Javascript nested for loop not working

Question

This is weird, I'm having issues with simple nested for loops in javascript.

My code is like this:

var a = 0, b = 2048;
var i = 0, l = 2048;

for(; a < b; a++) {
    for(; i < l; i++) {
        console.log(a, b, i, l);
    }
}

So while, I'm expecting an output like this (0..2047), 2048, (0..2047), 2048, I'm having this output: 0, 2048, 0..2047, 2048 where the first variable: a doesn't simply iterate from 0 to 2047. Rephrasing the concept: while the inner loop iterates correctly, the outer one is executed only once at index 0.

I'm sure it's a simple and silly issue, but I can't really spot that..

COMMENT

Thank you all for finding this issue, it's incredible how I couldn't see that. I'm accepting simon's answer because it seems cleaner and more elegant to me:

He doesn't reinitialize variable as in for(var i = 0;...) but just reset it
He includes the variable reset in the for statement rather than after every iteration
He doesn't declare variables var a = 0, i = 0 and then reset that in every for statement
He uses the regular increment
He declares every variable at the beginning of the snippet instead of declaring them at different times in the execution

Thanks again!

its simply because when i reaches 2048 the outer loop cannot get executed again — theodore hogberg
– theodore hogberg, Commented Jun 26, 2015 at 13:35

smnbbrv · Accepted Answer · 2015-06-26 13:23:05Z

2

Just initialize i every time before inner iteration:

var a = 0, b = 2048;
var i, l = 2048;

for(; a < b; a++) {
    for(i = 0; i < l; i++) {
        console.log(a, b, i, l);
    }
}

answered Jun 26, 2015 at 13:23

smnbbrv

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Comments

Samuel · Accepted Answer · 2015-06-26 13:22:37Z

2

You didn't reload i to 0 between a loops. Here the fix :

var a = 0, b = 2048;
var i = 0, l = 2048;

for(a = 0; a < b; a++) {
    for(i = 0; i < l; i++) {
        console.log(a, b, i, l);
    }
}

answered Jun 26, 2015 at 13:22

Samuel

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Comments

Nicholas Robinson · Accepted Answer · 2015-06-26 13:23:17Z

2

The reason this is happening is because you never reset i, so the inner loop will only happen during the first iteration on the outer loop. Thereafter i will always be greater than l.

Try this:

var a = 0, b = 2048;
var i = 0, l = 2048;

for(; a < b; a++) {
    for(; i < l; i++) {
        console.log(a, b, i, l);
    }
    i = 0;
}

answered Jun 26, 2015 at 13:23

Nicholas Robinson

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Comments

cs04iz1 · Accepted Answer · 2015-06-26 13:41:25Z

1

the loop goes like this:

for a = 0; itarates while i becomes 2048. then for a =1 the i is 2048, thus i not < l and thus not getting into the loop again.

var a = 0, b = 2048;

for(; a < b; a++) {
    var i = 0, l = 2048;
    for(; i < l; i++) {
        console.log(a, b, i, l);
    }
}

I believe this is what you want.

edited Jun 26, 2015 at 13:41

answered Jun 26, 2015 at 13:23

cs04iz1

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Comments

rob · Accepted Answer · 2016-12-07 22:39:13Z

1

~~This is a scope problem. Variables a and i are not defined in the scope of your for loops.~~ Define them inside this way:

var b = 2048,
    l = 2048;

for(var a = 0; a < b; a = a + 1) {
    for(var i = 0; i < l; i = i + 1) {
        console.log(a, b, i, l);
    }
}

edit: squint is absolutely correct, JavaScript doesn't use block scope, for which I mistook as the issue. Although, you should always consider defining the variables that increment inside the declaration of your for loops to eliminate confusion/ambiguity.

edited Dec 7, 2016 at 22:39

answered Jun 26, 2015 at 13:36

rob

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1 Comment

user1106925 Over a year ago

It's not a scope problem. Your code doesn't change the variable scope at all. The only problem was with the failure to reset i in the loop, which your answer does fix.

depperm · Accepted Answer · 2015-06-26 13:21:01Z

0

Try one loop

var a = 0, b = 2048;
var i = 0, l = 2048;

for(; a <= b&&i<=l; a++,i++) {
    console.log(a, b, i, l);
}

With two loops it i is never reset and so only get iterated once, hence the result of 0, 2048, 0..2047, 2048

edited Jun 26, 2015 at 13:21

answered Jun 26, 2015 at 13:19

depperm

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1 Comment

smnbbrv Over a year ago

this won't produce the expected output

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