For Loops and Logical AND + OR

Question

Why does this code take not run?

for(int i=0;i<11;i++){
    for(int j=0;j<11;j++){  

        if(i>0){
            if((staticWallLoc[i--][j]&4)>0){staticWallLoc[i][j]=staticWallLoc[i][j]|1;}
        }
        if(j<10){
            if((staticWallLoc[i][j++]&8)>0){staticWallLoc[i][j]=staticWallLoc[i][j]|2;}
        }
        if(i<10){
            if((staticWallLoc[i++][j]&1)>0){staticWallLoc[i][j]=staticWallLoc[i][j]|4;}
        }
        if(j>0){
            if((staticWallLoc[i][j--]&2)>0){staticWallLoc[i][j]=staticWallLoc[i][j]|8;}
        }

        System.out.println(i+" "+j);
    }
}

By the way, staticWallLoc[11][11] is a two dimensional array that has 11 indexes in each dimension (0-10).

Just wondering why my program literally cannot get passed this code.

everytime i see questions like this I'm baffled. Have you tried using a debugger?? — Amir Afghani, Jul 29 '12 at 23:13
Those post increments and decrements of loop indexes are horrible! just use i-1 and i+1 etc as the index. — John3136, Jul 29 '12 at 23:15
Take a look [at this thread](http://stackoverflow.com/questions/1094872/is-there-a-difference-between-x-and-x-in-java) regarding why you should not use the i-- & i++ indexes. — amiregelz, Jul 29 '12 at 23:17
It *does* run. It just never terminates,because you wrote it that way. — user207421, Jul 29 '12 at 23:49

score 2 · Answer 1 · answered Jul 29 '12 at 23:14

2

Dude - why are you decrementing in the loop body the same variables you're using to control the loop counter?!?

Sounds like "one step forward, two steps back", doesn't it ;)?

SUGGESTION:

Run the debugger, single step through a few iterations, and carefully note what happens to "i" and "j".

How soon do you think either or both will reach "11" ;)?

answered Jul 29 '12 at 23:14

paulsm4

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+1, this is some messed up code. – Kaleb Brasee Jul 29 '12 at 23:14

Keldon Alleyne · Accepted Answer · 2012-07-29T23:22:31.757

1

Inside your if(i > 0), i is decremented by staticWallLoc[i--]. Change it to staticWallLoc[i-1], and do the same for j.

if(i>0){
    // Try something like this instead
    if((staticWallLoc[i-1][j]&4)>0){
      staticWallLoc[i][j]=staticWallLoc[i][j]|1;
    }
}

You will also want to change your [i++] to [i+1].

edited Jul 29 '12 at 23:22

answered Jul 29 '12 at 23:17

Keldon Alleyne

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Thanks, I made a really really stupid mistake, I'm just used to using -- and ++, im a first time programmer, so these stupid mistakes are seldom but, I still make them! XD thanks! – Ben Hagel Jul 31 '12 at 02:06
Not a problem. Mistakes like these provide valuable experience. Now when you read a theoretical book you will at least have the benefit of having this memorable occasion to better understand the reasoning behind various programming practices, etc. – Keldon Alleyne Jul 31 '12 at 05:03

score 0 · Answer 3 · answered Jul 29 '12 at 23:15

In your loop, you have statements like if (i>0) {staticWallLoc[i--][j].... These are directly altering the loop index. So, when i is 1, it is changed back to 0. When the loop is incremented, it goes back to 1, and then 0 again, creating an infinite loop.

Do you perhaps mean to use staticWallLoc[i - 1][j]..., etc. ? This gets the desired index without changing the loop index.

score 0 · Answer 4 · answered Jul 29 '12 at 23:16

0

rather than using i++ i-- etc in your code, just use offsets i-1 or i+1.

General rule of thumb: in for loops, don't mess with the for loop variables within the body.

(there are exceptions to the rule, but this doesn't look like an exceptional case)

answered Jul 29 '12 at 23:16

Keith Nicholas

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score 0 · Answer 5 · answered Jul 29 '12 at 23:16

Without knowing what the heck the contents of your array is it's hard to say. Might it be because you're modifying the values of i and j on the fly with -- and ++? Do you mean to be keeping those values constant and doing something like:

if((staticWallLoc[i+1][j]&1)>0){staticWallLoc[i][j]=staticWallLoc[i][j]|4;} // changed i++ to i+1

score 0 · Answer 6 · answered Jul 29 '12 at 23:17

Once j reaches 10, it does not get incremented in the "if (j < 10)" section, but it gets decremented in the "if (j > 0)" section, so j will end the loop at 9, then get incremented back to 10 by the loop management. So you will have an infinite loop with i = -1/0 (since it gets decremented and incremented in the loop) and j = 9/10 (for the same reason).

score 0 · Answer 7 · answered Jul 29 '12 at 23:17

0

You do a j-- in the last if once j >=10. The loop will stuck at j==10.

It's not a very good idea to manipulate the loop variable :)

answered Jul 29 '12 at 23:17

Csongor Fagyal

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For Loops and Logical AND + OR

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