
}
That builds a recursive tree of Expression objects representing the compiled expression (an abstract syntax tree). Then you can compile it once and evaluate it repeatedly with different values:
public static void main(String[] args) {
Map<String,Double> variables = new HashMap<>();
Expression exp = parse("x^2 - x + 2", variables);
for (double x = -20; x <= +20; x++) {
variables.put("x", x);
System.out.println(x + " => " + exp.eval());
}
}
Different datatypes:
Instead of double, you could change the evaluator to use something more powerful like BigDecimal, or a class that implements complex numbers, or rational numbers (fractions). You could even use Object, allowing some mix of datatypes in expressions, just like a real programming language. :)
All code in this answer released to the public domain. Have fun!
For my university project, I was looking for a parser / evaluator supporting both basic formulas and more complicated equations (especially iterated operators). I found very nice open source library for JAVA and .NET called mXparser. I will give a few examples to make some feeling on the syntax, for further instructions please visit project website (especially tutorial section).
https://mathparser.org/
https://mathparser.org/mxparser-tutorial/
https://mathparser.org/api/
And few examples
1 - Simple furmula
Expression e = new Expression("( 2 + 3/4 + sin(pi) )/2");
double v = e.calculate()
2 - User defined arguments and constants
Argument x = new Argument("x = 10");
Constant a = new Constant("a = pi^2");
Expression e = new Expression("cos(a*x)", x, a);
double v = e.calculate()
3 - User defined functions
Function f = new Function("f(x, y, z) = sin(x) + cos(y*z)");
Expression e = new Expression("f(3,2,5)", f);
double v = e.calculate()
4 - Iteration
Expression e = new Expression("sum( i, 1, 100, sin(i) )");
double v = e.calculate()
Found recently - in case you would like to try the syntax (and see the advanced use case) you can download the Scalar Calculator app that is powered by mXparser.
The correct way to solve this is with a lexer and a parser. You can write simple versions of these yourself, or those pages also have links to Java lexers and parsers.
Creating a recursive descent parser is a really good learning exercise.
HERE is another open source library on GitHub named EvalEx.
Unlike the JavaScript engine this library is focused in evaluating mathematical expressions only. Moreover, the library is extensible and supports use of boolean operators as well as parentheses.
You can also try the BeanShell interpreter:
Interpreter interpreter = new Interpreter();
interpreter.eval("result = (7+21*6)/(32-27)");
System.out.println(interpreter.get("result"));
You can evaluate expressions easily if your Java application already accesses a database, without using any other JARs.
Some databases require you to use a dummy table (eg, Oracle's "dual" table) and others will allow you to evaluate expressions without "selecting" from any table.
For example, in Sql Server or Sqlite
select (((12.10 +12.0))/ 233.0) amount
and in Oracle
select (((12.10 +12.0))/ 233.0) amount from dual;
The advantage of using a DB is that you can evaluate many expressions at the same time. Also most DB's will allow you to use highly complex expressions and will also have a number of extra functions that can be called as necessary.
However performance may suffer if many single expressions need to be evaluated individually, particularly when the DB is located on a network server.
The following addresses the performance problem to some extent, by using a Sqlite in-memory database.
Here's a full working example in Java
Class. forName("org.sqlite.JDBC");
Connection conn = DriverManager.getConnection("jdbc:sqlite::memory:");
Statement stat = conn.createStatement();
ResultSet rs = stat.executeQuery( "select (1+10)/20.0 amount");
rs.next();
System.out.println(rs.getBigDecimal(1));
stat.close();
conn.close();
Of course you could extend the above code to handle multiple calculations at the same time.
ResultSet rs = stat.executeQuery( "select (1+10)/20.0 amount, (1+100)/20.0 amount2");
Another way is to use the Spring Expression Language or SpEL which does a whole lot more along with evaluating mathematical expressions, therefore maybe slightly overkill. You do not have to be using Spring framework to use this expression library as it is stand-alone. Copying examples from SpEL's documentation:
ExpressionParser parser = new SpelExpressionParser();
int two = parser.parseExpression("1 + 1").getValue(Integer.class); // 2
double twentyFour = parser.parseExpression("2.0 * 3e0 * 4").getValue(Double.class); //24.0
This article discusses various approaches. Here are the 2 key approaches mentioned in the article:
JEXL from Apache
Allows for scripts that include references to java objects.
// Create or retrieve a JexlEngine
JexlEngine jexl = new JexlEngine();
// Create an expression object
String jexlExp = "foo.innerFoo.bar()";
Expression e = jexl.createExpression( jexlExp );
// Create a context and add data
JexlContext jctx = new MapContext();
jctx.set("foo", new Foo() );
// Now evaluate the expression, getting the result
Object o = e.evaluate(jctx);
Use the javascript engine embedded in the JDK:
private static void jsEvalWithVariable()
{
List<String> namesList = new ArrayList<String>();
namesList.add("Jill");
namesList.add("Bob");
namesList.add("Laureen");
namesList.add("Ed");
ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine jsEngine = mgr.getEngineByName("JavaScript");
jsEngine.put("namesListKey", namesList);
System.out.println("Executing in script environment...");
try
{
jsEngine.eval("var x;" +
"var names = namesListKey.toArray();" +
"for(x in names) {" +
" println(names[x]);" +
"}" +
"namesListKey.add(\"Dana\");");
}
catch (ScriptException ex)
{
ex.printStackTrace();
}
}
if we are going to implement it then we can can use the below algorithm :--
While there are still tokens to be read in,
1.1 Get the next token.
1.2 If the token is:
1.2.1 A number: push it onto the value stack.
1.2.2 A variable: get its value, and push onto the value stack.
1.2.3 A left parenthesis: push it onto the operator stack.
1.2.4 A right parenthesis:
1 While the thing on top of the operator stack is not a
left parenthesis,
1 Pop the operator from the operator stack.
2 Pop the value stack twice, getting two operands.
3 Apply the operator to the operands, in the correct order.
4 Push the result onto the value stack.
2 Pop the left parenthesis from the operator stack, and discard it.
1.2.5 An operator (call it thisOp):
1 While the operator stack is not empty, and the top thing on the
operator stack has the same or greater precedence as thisOp,
1 Pop the operator from the operator stack.
2 Pop the value stack twice, getting two operands.
3 Apply the operator to the operands, in the correct order.
4 Push the result onto the value stack.
2 Push thisOp onto the operator stack.
While the operator stack is not empty,
1 Pop the operator from the operator stack.
2 Pop the value stack twice, getting two operands.
3 Apply the operator to the operands, in the correct order.
4 Push the result onto the value stack.
At this point the operator stack should be empty, and the value
stack should have only one value in it, which is the final result.
This is another interesting alternative
https://github.com/Shy-Ta/expression-evaluator-demo
The usage is very simple and gets the job done, for example:
ExpressionsEvaluator evalExpr = ExpressionsFactory.create("2+3*4-6/2");
assertEquals(BigDecimal.valueOf(11), evalExpr.eval());
It seems like JEP should do the job
It's too late to answer but I came across same situation to evaluate expression in java, it might help someone
MVEL does runtime evaluation of expressions, we can write a java code in String to get it evaluated in this.
String expressionStr = "x+y";
Map<String, Object> vars = new HashMap<String, Object>();
vars.put("x", 10);
vars.put("y", 20);
ExecutableStatement statement = (ExecutableStatement) MVEL.compileExpression(expressionStr);
Object result = MVEL.executeExpression(statement, vars);
Try the following sample code using JDK1.6's Javascript engine with code injection handling.
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
public class EvalUtil {
private static ScriptEngine engine = new ScriptEngineManager().getEngineByName("JavaScript");
public static void main(String[] args) {
try {
System.out.println((new EvalUtil()).eval("(((5+5)/2) > 5) || 5 >3 "));
System.out.println((new EvalUtil()).eval("(((5+5)/2) > 5) || true"));
} catch (Exception e) {
e.printStackTrace();
}
}
public Object eval(String input) throws Exception{
try {
if(input.matches(".*[a-zA-Z;~`#$_{}\\[\\]:\\\\;\"',\\.\\?]+.*")) {
throw new Exception("Invalid expression : " + input );
}
return engine.eval(input);
} catch (Exception e) {
e.printStackTrace();
throw e;
}
}
}
This is actually complementing the answer given by #Boann. It has a slight bug which causes "-2 ^ 2" to give an erroneous result of -4.0. The problem for that is the point at which the exponentiation is evaluated in his. Just move the exponentiation to the block of parseTerm(), and you'll be all fine. Have a look at the below, which is #Boann's answer slightly modified. Modification is in the comments.
public static double eval(final String str) {
return new Object() {
int pos = -1, ch;
void nextChar() {
ch = (++pos < str.length()) ? str.charAt(pos) : -1;
}
boolean eat(int charToEat) {
while (ch == ' ') nextChar();
if (ch == charToEat) {
nextChar();
return true;
}
return false;
}
double parse() {
nextChar();
double x = parseExpression();
if (pos < str.length()) throw new RuntimeException("Unexpected: " + (char)ch);
return x;
}
// Grammar:
// expression = term | expression `+` term | expression `-` term
// term = factor | term `*` factor | term `/` factor
// factor = `+` factor | `-` factor | `(` expression `)`
// | number | functionName factor | factor `^` factor
double parseExpression() {
double x = parseTerm();
for (;;) {
if (eat('+')) x += parseTerm(); // addition
else if (eat('-')) x -= parseTerm(); // subtraction
else return x;
}
}
double parseTerm() {
double x = parseFactor();
for (;;) {
if (eat('*')) x *= parseFactor(); // multiplication
else if (eat('/')) x /= parseFactor(); // division
else if (eat('^')) x = Math.pow(x, parseFactor()); //exponentiation -> Moved in to here. So the problem is fixed
else return x;
}
}
double parseFactor() {
if (eat('+')) return parseFactor(); // unary plus
if (eat('-')) return -parseFactor(); // unary minus
double x;
int startPos = this.pos;
if (eat('(')) { // parentheses
x = parseExpression();
eat(')');
} else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers
while ((ch >= '0' && ch <= '9') || ch == '.') nextChar();
x = Double.parseDouble(str.substring(startPos, this.pos));
} else if (ch >= 'a' && ch <= 'z') { // functions
while (ch >= 'a' && ch <= 'z') nextChar();
String func = str.substring(startPos, this.pos);
x = parseFactor();
if (func.equals("sqrt")) x = Math.sqrt(x);
else if (func.equals("sin")) x = Math.sin(Math.toRadians(x));
else if (func.equals("cos")) x = Math.cos(Math.toRadians(x));
else if (func.equals("tan")) x = Math.tan(Math.toRadians(x));
else throw new RuntimeException("Unknown function: " + func);
} else {
throw new RuntimeException("Unexpected: " + (char)ch);
}
//if (eat('^')) x = Math.pow(x, parseFactor()); // exponentiation -> This is causing a bit of problem
return x;
}
}.parse();
}
import java.util.*;
public class check {
int ans;
String str="7 + 5";
StringTokenizer st=new StringTokenizer(str);
int v1=Integer.parseInt(st.nextToken());
String op=st.nextToken();
int v2=Integer.parseInt(st.nextToken());
if(op.equals("+")) { ans= v1 + v2; }
if(op.equals("-")) { ans= v1 - v2; }
//.........
}
I think what ever way you do this it's going to involve a lot of conditional statements. But for single operations like in your examples you could limit it to 4 if statements with something like
String math = "1+4";
if (math.split("+").length == 2) {
//do calculation
} else if (math.split("-").length == 2) {
//do calculation
} ...
It gets a whole lot more complicated when you want to deal with multiple operations like "4+5*6".
If you are trying to build a calculator then I'd surgest passing each section of the calculation separatly (each number or operator) rather than as a single string.
You might have a look at the Symja framework:
ExprEvaluator util = new ExprEvaluator();
IExpr result = util.evaluate("10-40");
System.out.println(result.toString()); // -> "-30"
Take note that definitively more complex expressions can be evaluated:
// D(...) gives the derivative of the function Sin(x)*Cos(x)
IAST function = D(Times(Sin(x), Cos(x)), x);
IExpr result = util.evaluate(function);
// print: Cos(x)^2-Sin(x)^2
package ExpressionCalculator.expressioncalculator;
import java.text.DecimalFormat;
import java.util.Scanner;
public class ExpressionCalculator {
private static String addSpaces(String exp){
//Add space padding to operands.
//https://regex101.com/r/sJ9gM7/73
exp = exp.replaceAll("(?<=[0-9()])[\\/]", " / ");
exp = exp.replaceAll("(?<=[0-9()])[\\^]", " ^ ");
exp = exp.replaceAll("(?<=[0-9()])[\\*]", " * ");
exp = exp.replaceAll("(?<=[0-9()])[+]", " + ");
exp = exp.replaceAll("(?<=[0-9()])[-]", " - ");
//Keep replacing double spaces with single spaces until your string is properly formatted
/*while(exp.indexOf(" ") != -1){
exp = exp.replace(" ", " ");
}*/
exp = exp.replaceAll(" {2,}", " ");
return exp;
}
public static Double evaluate(String expr){
DecimalFormat df = new DecimalFormat("#.####");
//Format the expression properly before performing operations
String expression = addSpaces(expr);
try {
//We will evaluate using rule BDMAS, i.e. brackets, division, power, multiplication, addition and
//subtraction will be processed in following order
int indexClose = expression.indexOf(")");
int indexOpen = -1;
if (indexClose != -1) {
String substring = expression.substring(0, indexClose);
indexOpen = substring.lastIndexOf("(");
substring = substring.substring(indexOpen + 1).trim();
if(indexOpen != -1 && indexClose != -1) {
Double result = evaluate(substring);
expression = expression.substring(0, indexOpen).trim() + " " + result + " " + expression.substring(indexClose + 1).trim();
return evaluate(expression.trim());
}
}
String operation = "";
if(expression.indexOf(" / ") != -1){
operation = "/";
}else if(expression.indexOf(" ^ ") != -1){
operation = "^";
} else if(expression.indexOf(" * ") != -1){
operation = "*";
} else if(expression.indexOf(" + ") != -1){
operation = "+";
} else if(expression.indexOf(" - ") != -1){ //Avoid negative numbers
operation = "-";
} else{
return Double.parseDouble(expression);
}
int index = expression.indexOf(operation);
if(index != -1){
indexOpen = expression.lastIndexOf(" ", index - 2);
indexOpen = (indexOpen == -1)?0:indexOpen;
indexClose = expression.indexOf(" ", index + 2);
indexClose = (indexClose == -1)?expression.length():indexClose;
if(indexOpen != -1 && indexClose != -1) {
Double lhs = Double.parseDouble(expression.substring(indexOpen, index));
Double rhs = Double.parseDouble(expression.substring(index + 2, indexClose));
Double result = null;
switch (operation){
case "/":
//Prevent divide by 0 exception.
if(rhs == 0){
return null;
}
result = lhs / rhs;
break;
case "^":
result = Math.pow(lhs, rhs);
break;
case "*":
result = lhs * rhs;
break;
case "-":
result = lhs - rhs;
break;
case "+":
result = lhs + rhs;
break;
default:
break;
}
if(indexClose == expression.length()){
expression = expression.substring(0, indexOpen) + " " + result + " " + expression.substring(indexClose);
}else{
expression = expression.substring(0, indexOpen) + " " + result + " " + expression.substring(indexClose + 1);
}
return Double.valueOf(df.format(evaluate(expression.trim())));
}
}
}catch(Exception exp){
exp.printStackTrace();
}
return 0.0;
}
public static void main(String args[]){
Scanner scanner = new Scanner(System.in);
System.out.print("Enter an Mathematical Expression to Evaluate: ");
String input = scanner.nextLine();
System.out.println(evaluate(input));
}
}
A Java class that can evaluate mathematical expressions:
package test;
public class Calculator {
public static Double calculate(String expression){
if (expression == null || expression.length() == 0) {
return null;
}
return calc(expression.replace(" ", ""));
}
public static Double calc(String expression) {
String[] containerArr = new String[]{expression};
double leftVal = getNextOperand(containerArr);
expression = containerArr[0];
if (expression.length() == 0) {
return leftVal;
}
char operator = expression.charAt(0);
expression = expression.substring(1);
while (operator == '*' || operator == '/') {
containerArr[0] = expression;
double rightVal = getNextOperand(containerArr);
expression = containerArr[0];
if (operator == '*') {
leftVal = leftVal * rightVal;
} else {
leftVal = leftVal / rightVal;
}
if (expression.length() > 0) {
operator = expression.charAt(0);
expression = expression.substring(1);
} else {
return leftVal;
}
}
if (operator == '+') {
return leftVal + calc(expression);
} else {
return leftVal - calc(expression);
}
}
private static double getNextOperand(String[] exp){
double res;
if (exp[0].startsWith("(")) {
int open = 1;
int i = 1;
while (open != 0) {
if (exp[0].charAt(i) == '(') {
open++;
} else if (exp[0].charAt(i) == ')') {
open--;
}
i++;
}
res = calc(exp[0].substring(1, i - 1));
exp[0] = exp[0].substring(i);
} else {
int i = 1;
if (exp[0].charAt(0) == '-') {
i++;
}
while (exp[0].length() > i && isNumber((int) exp[0].charAt(i))) {
i++;
}
res = Double.parseDouble(exp[0].substring(0, i));
exp[0] = exp[0].substring(i);
}
return res;
}
private static boolean isNumber(int c) {
int zero = (int) '0';
int nine = (int) '9';
return (c >= zero && c <= nine) || c =='.';
}
public static void main(String[] args) {
System.out.println(calculate("(((( -6 )))) * 9 * -1"));
System.out.println(calc("(-5.2+-5*-5*((5/4+2)))"));
}
}
How about something like this:
String st = "10+3";
int result;
for(int i=0;i<st.length();i++)
{
if(st.charAt(i)=='+')
{
result=Integer.parseInt(st.substring(0, i))+Integer.parseInt(st.substring(i+1, st.length()));
System.out.print(result);
}
}
and do the similar thing for every other mathematical operator accordingly ..
It is possible to convert any expression string in infix notation to a postfix notation using Djikstra's shunting-yard algorithm. The result of the algorithm can then serve as input to the postfix algorithm with returns the result of the expression.
I wrote an article about it here, with an implementation in java
Yet another option: https://github.com/stefanhaustein/expressionparser
I have implemented this to have a simple but flexible option to permit both:
Immediate processing (Calculator.java, SetDemo.java)
Building and processing a parse tree (TreeBuilder.java)
The TreeBuilder linked above is part of a CAS demo package that does symbolic derivation. There is also a BASIC interpreter example and I have started to build a TypeScript interpreter using it.
External library like RHINO or NASHORN can be used to run javascript. And javascript can evaluate simple formula without parcing the string. No performance impact as well if code is written well.
Below is an example with RHINO -
public class RhinoApp {
private String simpleAdd = "(12+13+2-2)*2+(12+13+2-2)*2";
public void runJavaScript() {
Context jsCx = Context.enter();
Context.getCurrentContext().setOptimizationLevel(-1);
ScriptableObject scope = jsCx.initStandardObjects();
Object result = jsCx.evaluateString(scope, simpleAdd , "formula", 0, null);
Context.exit();
System.out.println(result);
}
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
public class test2 {
public static void main(String[] args) throws ScriptException {
String s = "10+2";
ScriptEngineManager mn = new ScriptEngineManager();
ScriptEngine en = mn.getEngineByName("js");
Object result = en.eval(s);
System.out.println(result);
}
}
I have done using iterative parsing and shunting Yard algorithm and i have really enjoyed developing the expression evaluator ,you can find all the code here
https://github.com/nagaraj200788/JavaExpressionEvaluator
Has 73 test cases and even works for Bigintegers,Bigdecimals
supports all relational, arithmetic expression and also combination of both .
even supports ternary operator .
Added enhancement to support signed numbers like -100+89 it was intresting, for details check TokenReader.isUnaryOperator() method and i have updated code in above Link

Views not removing as expected

So this function is intended to remove a Unit object, and its corresponding view from a list of views. It then checks the rest of the Units, and if the previously removed one was a pre-requisite for another Unit, it recurses and removes that unit too, and so on.
The Units are removed correctly from storage, and displayed back in the pending display, but not all of the removed units' views are removed from "temp".
Edit: The function is working entirely when a maximum of one unit is in each semester, but leaves things behind otherwise.
Any insight you could offer into why this is occuring would be muchly appreciated.
void removeLinkedUnits(Unit inUnit)
{
int sem;
for(sem = semesters.size() - 1; sem >= 0; sem--) //iterate through each semester containing units
{
int unit;
for(unit = semesters.get(sem).getUnits().size() - 1; unit >= 0; unit--) //iterate through each unit in a semester
{
String[] pres = semesters.get(sem).getUnit(unit).getPrerequisites();
int i;
boolean toRemove = false;
for(i = 0; i < pres.length; i++) //Compare list of pre-requisites against removed unit.
{
if(pres[i].contains(inUnit.getUnitID()))
{
toRemove = true;
}
}
if(semesters.get(sem).getUnits().get(unit).getCorequisites().contains(inUnit.getUnitID()))
{
toRemove = true;
}
if(toRemove) //Unit relies on previously removed unit
{
Unit unitx = semesters.get(sem).getUnit(unit);
semesters.get(sem).remove(unitx);
LinearLayout temp = vertUnitLayouts.get(sem);
temp.removeViewAt(unit);
scheduledUnits.remove(unitx.getUnitID());
removeLinkedUnits(unitx);
redrawPendingSpinners();
pendingUnits.add(unitx);
LinearLayout pendingLinear = (LinearLayout) findViewById(R.id.pendingLinear);
pendingLinear.addView(makePendingView(unitx));
}
}
}
}
Figured out the problem.
temp (vertUnitsLayout) contained 2 items per Unit, not the 1 I was expecting. Misunderstood how my partner was constructing his views.
I need to use debugger more :S
Replaced:
temp.removeViewAt(unit);
With:
temp.removeViewAt(unit*2);
temp.removeViewAt(unit*2);

Update item ID when deleting

I have 5 empty TextViews where I add the names. After adding a name, it is stored in a database. The database consist on 2 columns, the item ID and the item NAME. This is an example of what I'm doing:
- Mark1 //ID=1, NAME= Mark1
- Mark2 //ID=2, NAME= Mark2
- Mark3 //ID=3, NAME= Mark3
- Empty
- Empty
I add and edit perfectly the textViews, but I'm facing a problem when deleting. This has something to do with the way I'm getting the values from the database, I'll explain:
Every time the app starts, or I edit, add or delete one element, what I do is get the items from the database, get them into a Map, and copy them into the textviews (whose at a first time are invisible) making visible just the ones that have a name setted.
This is the code I use to do that:
public void getTravelers() {
/*Create map where store items*/
Map<Integer, String> nameList = new HashMap<Integer, String>();
/*Lon in providers query() method to get database's items and save them into the map*/
Cursor c = getContentResolver().query(TravelersProvider.CONTENT_URI, PROJECTION, null, null, null);
if (c.moveToFirst()) {
do {
nameList.put(Integer.parseInt(c.getString(c.getColumnIndex(Travelers._ID))), c.getString(c.getColumnIndex(Travelers.NAME)));
}while(c.moveToNext());
}
if (c != null && !c.isClosed()) {
c.close();
}
/*Check size*/
int size = nameList.size();
if (size >= 1) {
/*Save items in TextViews*/
//TODO: This is the code I should fix
for (int i = 0; i <= size; i++) {
if (i==1) {
traveler1.setText(nameList.get(i).toString());
traveler1.setVisibility(View.VISIBLE);
}
if (i==2) {
traveler2.setText(nameList.get(i).toString());
traveler2.setVisibility(View.VISIBLE);
}
if (i==3) {
traveler3.setText(nameList.get(i).toString());
traveler3.setVisibility(View.VISIBLE);
}
if (i==4) {
traveler4.setText(nameList.get(i).toString());
traveler4.setVisibility(View.VISIBLE);
}
if (i==5) {
traveler5.setText(nameList.get(i).toString());
traveler5.setVisibility(View.VISIBLE);
}
}
}
}
The problem comes in the for loop. Let's supposse that from the items named above, I want to delete Mark2 with ID=2, so then the size of the new Map would be 2, and it would enter to (i == 1) and (i == 2). But when entering to this last one, it would do traveler2.setText(nameList.get(2).toString()); and as seen, there is no element existing with the ID=2 because that is the one that I've deleted and it throws a NPE.
So my question is, what would be the right way to do this without facing this problem?
You should go for switch case other than for loop. Than code will not be in loop.
Finally I get what I need just changing the Key value of the Map that was the same as the ID of the database:
if (c.moveToFirst()) {
int key = 0;
do {
key++;
nameList.put(key, c.getString(c.getColumnIndex(Travelers.NAME)));
}while(c.moveToNext());
}
if (c != null && !c.isClosed()) {
c.close();
}
Basically this way I don't need to change nothing more as now the key value of the Map will match with the Textview position

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