How do I shift an array of items up by 4 places in Javascript

Question

How do I shift an array of items up by 4 places in Javascript?

I have the following string array:

var array1 = ["t0","t1","t2","t3","t4","t5"];

I need a function convert "array1" to result in:

// Note how "t0" moves to the fourth position for example
var array2 = ["t3","t4","t5","t0","t1","t2"];

Thanks in advance.

+1 Peter b/c I think this is a case where precision (using the correct word) _is_ important. Array.shift() already defines the meaning of shifting an array in the context of JavaScript. That said, here is a JS function for rotating an array. http://stackoverflow.com/a/1985471/740639 — Walter Stabosz, Jun 25 '13 at 20:57

score 35 · Accepted Answer · answered Oct 05 '09 at 16:39

35

array1 = array1.concat(array1.splice(0,3));

run the following in Firebug to verify

var array1 = ["t0","t1","t2","t3","t4","t5"];
console.log(array1);
array1 = array1.concat(array1.splice(0,3));
console.log(array1);

results in

["t0", "t1", "t2", "t3", "t4", "t5"]
["t3", "t4", "t5", "t0", "t1", "t2"]

answered Oct 05 '09 at 16:39

Russ Cam

124,184
33
204
266

2

as a function- `function reorderArray(arr, startIndex, howMany) { return arr.concat(arr.splice(startIndex,howMany)); }` would need error checking as well :) – Russ Cam Oct 05 '09 at 16:51
1

Be aware, Array.concat() create a whole new array using the elements from the source arrays. Moot for small arrays, but something to consider. – Walter Stabosz Jun 25 '13 at 21:02

score 19 · Answer 2 · answered Oct 05 '09 at 16:35

19

You can slice the array and then join it in reversed order:

var array2 = array1.slice(3).concat(array1.slice(0, 3));

answered Oct 05 '09 at 16:35

Lukáš Lalinský

40,587
6
104
126

2

This is nice and concise. If you're going to do this frequently, and need to shift it by different amounts, you could put this into a function and pass in a variable to plug in where the 3 is. (I use the word "shift" hesitantly, since that's a Javascript array method, too.) – Nathan Long Oct 05 '09 at 16:44
Yep this is my favorite of the answers, nice! – Eric Bishard Aug 01 '15 at 08:34
1

This is great because it doesn't modify array1. – tjklemz May 24 '16 at 18:05

score 4 · Answer 3 · answered Oct 05 '09 at 16:43

function shiftArray(theArray, times) {
    // roll over when longer than length
    times = times % theArray.length;
    var newArray = theArray.slice(times);
    newArray = newArray.concat(theArray.slice(0, times));
    return newArray;
}

var array1 = ["t0","t1","t2","t3","t4","t5"];
var array2 = shiftArray(array1, 3);
alert(array2); // ["t3","t4","t5","t0","t1","t2"]

score 2 · Answer 4 · answered Oct 05 '09 at 16:37

2

One more way would be this:

var array2 = array1.slice(0);

for (var i = 0; i < 3; i++) {
    array2.push(array2.shift());
}

answered Oct 05 '09 at 16:37

slikts

8,020
1
27
47

NickFitz · Answer 5 · 2009-10-05T16:54:12.563

0

Another way - paste the following code into the large Firebug console to confirm it works:

var a = [0, 1, 2, 3, 4, 5];
for (var i = 0; i < 3; i++) {
    a.unshift(a.pop());
}
// the next line is to show it in the Firebug console; variable "a" holds the array
a.toString(",");

edited Oct 05 '09 at 16:54

answered Oct 05 '09 at 16:46

NickFitz

34,537
8
43
40

@Nathan: the end result *is* an array, held in the variable "a"; the line outputting it as a string is for illustrative purposes, which is why it's not assigned to anything (it goes straight to the console in Firebug). I'll correct my answer to make it clear that I meant the Firebug console... sometimes I forget it's not built-in to the browser ;-) – NickFitz Oct 05 '09 at 16:53

How do I shift an array of items up by 4 places in Javascript

5 Answers5