Removing Element From Array of Arrays with Regex

Question

I'm using regex to test certain elements in an array of arrays. If an inner array doesn't follow the desired format, I'd like to remove it from the main/outer array. The regex I'm using is working correctly. I am not sure why it isn't removing - can anyone advise or offer any edits to resolve this problem?

for (var i = arr.length-1; i>0; i--) {

      var a = /^\w+$/;
      var b = /^\w+$/;
      var c = /^\w+$/;  

    var first = a.test(arr[i][0]);
    var second = b.test(arr[i][1]);
    var third = c.test(arr[i][2]);

if ((!first) || (!second) || (!third)){
arr.splice(i,1);
}

have you tried the regex without trying to match start and end of line, i.e. ^ and $? — Kopernik
– Kopernik, Commented Apr 28, 2015 at 1:27

Leo · Accepted Answer · 2015-04-28 02:02:39Z

4

When you cast splice method on an array, its length is updated immediately. Thus, in future iterations, you will probably jump over some of its members.

For example:

var arr = ['a','b','c','d','e','f','g']

for(var i = 0; i < arr.length; i++) {
  console.log(i, arr)
  if(i%2 === 0) {
    arr.splice(i, 1) // remove elements with even index
  }
}

console.log(arr)

It will output:

0 ["a", "b", "c", "d", "e", "f", "g"]
1 ["b", "c", "d", "e", "f", "g"]
2 ["b", "c", "d", "e", "f", "g"]
3 ["b", "c", "e", "f", "g"]
4 ["b", "c", "e", "f", "g"]

["b", "c", "e", "f"]

My suggestion is, do not modify the array itself if you still have to iterate through it. Use another variable to save it.

var arr = ['a','b','c','d','e','f','g']
var another = []

for(var i = 0; i < arr.length; i++) {
  if(i%2) {
    another.push(arr[i]) // store elements with odd index
  }
}

console.log(another) // ["b", "d", "f"]

Or you could go with Array.prototype.filter, which is much simpler:

arr.filter(function(el, i) {
  return i%2 // store elements with odd index
})

It also outputs:

["b", "d", "f"]

edited Apr 28, 2015 at 2:02

answered Apr 28, 2015 at 1:18

Leo

13.9k5 gold badges47 silver badges64 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

Leo Over a year ago

@djfkdjfkd39939 No, with i < arr.length you are getting the length in every iteration. If you cache the length like var i = 0, l = arr.length then i < l, you will probably encounter errors if you visit some index that was not existing yet.

Leo Over a year ago

@djfkdjfkd39939 updated my example so that you could see what happens to the array when you modify it in iterations.

Leo Over a year ago

@djfkdjfkd39939 It should work. The root cause you encountered has nothing to do with array nesting.

Leo Over a year ago

@djfkdjfkd39939 Yes. You were manipulating the array when iterating through it. Its length and members are changing every time splice was called. Read the first example in my answer.

Useless Code Over a year ago

In your example, the issue isn't really that the length keeps changing per se, things get skipped because when you remove the current item the "next" item has the same index as the current one did. The for statement will keep incrementing i every round even if you remove an item, which is what causes the skip. The code in the question goes over the array starting at the end and decrementing each round, it shouldn't suffer from this behavior since the lower indexes won't change when removing higher indexes.

|

Useless Code · Accepted Answer · 2015-05-01 09:14:50Z

Your code seems to work to me. The code in your post was missing a } to close the for statement but that should have caused the script to fail to parse and not even run at all.

I do agree with Leo that it would probably be cleaner to rewrite it using Array.prototype.filter though.

The code in your question would look something like this as a filter:

arr = arr.filter(function (row) {
  return /^\w+$/.test(row[0]) && /^\w+$/.test(row[1]) && /^\w+$/.test(row[2]);
});

jsFiddle

I'm assuming it is 3 different regular expressions in your actual code, if they are all identical in your code you can save a little overhead by defining the RegExp literal once:

arr = arr.filter(function (row) {
  var rxIsWord = /^\w+$/;
  return rxIsWord.test(row[0]) && rxIsWord.test(row[1]) && rxIsWord.test(row[2]);
});

Collectives™ on Stack Overflow

Removing Element From Array of Arrays with Regex

2 Answers 2

7 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

7 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related