嵌套的Java循环无效

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!

Answer 1

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);
    }
}

Answer 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);
    }
}

Answer 3

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;
}

Answer 4

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.

Answer 5

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.

Answer 6

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