I am trying to delete all even numbers from the array but it just doesnt delete all of them . not sure why

 var arr = [3,45,56,7,88,56,34,345];    
    for (var i = 0; i < arr.length; i++) {
      if (arr[i] % 2 === 0) {
        arr.splice(i,1);
      }
    }

console.log(arr); gives this - [3, 45, 7, 56, 345] instead of this [3, 45, 7, 345]

Any ideas?

I am trying to delete all even numbers from the array but it just doesnt delete all of them . not sure why

 var arr = [3,45,56,7,88,56,34,345];    
    for (var i = 0; i < arr.length; i++) {
      if (arr[i] % 2 === 0) {
        arr.splice(i,1);
      }
    }

console.log(arr); gives this - [3, 45, 7, 56, 345] instead of this [3, 45, 7, 345]

Any ideas?

• javascript
• arrays

• The classic for-loop through array problem. – Zhouster Commented Aug 15, 2014 at 0:10
• 2 add the numbers you want to keep to a new array, when done remove old arr and use new one. – RST Commented Aug 15, 2014 at 0:16

5 Answers 5

Yes, that is because when you splice out 1 element from an array the length of array is changed. Try this -

var arr = [3,45,56,7,88,56,34,345];

for (var i = 0; i < arr.length; i++) {
  if (arr[i] % 2 === 0) {
    arr.splice(i,1);
    i = i-1; // SINCE YOU DELETED ONE ELEMENT, 
             // THE NEXT ELEMENT IN THE ARRAY GETS SHIFTED TO ONE POSITION ABOVE 
             //(SAY, FROM INDEX 4 TO 3). SO, YOU WILL NEED TO CHECK FROM INDEX 3 NOT 4
  }
}

When you delete 88, the 56 following it moves up one position, and your loop skips it (it just does i++).

You need to be careful when updating an array while iterating over it.

You could iterate the array backwards in this case (start at the end, do i--). Depending on how splice is implemented, this could even be a bit faster (potentially fewer array elements are getting copied around).

The length of your array changes as you delete elements from it.

I would suggest using Array.prototype.filter to achieve this (which is easily shimmed for IE <= 8):

var arr = [3,45,56,7,88,56,34,345];    
arr = arr.filter( function is_odd(el) { return el % 2; } );

You are looping through your array via i which is incremented every time. After it gets to 88, it splices it out, and still increases the array, which will skip over the 56, the next i will refer to 34.

You either need to not increment i when you splice, or, when you splice, simply i--;

arr.filter(function(item){return item % 2;}));