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[0] would = 'a', arr[1] = 'd' arr[2] = 'b', arr[3] = 'c', arr[4] = 'e'
This seems like it should be pretty simple, but I can't wrap my head around it.
This question is related to
javascript
arrays
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'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]
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.
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);
});
});
Another pure JS variant using ES6 array spread operator with no mutation
const reorder = (array, sourceIndex, destinationIndex) => {_x000D_
const smallerIndex = Math.min(sourceIndex, destinationIndex);_x000D_
const largerIndex = Math.max(sourceIndex, destinationIndex);_x000D_
_x000D_
return [_x000D_
...array.slice(0, smallerIndex),_x000D_
...(sourceIndex < destinationIndex_x000D_
? array.slice(smallerIndex + 1, largerIndex + 1)_x000D_
: []),_x000D_
array[sourceIndex],_x000D_
...(sourceIndex > destinationIndex_x000D_
? array.slice(smallerIndex, largerIndex)_x000D_
: []),_x000D_
...array.slice(largerIndex + 1),_x000D_
];_x000D_
}_x000D_
_x000D_
// returns ['a', 'c', 'd', 'e', 'b', 'f']_x000D_
console.log(reorder(['a', 'b', 'c', 'd', 'e', 'f'], 1, 4))_x000D_
_x000D_
_x000D_
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.
I like this way. It's concise and it works.
function arraymove(arr, fromIndex, toIndex) {
var element = arr[fromIndex];
arr.splice(fromIndex, 1);
arr.splice(toIndex, 0, element);
}
Note: always remember to check your array bounds.
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) );
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);
}, []);
};
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
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
};
const move = (from, to, ...a) =>from === to ? a : (a.splice(to, 0, ...a.splice(from, 1)), a);_x000D_
const moved = move(0, 2, ...['a', 'b', 'c']);_x000D_
console.log(moved)
_x000D_
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(','));
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) {_x000D_
new_index =((new_index % arr.length) + arr.length) % arr.length;_x000D_
arr.splice(new_index, 0, arr.splice(old_index, 1)[0]);_x000D_
return arr; // for testing_x000D_
}_x000D_
_x000D_
// returns [2, 1, 3]_x000D_
console.log(array_move([1, 2, 3], 0, 1));
_x000D_
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.
If the object is nested:
let array = ['a', 'b', 'c', 'd', 'e'];
let existingElement = JSON.parse(JSON.stringify(array[3]));
array.splice(1, 0, existingElement);
array.splice(4, 1);
console.log(array)
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' ]
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]);
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);
}
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]
This method will preserve the original array, and check for bounding errors.
const move = (from, to, arr) => {
to = Math.max(to,0)
from > to
? [].concat(
arr.slice(0,to),
arr[from],
arr.filter((x,i) => i != from).slice(to))
: to > from
? [].concat(
arr.slice(0, from),
arr.slice(from + 1, to + 1),
arr[from],
arr.slice(to + 1))
: arr}
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;
}
});
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.
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 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;
}
}
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;
}
let oldi, newi, arr;_x000D_
_x000D_
if(newi !== oldi) {_x000D_
let el = this.arr.splice(oldi, 1);_x000D_
if(newi > oldi && newi === (this.arr.length + 2)) {_x000D_
this.arr.push("");_x000D_
}_x000D_
this.arr.splice(newi, 0, el);_x000D_
if(newi > oldi && newi === (this.arr.length + 2)) {_x000D_
this.arr.pop();_x000D_
}_x000D_
}
_x000D_
This is a really simple method using splice
Array.prototype.moveToStart = function(index) {
this.splice(0, 0, this.splice(index, 1)[0]);
return this;
};
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 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.
In your example, because is an array of string
we can use a ranking object to reorder the string array:
let rank = { 'a': 0, 'b': 1, 'c': 2, 'd': 0.5, 'e': 4 };
arr.sort( (i, j) => rank[i] - rank[j] );
We can use this approach to write a move
function that works on a string array:
function stringArrayMove(arr, from, to)
{
let rank = arr.reduce( (p, c, i) => ( p[c] = i, p ), ({ }) );
// rank = { 'a': 0, 'b': 1, 'c': 2, 'd': 3, 'e': 4 }
rank[arr[from]] = to - 0.5;
// rank = { 'a': 0, 'b': 1, 'c': 2, 'd': 1.5, 'e': 4 }
arr.sort( (i, j) => rank[i] - rank[j] );
// arr = [ 'a', 'd', 'b', 'c', 'e' ];
}
let arr = [ 'a', 'b', 'c', 'd', 'e' ];
stringArrayMove(arr, 3, 1);
console.log( JSON.stringify(arr) );
_x000D_
If, however, the thing we wanted to sort is an array of object, we can introduce the ranking as a new property of each object, i.e.
let arr = [ { value: 'a', rank: 0 },
{ value: 'b', rank: 1 },
{ value: 'c', rank: 2 },
{ value: 'd', rank: 0.5 },
{ value: 'e', rank: 4 } ];
arr.sort( (i, j) => i['rank'] - j['rank'] );
We can use Symbol
to hide the visibility of this property, i.e. it will not be shown in JSON.stringify
. We can generalize this in an objectArrayMove
function:
function objectArrayMove(arr, from, to) {
let rank = Symbol("rank");
arr.forEach( (item, i) => item[rank] = i );
arr[from][rank] = to - 0.5;
arr.sort( (i, j) => i[rank] - j[rank]);
}
let arr = [ { value: 'a' }, { value: 'b' }, { value: 'c' }, { value: 'd' }, { value: 'e' } ];
console.log( 'array before move: ', JSON.stringify( arr ) );
// array before move: [{"value":"a"},{"value":"b"},{"value":"c"},{"value":"d"},{"value":"e"}]
objectArrayMove(arr, 3, 1);
console.log( 'array after move: ', JSON.stringify( arr ) );
// array after move: [{"value":"a"},{"value":"d"},{"value":"b"},{"value":"c"},{"value":"e"}]
_x000D_
var ELEMS = ['a', 'b', 'c', 'd', 'e'];_x000D_
/*_x000D_
Source item will remove and it will be placed just after destination_x000D_
*/_x000D_
function moveItemTo(sourceItem, destItem, elements) {_x000D_
var sourceIndex = elements.indexOf(sourceItem);_x000D_
var destIndex = elements.indexOf(destItem);_x000D_
if (sourceIndex >= -1 && destIndex > -1) {_x000D_
elements.splice(destIndex, 0, elements.splice(sourceIndex, 1)[0]);_x000D_
}_x000D_
return elements;_x000D_
}_x000D_
console.log('Init: ', ELEMS);_x000D_
var result = moveItemTo('a', 'c', ELEMS);_x000D_
console.log('BeforeAfter: ', result);
_x000D_
Source: Stackoverflow.com