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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!

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!

• javascript
• node.js
• loops
• for-loop

Share Improve this question Follow edited Jun 26, 2015 at 14:20 XCore asked Jun 26, 2015 at 13:17 XCoreXCore 11733 silver badges99 bronze badges 1

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

Add a ment  | 

6 Answers 6

Sorted by: Reset to default 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);
    }
}

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);
    }
}

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;
}

the loop goes like this:

for a = 0; itarates while i bees 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.

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.

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

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