Write a function that doubles up each item in an array

Question

This is in javascript. So far, I've got this:

 var double = function (array) {
     var array = [];
     for(var i = 0; i<array.length; i++) {
         var sum = array[i] + array[i];
     }
     return sum;
};

...Basically, if I entered in this area:

var a = [1,2,3];

I would want the function to return:

[1,1,2,2,3,3]

...etc

use `double` as variable name? It's a reserved word in ECMAScript standard 1 ~ 3. See [this](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar) — Raptor, Dec 11 '14 at 06:37
You are just summing some elements of the array... You need to (1) iterate through the array (2) insert the encountered element twice in another array (3à) return the newly constructed array. The parameter `array` is the one to iterate through, and you must construct a local one with a different name, say `myDoublingArray`, write the code is suggested you and `return myDoublingArray`. — Jean-Baptiste Yunès, Dec 11 '14 at 06:40
http://stackoverflow.com/questions/12503146/create-an-array-with-same-element-repeated-multiple-times-in-javascript — Paul, Dec 11 '14 at 06:41
@Raptor: You mean it *was* a reserved keyword until edition 3? Don't bother any more. — Bergi, Dec 11 '14 at 06:48

Evgeniy · Answer 1 · 2018-12-05T10:01:04.083

8

Way with reduce, maybe the shortest one

[1,2,3].reduce(function(m,i) { return m.concat([i,i]); }, []);

and ES6 version

[1,2,3].reduce((m,i) => m.concat([i,i]), []);

edited Dec 05 '18 at 10:01

answered Dec 11 '14 at 06:47

Evgeniy

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JLRishe · Accepted Answer · 2019-06-09T03:24:32.647

Here's a concise way to do this:

function doubleValues(array) {
   return array.concat.apply([], array.map(function (el) { return [el, el] }));
}

console.log(doubleValues([1, 2, 3]));

A somewhat more easy-to-understand approach:

function doubleValues(array) {
  var newArray = [];
  array.forEach(function (el) { newArray.push(el, el); });    
  return newArray;
}

console.log(doubleValues([1,2,3]));

And an ESNext version:

const doubleValues = array => array.flatMap(el => [el,el]);

console.log(doubleValues([1, 2, 3]));

score 2 · Answer 3 · answered Dec 11 '14 at 06:43

Untested code, I'm not overly familiar with javascript, so this might have some syntactical errors:

var duplicateArrayMembers = function (array) {
  var result = [];
  for (var i = 0; i < array.length; ++i) {
    result.push(array[i]);
    result.push(array[i]);
  }
  return result;
}

var myArray = [1, 2, 3];
document.write(duplicateArrayMembers(myArray));

First, we declare a function with a parameter named array. Because array is a parameter, we don't need to declare it in the function body.

Next, we declare the array where we will put the result of our duplication. Let's name this simply result.

Next, we loop through the array. And for every member of the array we push the element to the end of the result. Which means if the first member is 1 we push 1 two different times ending up with [1,1] after the first step.

Finally, after we have looped through the array, we return our result.

Amit Joki · Answer 4 · 2014-12-11T06:58:58.747

2

Here's more concise way do to it.

function doubleArr(arr){
   var rtn = []; 
   return arr.forEach(function(x){ rtn.push(x, x); }), rtn;
}

Calling it

console.log(doubleArr([1,2,3])); // [1, 1, 2, 2, 3, 3]

Here's a simple one-liner if only array of Numbers is concerned.

function doubleArr(arr){
   return arr.map(function(x){ return [x,x]}).join().split(",").map(Number);
}

edited Dec 11 '14 at 06:58

answered Dec 11 '14 at 06:43

Amit Joki

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1

Your simple one-liner will break if any of the original array's values contain commas. – JLRishe Dec 11 '14 at 06:51
@JLRishe OP is passing numbers. – Amit Joki Dec 11 '14 at 06:52
That doesn't mean OP will always be passing numbers. And even if they are all numbers, the values in the result array will all be strings. That doesn't really qualify as doubling all of the array's values. – JLRishe Dec 11 '14 at 06:56
@JLRishe that's why I've put it down with "only if array of Numbers is concerned" – Amit Joki Dec 11 '14 at 06:57
What about the second part of my comment? – JLRishe Dec 11 '14 at 06:58
@JLRishe now got what you meant – Amit Joki Dec 11 '14 at 06:59

score 1 · Answer 5 · answered Dec 11 '14 at 06:42

1

Try this:

var dob = function (array) {
    var test = [];
   for(var i = 0; i<array.length; i++) {
       var sum = array[i];
       test.push(sum);test.push(sum);
  }
   console.log(test);
  };

dob([1,2,3])

FIDDLE

answered Dec 11 '14 at 06:42

Aditya

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score 1 · Answer 6 · answered Dec 11 '14 at 06:43

I changed your function a little bit.

The idea is to .push(item) into another array TWICE.

function double(array) {
  var output = [];
  for(var i = 0; i < array.length; i++) {
    output.push(array[i]);
    output.push(array[i]);
  }
  return output;
};

var input = [1,2,3];

var output = double(input);

alert(output);

score 0 · Answer 7 · answered Dec 11 '14 at 07:03

Apart from all the ways of doing it with pure Javascript that you can find in other answers, you could use a library like underscore or lo-dash, or similar, in which case you could write it as:

_.chain(array).map(function(x) {return [x,x]}).flatten().value()

The chain function will wrap your array in an object, so you can apply successive functions on it. If you start with [1, 2, 3] then map will return [[1,1],[2,2],[3,3]]. After that flatten will flatten your array to [1,1,2,2,3,3], and value() just unwraps it from the object so you can use it.

score 0 · Answer 8 · answered Jun 04 '19 at 08:37

This one worked for me:

Given an array of integers, return a new array with each value doubled.

For example:

[1, 2, 3] --> [2, 4, 6]

For the beginner, try to use the map method - it comes in very handy quite a lot so is a good one to know.

function maps(integers) {
return integers.concat.apply([],
integers.map(function (n) { 
return [n*2] }));
};

Write a function that doubles up each item in an array

8 Answers8