Unsetting javascript array in a loop?

Question

I have the following code:

  $.each(current, function(index, value) {
     thisArray = JSON.parse(value);
     if (thisArray.ttl + thisArray.now > now()) {
        banned.push(thisArray.foodID);
     }
     else{
        current.splice(index,1);
     }
  });

There's a problem with the following line:

current.splice(index,1);

What happens is that it (probably) unsets the first case which fits the else condition and then when it has to happen again, the keys don't match anymore and it cannot unset anything else. It works once, but not in the following iterations.

Is there a fix for this?

exact duplicate of Removing Multiple Objects from Javascript Array Breaks Half Way Through. You are not allowed to mutate an array that is looped over by each — Bergi
– Bergi, Commented Aug 17, 2013 at 20:26

bfavaretto · Accepted Answer · 2013-08-17 20:17:15Z

2

You can use a regular for loop, and loop backwards:

for(var i=current.length-1; i>= 0; i--) {
   var thisArray = JSON.parse(current[i]);
   if (thisArray.ttl + thisArray.now > now()) {
      banned.push(thisArray.foodID);
   } else {
      current.splice(i, 1);
   }
});

Also it seems thisArray is actually an object, not an array...

answered Aug 17, 2013 at 20:17

bfavaretto

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Comments

ViliusL · Accepted Answer · 2013-08-17 20:25:17Z

0

you can use regular for loop, but after splice, decrease index by 1

for(var i=0; i<current.length; i++){
    current.splice(i, 1); i--;
    }

answered Aug 17, 2013 at 20:25

ViliusL

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Comments

6502 · Accepted Answer · 2013-08-17 20:25:23Z

You should pay attention if you mutate an array (by moving objects) while it's being traversed because some element may get skipped like in your case.

When you call current.splice(index, 1) element index will be removed and element index+1 will take its place. But then index will be incremented, thus skipping one element.

A better solution is IMO the read-write approach. You keep one index as the "read pointer" and you always increment it, and another index (the "write pointer") that is incremented only when you decide an element should be kept in the array:

var wp = 0;  // The "write pointer"
for (var rp=0; rp<a.length; rp++) {
    if (... i want to keep element a[rp] ...) {
        a[wp++] = a[rp];
    }
}
a.splice(wp); // Remove all elements after wp

This is a o(N) operation and will move each element at most once. Other approaches like starting from the end and using the same splice(i, 1) approach instead will keep moving all elements after i each time an element needs to be removed.

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