Javascript子数组

Question

I recently ran into the problem where I would like to select multiple elements from an array, to return a sub-array. For example, given the array:

a = [1, 5, 1, 6, 2, 3, 7, 8, 3]

And the index array of:

i = [3, 5, 6]

I want to select all elements in a, who's index appears in i. So the output in my simple example would be:

[6, 3, 7]

I completely realise I could use a for loop over i and construct a new array then use Array.push(a[index (in i)]) to add in each, but I was wondering if there was a clearer/cleaner way to achieve this (possibly using underscore.js, or something similar).

Answer 1

i.map(function(x) { return a[x]; })
// => [6, 3, 7]

Answer 2

You can try this

a = [1, 5, 1, 6, 2, 3, 7, 8, 3];
i = [3,5,6];
var res= [];     //for show result array
for (var n in a){      //loop a[]
 for(var index in i){     //loop i[]
     if(  n == i[index] )     //n is index of a[]
     res.push(a[n]);          //add array if equal n index and i[] value
 }
 }
 alert(res);    // 6,3,7 

Answer 3

You could use map function to achieve your desired result.

var a = [1, 5, 1, 6, 2, 3, 7, 8, 3];

var i = [3, 5, 6];

var mapped = i.map(function(index) { 
    return a[index]; 
});

console.log(mapped);

Here is the working jsfiddle .

However with above example, map not be available in all browsers yet. Here is the quote from documentation of map .

map was added to the ECMA-262 standard in the 5th edition; as such it may not be present in all implementations of the standard.

If your code will be running in old browsers then you will need to add a polyfill . However there are libraries that give you similar functionality with polyfills for older browsers. Along with map function, underscodejs has tons of other helpful functions. I higly recommend you to look at what underscorejs has to offer. It provides tons of helper functions and has quite wide range browser support.

You would do following in underscorejs and wont have to worry if your code works in cross browsers.

var a = [1, 5, 1, 6, 2, 3, 7, 8, 3];

var mapped = _.map([3, 5, 6], function(index) {
    return a[index];
});

alert(mapped);

Here is jsfiddle for that.