JavaScript-是否可以在for循环中递减计数器？

Question

I'm trying to iterate over an array and in certain situations need to decrement my counter.

My goal is to rearrange divs into two columns with a fixed height.

It looks like this:

var divs = document.getElementsByTagName('div');

for(var i = 0; i < divs.length; i++) {
    if(left column is not full) {
        left.append(divs[i]);
    }
    else if(right column is not full){
        right.append(divs[i]);
    }
    else {
        create 2 new columns (left & right);
        i--; //so that the element doesn't get lost
    }
}

I don't end up in an infinite loop, but the result isn't quite what i expected. Is there an error in my logic or is it simply impossible/not allowed to decrement the counter from within the loop?

Answer 1

It is a bad practice to modify a counter within a loop.

Use while instead. Then you will be able to define your custom counter modifying logics.

For example, the snippet below removes all negative items from an array using the following algorithm:

• if an item is positive - continue
• otherwise, remove an item in position i

It isn't possible to do this with a for loop without either additional array or manual counter decrement, as for loop would skip the value after a removed one every time.

 var items = [0, 3, 5, -3, -5, -7, 11]; var i = 0; while (i < items.length) { if (items[i] < 0) items.splice(i, 1); else i++; } document.body.innerText = JSON.stringify(items); 

Answer 2

you can decrement counters in for loops but you will find it hard to keep track on them. for your case it's simply better to use a while loop.

how about this:

var i = 0;
while(i < length) {
    if(a) {
        //do something
        i++;
    }
    else {
        //do something else
        i--;
    }
}