I'm having a hard time figuring out how to move an array element. For example, given the following:
var arr = [ 'a', 'b', 'c', 'd', 'e'];
How can I write a function to move 'd' before 'b'?
Or 'a' after 'c'?
After the move, the indices of the rest of the elements should be updated. This means in the first example after the move,
arr = ['a', 'd', 'b', 'c', 'e']
This seems like it should be pretty simple, but I can't wrap my head around it.
If you'd like a version on npm, array-move is the closest to this answer, although it's not the same implementation. See its usage section for more details. The previous version of this answer (that modified Array.prototype.move) can be found on npm at array.prototype.move.
I had fairly good success with this function:
function array_move(arr, old_index, new_index) {
if (new_index >= arr.length) {
var k = new_index - arr.length + 1;
while (k--) {
arr.push(undefined);
}
}
arr.splice(new_index, 0, arr.splice(old_index, 1)[0]);
return arr; // for testing
};
// returns [2, 1, 3]
console.log(array_move([1, 2, 3], 0, 1)); Note that the last return is simply for testing purposes: splice performs operations on the array in-place, so a return is not necessary. By extension, this move is an in-place operation. If you want to avoid that and return a copy, use slice.
Stepping through the code:
new_index is greater than the length of the array, we want (I presume) to pad the array properly with new undefineds. This little snippet handles this by pushing undefined on the array until we have the proper length.arr.splice(old_index, 1)[0], we splice out the old element. splice returns the element that was spliced out, but it's in an array. In our above example, this was [1]. So we take the first index of that array to get the raw 1 there.splice to insert this element in the new_index's place. Since we padded the array above if new_index > arr.length, it will probably appear in the right place, unless they've done something strange like pass in a negative number. A fancier version to account for negative indices:
function array_move(arr, old_index, new_index) {
while (old_index < 0) {
old_index += arr.length;
}
while (new_index < 0) {
new_index += arr.length;
}
if (new_index >= arr.length) {
var k = new_index - arr.length + 1;
while (k--) {
arr.push(undefined);
}
}
arr.splice(new_index, 0, arr.splice(old_index, 1)[0]);
return arr; // for testing purposes
};
// returns [1, 3, 2]
console.log(array_move([1, 2, 3], -1, -2));Which should account for things like array_move([1, 2, 3], -1, -2) properly (move the last element to the second to last place). Result for that should be [1, 3, 2].
Either way, in your original question, you would do array_move(arr, 0, 2) for a after c. For d before b, you would do array_move(arr, 3, 1).
Here's a one liner I found on JSPerf....
Array.prototype.move = function(from, to) {
this.splice(to, 0, this.splice(from, 1)[0]);
};
which is awesome to read, but if you want performance (in small data sets) try...
Array.prototype.move2 = function(pos1, pos2) {
// local variables
var i, tmp;
// cast input parameters to integers
pos1 = parseInt(pos1, 10);
pos2 = parseInt(pos2, 10);
// if positions are different and inside array
if (pos1 !== pos2 && 0 <= pos1 && pos1 <= this.length && 0 <= pos2 && pos2 <= this.length) {
// save element from position 1
tmp = this[pos1];
// move element down and shift other elements up
if (pos1 < pos2) {
for (i = pos1; i < pos2; i++) {
this[i] = this[i + 1];
}
}
// move element up and shift other elements down
else {
for (i = pos1; i > pos2; i--) {
this[i] = this[i - 1];
}
}
// put element from position 1 to destination
this[pos2] = tmp;
}
}
I can't take any credit, it should all go to Richard Scarrott. It beats the splice based method for smaller data sets in this performance test. It is however significantly slower on larger data sets as Darwayne points out.
The splice() method adds/removes items to/from an array, and returns the removed item(s).
Note: This method changes the original array. /w3schools/
Array.prototype.move = function(from,to){
this.splice(to,0,this.splice(from,1)[0]);
return this;
};
var arr = [ 'a', 'b', 'c', 'd', 'e'];
arr.move(3,1);//["a", "d", "b", "c", "e"]
var arr = [ 'a', 'b', 'c', 'd', 'e'];
arr.move(0,2);//["b", "c", "a", "d", "e"]
as the function is chainable this works too:
alert(arr.move(0,2).join(','));
My 2c. Easy to read, it works, it's fast, it doesn't create new arrays.
function move(array, from, to) {
if( to === from ) return array;
var target = array[from];
var increment = to < from ? -1 : 1;
for(var k = from; k != to; k += increment){
array[k] = array[k + increment];
}
array[to] = target;
return array;
}
Got this idea from @Reid of pushing something in the place of the item that is supposed to be moved to keep the array size constant. That does simplify calculations. Also, pushing an empty object has the added benefits of being able to search for it uniquely later on. This works because two objects are not equal until they are referring to the same object.
({}) == ({}); // false
So here's the function which takes in the source array, and the source, destination indexes. You could add it to the Array.prototype if needed.
function moveObjectAtIndex(array, sourceIndex, destIndex) {
var placeholder = {};
// remove the object from its initial position and
// plant the placeholder object in its place to
// keep the array length constant
var objectToMove = array.splice(sourceIndex, 1, placeholder)[0];
// place the object in the desired position
array.splice(destIndex, 0, objectToMove);
// take out the temporary object
array.splice(array.indexOf(placeholder), 1);
}
This is based on @Reid's solution. Except:
Array prototype.undefined items, it just moves the item to the right-most position.Function:
function move(array, oldIndex, newIndex) {
if (newIndex >= array.length) {
newIndex = array.length - 1;
}
array.splice(newIndex, 0, array.splice(oldIndex, 1)[0]);
return array;
}
Unit tests:
describe('ArrayHelper', function () {
it('Move right', function () {
let array = [1, 2, 3];
arrayHelper.move(array, 0, 1);
assert.equal(array[0], 2);
assert.equal(array[1], 1);
assert.equal(array[2], 3);
})
it('Move left', function () {
let array = [1, 2, 3];
arrayHelper.move(array, 1, 0);
assert.equal(array[0], 2);
assert.equal(array[1], 1);
assert.equal(array[2], 3);
});
it('Move out of bounds to the left', function () {
let array = [1, 2, 3];
arrayHelper.move(array, 1, -2);
assert.equal(array[0], 2);
assert.equal(array[1], 1);
assert.equal(array[2], 3);
});
it('Move out of bounds to the right', function () {
let array = [1, 2, 3];
arrayHelper.move(array, 1, 4);
assert.equal(array[0], 1);
assert.equal(array[1], 3);
assert.equal(array[2], 2);
});
});
Here is my one liner ES6 solution with an optional parameter on.
if (typeof Array.prototype.move === "undefined") {
Array.prototype.move = function(from, to, on = 1) {
this.splice(to, 0, ...this.splice(from, on))
}
}
Adaptation of the first solution proposed by digiguru
The parameter on is the number of element starting from from you want to move.
Here is a chainable variation of this:
if (typeof Array.prototype.move === "undefined") {
Array.prototype.move = function(from, to, on = 1) {
return this.splice(to, 0, ...this.splice(from, on)), this
}
}
[3, 4, 5, 1, 2].move(3, 0, 2) // => [1, 2, 3, 4, 5]
If you'd like to avoid prototype pollution, here's a stand-alone function:
function move(array, from, to, on = 1) {
return array.splice(to, 0, ...array.splice(from, on)), array
}
move([3, 4, 5, 1, 2], 3, 0, 2) // => [1, 2, 3, 4, 5]
And finally, here's a pure function that doesn't mutate the original array:
function moved(array, from, to, on = 1) {
return array = array.slice(), array.splice(to, 0, ...array.splice(from, on)), array
}
This should cover basically every variation seen in every other answer.
I've implemented an immutable ECMAScript 6 solution based off of @Merc's answer over here:
const moveItemInArrayFromIndexToIndex = (array, fromIndex, toIndex) => {
if (fromIndex === toIndex) return array;
const newArray = [...array];
const target = newArray[fromIndex];
const inc = toIndex < fromIndex ? -1 : 1;
for (let i = fromIndex; i !== toIndex; i += inc) {
newArray[i] = newArray[i + inc];
}
newArray[toIndex] = target;
return newArray;
};
The variable names can be shortened, just used long ones so that the code can explain itself.
The splice method of Array might help: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/splice
Just keep in mind it might be relatively expensive since it has to actively re-index the array.
One approach would be to create a new array with the pieces in the order you want, using the slice method.
Example
var arr = [ 'a', 'b', 'c', 'd', 'e'];
var arr2 = arr.slice(0,1).concat( ['d'] ).concat( arr.slice(2,4) ).concat( arr.slice(4) );
You can implement some basic Calculus and create a universal function for moving array element from one position to the other.
For JavaScript it looks like this:
function magicFunction (targetArray, indexFrom, indexTo) {
targetElement = targetArray[indexFrom];
magicIncrement = (indexTo - indexFrom) / Math.abs (indexTo - indexFrom);
for (Element = indexFrom; Element != indexTo; Element += magicIncrement){
targetArray[Element] = targetArray[Element + magicIncrement];
}
targetArray[indexTo] = targetElement;
}
Check out "moving array elements" at "gloommatter" for detailed explanation.
http://www.gloommatter.com/DDesign/programming/moving-any-array-elements-universal-function.html
I needed an immutable move method (one that didn't change the original array), so I adapted @Reid's accepted answer to simply use Object.assign to create a copy of the array before doing the splice.
Array.prototype.immutableMove = function (old_index, new_index) {
var copy = Object.assign([], this);
if (new_index >= copy.length) {
var k = new_index - copy.length;
while ((k--) + 1) {
copy.push(undefined);
}
}
copy.splice(new_index, 0, copy.splice(old_index, 1)[0]);
return copy;
};
Here is a jsfiddle showing it in action.
Another pure JS variant using ES6 array spread operator with no mutation
const reorder = (array, sourceIndex, destinationIndex) => {
const smallerIndex = Math.min(sourceIndex, destinationIndex);
const largerIndex = Math.max(sourceIndex, destinationIndex);
return [
...array.slice(0, smallerIndex),
...(sourceIndex < destinationIndex
? array.slice(smallerIndex + 1, largerIndex + 1)
: []),
array[sourceIndex],
...(sourceIndex > destinationIndex
? array.slice(smallerIndex, largerIndex)
: []),
...array.slice(largerIndex + 1),
];
}
// returns ['a', 'c', 'd', 'e', 'b', 'f']
console.log(reorder(['a', 'b', 'c', 'd', 'e', 'f'], 1, 4))
Array.prototype.moveUp = function (value, by) {
var index = this.indexOf(value),
newPos = index - (by || 1);
if (index === -1)
throw new Error("Element not found in array");
if (newPos < 0)
newPos = 0;
this.splice(index, 1);
this.splice(newPos, 0, value);
};
Array.prototype.moveDown = function (value, by) {
var index = this.indexOf(value),
newPos = index + (by || 1);
if (index === -1)
throw new Error("Element not found in array");
if (newPos >= this.length)
newPos = this.length;
this.splice(index, 1);
this.splice(newPos, 0, value);
};
var arr = ['banana', 'curyWurst', 'pc', 'remembaHaruMembaru'];
alert('withiout changes= '+arr[0]+' ||| '+arr[1]+' ||| '+arr[2]+' ||| '+arr[3]);
arr.moveDown(arr[2]);
alert('third word moved down= '+arr[0] + ' ||| ' + arr[1] + ' ||| ' + arr[2] + ' ||| ' + arr[3]);
arr.moveUp(arr[2]);
alert('third word moved up= '+arr[0] + ' ||| ' + arr[1] + ' ||| ' + arr[2] + ' ||| ' + arr[3]);
I ended up combining two of these to work a little better when moving both small and large distances. I get fairly consistent results, but this could probably be tweaked a little bit by someone smarter than me to work differently for different sizes, etc.
Using some of the other methods when moving objects small distances was significantly faster (x10) than using splice. This might change depending on the array lengths though, but it is true for large arrays.
function ArrayMove(array, from, to) {
if ( Math.abs(from - to) > 60) {
array.splice(to, 0, array.splice(from, 1)[0]);
} else {
// works better when we are not moving things very far
var target = array[from];
var inc = (to - from) / Math.abs(to - from);
var current = from;
for (; current != to; current += inc) {
array[current] = array[current + inc];
}
array[to] = target;
}
}
It is stated in many places (adding custom functions into Array.prototype) playing with the Array prototype could be a bad idea, anyway I combined the best from various posts, I came with this, using modern Javascript:
Object.defineProperty(Array.prototype, 'immutableMove', {
enumerable: false,
value: function (old_index, new_index) {
var copy = Object.assign([], this)
if (new_index >= copy.length) {
var k = new_index - copy.length;
while ((k--) + 1) { copy.push(undefined); }
}
copy.splice(new_index, 0, copy.splice(old_index, 1)[0]);
return copy
}
});
//how to use it
myArray=[0, 1, 2, 3, 4];
myArray=myArray.immutableMove(2, 4);
console.log(myArray);
//result: 0, 1, 3, 4, 2
Hope can be useful to anyone
This version isn't ideal for all purposes, and not everyone likes comma expressions, but here's a one-liner that's a pure expression, creating a fresh copy:
const move = (from, to, ...a) => (a.splice(to, 0, ...a.splice(from, 1)), a)
A slightly performance-improved version returns the input array if no move is needed, it's still OK for immutable use, as the array won't change, and it's still a pure expression:
const move = (from, to, ...a) =>
from === to
? a
: (a.splice(to, 0, ...a.splice(from, 1)), a)
The invocation of either is
const shuffled = move(fromIndex, toIndex, ...list)
i.e. it relies on spreading to generate a fresh copy. Using a fixed arity 3 move would jeopardize either the single expression property, or the non-destructive nature, or the performance benefit of splice. Again, it's more of an example that meets some criteria than a suggestion for production use.
Here's one way to do it. It handles negative numbers as well as an added bonus.
const numbers = [1, 2, 3];
const moveElement = (array, from, to) => {
const copy = [...array];
const valueToMove = copy.splice(from, 1)[0];
copy.splice(to, 0, valueToMove);
return copy;
};
console.log(moveElement(numbers, 0, 2))
// > [2, 3, 1]
console.log(moveElement(numbers, -1, -3))
// > [3, 1, 2]
Moves elements within an array, returning an array containing the moved elements.
array.move(index, howMany, toIndex);
index: Index at which to move elements. If negative, index will start from the end.
howMany: Number of elements to move from index.
toIndex: Index of the array at which to place the moved elements. If negative, toIndex will start from the end.
array = ["a", "b", "c", "d", "e", "f", "g"];
array.move(3, 2, 1); // returns ["d","e"]
array; // returns ["a", "d", "e", "b", "c", "f", "g"]
Array.prototype.move || Object.defineProperty(Array.prototype, "move", {
value: function (index, howMany, toIndex) {
var
array = this,
index = parseInt(index) || 0,
index = index < 0 ? array.length + index : index,
toIndex = parseInt(toIndex) || 0,
toIndex = toIndex < 0 ? array.length + toIndex : toIndex,
toIndex = toIndex <= index ? toIndex : toIndex <= index + howMany ? index : toIndex - howMany,
moved;
array.splice.apply(array, [toIndex, 0].concat(moved = array.splice(index, howMany)));
return moved;
}
});
I used the nice answer of @Reid, but struggled with moving an element from the end of an array one step further - to the beginning (like in a loop). E.g. ['a', 'b', 'c'] should become ['c', 'a', 'b'] by calling .move(2,3)
I achieved this by changing the case for new_index >= this.length.
Array.prototype.move = function (old_index, new_index) {
console.log(old_index + " " + new_index);
while (old_index < 0) {
old_index += this.length;
}
while (new_index < 0) {
new_index += this.length;
}
if (new_index >= this.length) {
new_index = new_index % this.length;
}
this.splice(new_index, 0, this.splice(old_index, 1)[0]);
return this; // for testing purposes
};
As an addition to Reid's excellent answer (and because I cannot comment); You can use modulo to make both negative indices and too large indices "roll over":
function array_move(arr, old_index, new_index) {
new_index =((new_index % arr.length) + arr.length) % arr.length;
arr.splice(new_index, 0, arr.splice(old_index, 1)[0]);
return arr; // for testing
}
// returns [2, 1, 3]
console.log(array_move([1, 2, 3], 0, 1)); let ar = ['a', 'b', 'c', 'd'];
function change( old_array, old_index , new_index ){
return old_array.map(( item , index, array )=>{
if( index === old_index ) return array[ new_index ];
else if( index === new_index ) return array[ old_index ];
else return item;
});
}
let result = change( ar, 0, 1 );
console.log( result );
result:
["b", "a", "c", "d"]
I thought this was a swap problem but it's not. Here's my one-liner solution:
const move = (arr, from, to) => arr.map((item, i) => i === to ? arr[from] : (i >= Math.min(from, to) && i <= Math.max(from, to) ? arr[i + Math.sign(to - from)] : item));
Here's a small test:
let test = ['a', 'b', 'c', 'd', 'e'];
console.log(move(test, 0, 2)); // [ 'b', 'c', 'a', 'd', 'e' ]
console.log(move(test, 1, 3)); // [ 'a', 'c', 'd', 'b', 'e' ]
console.log(move(test, 2, 4)); // [ 'a', 'b', 'd', 'e', 'c' ]
console.log(move(test, 2, 0)); // [ 'c', 'a', 'b', 'd', 'e' ]
console.log(move(test, 3, 1)); // [ 'a', 'd', 'b', 'c', 'e' ]
console.log(move(test, 4, 2)); // [ 'a', 'b', 'e', 'c', 'd' ]
console.log(move(test, 4, 0)); // [ 'e', 'a', 'b', 'c', 'd' ]
var ELEMS = ['a', 'b', 'c', 'd', 'e'];
/*
Source item will remove and it will be placed just after destination
*/
function moveItemTo(sourceItem, destItem, elements) {
var sourceIndex = elements.indexOf(sourceItem);
var destIndex = elements.indexOf(destItem);
if (sourceIndex >= -1 && destIndex > -1) {
elements.splice(destIndex, 0, elements.splice(sourceIndex, 1)[0]);
}
return elements;
}
console.log('Init: ', ELEMS);
var result = moveItemTo('a', 'c', ELEMS);
console.log('BeforeAfter: ', result);Immutable version without array copy:
const moveInArray = (arr, fromIndex, toIndex) => {
if (toIndex === fromIndex || toIndex >= arr.length) return arr;
const toMove = arr[fromIndex];
const movedForward = fromIndex < toIndex;
return arr.reduce((res, next, index) => {
if (index === fromIndex) return res;
if (index === toIndex) return res.concat(
movedForward ? [next, toMove] : [toMove, next]
);
return res.concat(next);
}, []);
};
I think the best way is define a new property for Arrays
Object.defineProperty(Array.prototype, 'move', {
value: function (old_index, new_index) {
while (old_index < 0) {
old_index += this.length;
}
while (new_index < 0) {
new_index += this.length;
}
if (new_index >= this.length) {
let k = new_index - this.length;
while ((k--) + 1) {
this.push(undefined);
}
}
this.splice(new_index, 0, this.splice(old_index, 1)[0]);
return this;
}
});
console.log([10, 20, 30, 40, 50].move(0, 1)); // [20, 10, 30, 40, 50]
console.log([10, 20, 30, 40, 50].move(0, 2)); // [20, 30, 10, 40, 50]
const changeValuePosition = (arr, init, target) => {[arr[init],arr[target]] = [arr[target],arr[init]]; return arr}– muhsalaainitandtarget. – Matt F.