How to splice an element to the start of an array?

Question

I'm new to Javascript and I'm trying to create a function that rotates the array depending on how many times num is === to. So if num = 2 ["Harry", "Sarah", "Oscar", "Tina"] becomes ["Oscar", "Tina", "Harry", "Sarah"]

Here is my code so far:

var count = 0;

function rotate(arr, num) {
  while (count < num) {
    arr.splice(0,0, "Tina");
    arr.pop();
    count++
  }
  return arr
}

console.log(rotate(["Harry", "Sarah", "Oscar", "Tina"], 2));

For this Line - arr.splice(0,0, "Tina"); I want it to be so that it will bring whatever name is the fourth element to the front of the array, I'm not sure if this is possible? I am suppposed to do this method using splice. Thanks for any help?! :)

Edit: This question is different to other questions. I don't want a full blown solution for rotation, I just want to know if it's possible to splice the fourth element to the beginning?

Eerily similar to [Javascript rotating an array using a function with splice?](http://stackoverflow.com/q/29775123/710446) but the mistakes in the code here are distinct from the mistakes in the other question. — apsillers, Apr 21 '15 at 16:16
*" I just want to know if it's possible to splice the fourth element to the beginning?"* Pretty simple: `arr.unshift(arr.pop())`. — Felix Kling, Apr 21 '15 at 16:30

A.J. Uppal · Accepted Answer · 2015-04-21T16:30:24.490

1

Try shifting the array in a for loop:

function rotate(arr, num){
    for(var i = 0; i < num; i++){
        item = arr[arr.length-1]
        arr.splice(arr.length-1, 1);
        arr.unshift(item)
    }
    return arr
}

alert(JSON.stringify(rotate(["Harry", "Sarah", "Oscar", "Tina"], 2)));
alert(JSON.stringify(rotate(["Harry", "Sarah", "Oscar", "Tina"], 1)));

edited Apr 21 '15 at 16:30

answered Apr 21 '15 at 16:15

A.J. Uppal

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This works, thank you :) – Austin Edwards Apr 21 '15 at 16:22
@AustinEdwards no problem :) – A.J. Uppal Apr 21 '15 at 16:22
I don't believe this is the correct implementation, it just coincidentally produces the expected result for `2`. But for `1`, it produces `Sarah,Oscar,Tina,Harry`, instead of `Tina, Harry, Sarah, Oscar` -- which seems what the OP really wants, according to my interpretation of *"I want it to be so that it will bring whatever name is the fourth element to the front of the array,"* – Felix Kling Apr 21 '15 at 16:28
@FelixKling ah, my code was switched I guess, let me edit. – A.J. Uppal Apr 21 '15 at 16:28
(Then again, the OP's question is not 100% clear to me) – Felix Kling Apr 21 '15 at 16:29
@FelixKling, fixed, I wasn't sure either, but I hope this is correct now :) – A.J. Uppal Apr 21 '15 at 16:30
Yeah, when I was testing it out I noticed that, the updated version is exactly what I was trying to do, I was unaware of unshift, thank you! :) – Austin Edwards Apr 21 '15 at 16:39

score 0 · Answer 2 · answered Apr 21 '15 at 16:19

You don't need a loop. First splice the last num elements off the end of the array, then splice them all onto the front.

function rotate(arr, num) {
        var lastN = arr.splice(-num, num);
        [].splice.apply(arr, [0, 0].concat(lastN));
        return arr;
    }
document.getElementById("result").innerHTML = JSON.stringify(rotate(["Harry", "Sarah", "Oscar", "Tina"], 2));

<div id="result"></div>

How to splice an element to the start of an array?

2 Answers2