Loop through two arrays and get a desired output using javascript

Question

I have two arrays as follows

const arr1 = ['john', 'robot'];

const arr2 = [{name : 'john'}, {name : 'kevin'}, {name : 'james}];

My desired output is to manipulate array arr1 such that if value of arr1 is not present as a name property in arr2 it should get removed from arr1;

So in the above example after the operation, the output should be

console.log(arr1)  // ['john']

as robot is not present in arr2, it gets removed.

Likewise if I have following set of arrays

const names = ['sachin', 'sehwag'];

const players = [{name : 'dhoni'}, {name : 'dravid'}, {name : 'ganguly'} , {name : 'laxman}];

names array should be manipulated to

console.log(names) // []

as both sachin and sehwag is not present in players array

Please help.Thanks in advance.

i tried looping names array with map method and inside the callback of map did a filter on the players array.However got stuck in getting the desired result. — Shijil Narayanan
– Shijil Narayanan, Commented Aug 19, 2018 at 16:42

Mark · Accepted Answer · 2018-08-19 16:47:31Z

3

If you can use es6, a Set is good for this. They only store unique values and are efficient at lookup. They give constant time lookups, which looping through an array doesn't:

const arr1 = ['john', 'robot'];
const arr2 = [{name : 'john'}, {name : 'kevin'}, {name : 'james'}];

// create a set with names
let testSet = arr2.reduce((set, obj) => set.add(obj.name), new Set)

let filtered = arr1.filter(item => testSet.has(item))
console.log(filtered)

edited Aug 19, 2018 at 16:47

answered Aug 19, 2018 at 16:39

Mark

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1 Comment

Shijil Narayanan Over a year ago

I am using ES6, hence using Set seems nice.Your solution worked for me.Thanks

Arup Rakshit · Accepted Answer · 2018-08-19 16:34:03Z

2

You can first grab all the names from array of objects arr2, and then filter the arr1 based on names list..

var arr1 = ["john", "robot"];
var arr2 = [{ name: "john" }, { name: "kevin" }, { name: "james" }];
var names = arr2.map(ob => ob.name);
console.log(arr1.filter(name => names.includes(name)));

var arr1 = ["sachin", "sehwag"];
var arr2 = [
  { name: "dhoni" },
  { name: "dravid" },
  { name: "ganguly" },
  { name: "laxman" }
];
var names = arr2.map(ob => ob.name);
console.log(arr1.filter(name => names.includes(name)));

answered Aug 19, 2018 at 16:34

Arup Rakshit

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1 Comment

wizzwizz4 Over a year ago

Your answer is clear, concise and informative. Unfortunately, it doesn't work in IE11. *ducks and runs away*

Saurabh Yadav · Accepted Answer · 2018-08-19 16:33:50Z

1

       const arr1 = ['john', 'robot'];
       const arr2 = [{name : 'john'}, {name : 'kevin'}, {name : 'james'}];
        var newArray= [];
        for(var i in arr2) {
            var content = arr2[i]['name'];
              if(arr1.indexOf(content) > -1){
                   newArray.push(content);
                }
        }

    console.log(newArray);

Hope it helps you!

answered Aug 19, 2018 at 16:33

Saurabh Yadav

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Comments

wizzwizz4 · Accepted Answer · 2018-08-19 16:36:28Z

-1

This sounds like a job for [].filter:

function doTheThingYouWantItToDo(a, b) {
    return a.filter(function(elem) {
        for (var i = 0; i < b.length; i+=1) {
            if (b[i].name == elem) {
                return true;
            }
        }
        return false;
    });
}

answered Aug 19, 2018 at 16:36

wizzwizz4

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3 Comments

Derek 朕會功夫 Over a year ago

Nested loops... ouch.

wizzwizz4 Over a year ago

I forgot about [].indexOf, which I should've used instead.

Derek 朕會功夫 Over a year ago

.indexOf is still a loop over the entire array, so the complexity is still O(nm). Using a set however is only O(n+m) which is considerably faster and more efficient.

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