Altering the array while an outer loop is still running on that same array

Question

I have a loop that goes through an array and removes some elements from it.

But since it removes the elements from the same array it's looping on, it creates some problems.

Here I have Players from which I wanna remove a player2

Players = [];
Players.push('player1');
Players.push('player2');
Players.push('player2');
Players.push('player2');
Players.push('player3');
Players.push('player2');
Players.push('player2');
Players.push('player2');

function check() {
    for (var i = 0; i < Players.length; i++) {
        if (Players[i] == 'player2')
            kick(Players[i])
    };
}

function kick(player) {
    for (var i = 0; i < Players.length; i++) {
        if (player == Players[i]) {
            Players.splice(i, 1);
            break;
        }
    };
}

but

check();

console.info(util.inspect(Players));

outputs

[ 'player1', 'player3', 'player2', 'player2' ]

What could I do to correct this?

Why don't you just remove them? if (Players[i] == 'player2') { delete Players[i]; } — feeela
– feeela, Commented Apr 7, 2014 at 20:46
@feeela It makes my array sparse. >[ 'player1', , , , 'player3', , , ] — laggingreflex
– laggingreflex, Commented Apr 7, 2014 at 20:54
It's redundant to search for the entry twice. After the first search, you know exactly where the thing is. — Pointy
– Pointy, Commented Apr 7, 2014 at 20:58

Mike 'Pomax' Kamermans · Accepted Answer · 2014-04-08 00:35:25Z

6

mind blowing trick: run through the array backwards.

You're hitting the "concurrent modification" scenario (splicing out an element at position i changes the position for all elements after it), so avoid that with:

for(i=arr.length-1; i >=0; i--) {
  // ...
  if (shouldberemoved) {
    arr.splice(i,1);
  }
}

now you can remove as many elements as you like without affecting the elements preceding it.

edited Apr 8, 2014 at 0:35

answered Apr 7, 2014 at 20:47

Mike 'Pomax' Kamermans

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Comments

Anthony Horne · Accepted Answer · 2014-04-07 20:50:06Z

2

Why don't you run the loop in reverse order.

for (var i = Player.lenght; i < 0; i--)

then if you remove, you will not be looking for those elements.

answered Apr 7, 2014 at 20:50

Anthony Horne

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