While loop stop at first false condition

Question

I have this while loop:

//All of this code is inside a for loop
positionUp = i - 1;

while ((positionUp > 0) && boardMatrix[positionUp][j] == boardMatrix[i][j]) {

    //do something

    positionUp--;
}

At some point, it is possible that positionUp it's assigned with the value -1 (when i=0 )

I thought that the while loop will stop at the first false evaluation thus not evaluating boardMatrix[positionUp][j] and not getting java.lang.ArrayIndexOutOfBoundsException: -1

I'm not seeing how can I solve this. Can someone point me in the way?

Answer 1

Change your loop (temporarily) to:

System.out.println ("pos="+positionUp+",i="+i+",j="+j);
while ((positionUp > 0) && boardMatrix[positionUp][j] == boardMatrix[i][j]) {
    positionUp--;
    System.out.println ("pos="+positionUp+",i="+i+",j="+j);
}

to see which variable is causing the problem. The Java logical operators do short-circuit so the problem most likely lies with the other variables, depending on how they change in the loop.

Answer 2

your problem is: the while loop trys to resolve your condition. your condition contains an access to a array-index which not exists. so before your condition turns "false", an ArrayOutOfBoundsException is thrown.

to solve this you could go:

while ((positionUp > 0) && 
       null != boardMatrix &&
       null != boardMatrix[positionUp][j] && 
       null != boardMatrix[i][j] && 
       boardMatrix[positionUp][j] == boardMatrix[i][j]) {

    //do something

    positionUp--;
}

Answer 3

Java does have short-circuit operators (&& and || - see Java logical operator short-circuiting ). So as long as positionUp is <= 0, the second part will not execute.

This strongly suggests the ArrayOutOfBoundsException originates in your i and j variables.

I would output them (before they are used in the array) so you can see what values they have when the exception is thrown.