Filter array based on an array of index

问题

First I apologize if it's a duplicate (I searched but did not find this simple example...), but I want to select elements of arr1 based on the index in arr2:

arr1 = [33,66,77,8,99]
arr2 = [2,0,3]

I am using underscore.js but the 0 index is not retrieved (seems to be considered as false):

res = _.filter(arr1, function(value, index){
    if(_.contains(arr2, index)){
        return index;
    }
});

Which returns:

# [77, 8]

How could I fix this, and is there a simpler way to filter using an array of indexes? I am expecting the following result:

# [77, 33, 8]

回答1:

The simplest way is to use _.map on arr2, like this

console.log(_.map(arr2, function (item) {
  return arr1[item];
}));
// [ 77, 33, 8 ]

Here, we iterate the indexes and fetching the corresponding values from arr1 and creating a new array.

If your environment supports ECMA Script 6's Arrow functions, then you can simply do

console.log(_.map(arr2, (item) => arr1[item]));
// [ 77, 33, 8 ]

Moreover, you can use the native Array.protoype.map itself, if your target environment supports them, like this

console.log(arr2.map((item) => arr1[item]));
// [ 77, 33, 8 ]

回答2:

You are returning index, so in your case 0 treated as false. So you need to return true instead

res = _.filter(arr1, function(value, index){
    if(_.contains(arr2, index)){
        return true;
    }
});

or just return _.contains()

res = _.filter(arr1, function(value, index){
   return _.contains(arr2, index);
});

回答3:

_.contains returns a boolean. You should return that from the filter predicate, rather than the index because 0 is a falsy value.

res = _.filter(arr1, function(value, index)) {
  return _.contains(arr2, index);
});

As an aside, JavaScript arrays have a native filter method so you could use:

res = arr1.filter(function(value, index)) {
  return _.contains(arr2, index);
});

回答4:

Isn't it better to iterate through indices array as main loop?

var arr1 = [33,66,77,8,99]
var arr2 = [2,0,3] 
var result = [];
for(var i=0; i<arr2.length; i++) {
   var index = arr2[i];
   result.push(arr1[index]);
}

console.log(result);

来源：https://stackoverflow.com/questions/33211799/filter-array-based-on-an-array-of-index

标签

javascript

arrays

underscore.js