CODEWITHSUNDEEP
javascriptarrays
Vicheanak
Vicheanak

Reputation: 6684

How can I find the common elements in a list of arrays?

a = [1, 2, 3, 4, 5]
b = [1, 3, 6, 4, 5, 9]
c = [5, 4, 7, 9]
d = [1, 7, 5, 6, 9, 4]
e = [4, 3, 5, 7, 1]
f = [...]
.
.
(n = [n,n,n])

For 1 out of many cases, we have 5 variables from a to e and we would like to get the intersection element out of these 5 arrays, without having to write nested for..loop for each case.

Please suggest the ideal solution to this problem.

Upvotes: 1

Views: 1794

Answers (2)

Andy
Andy

Reputation: 63524

Here's the non-Mr.Fancy-Pants version :)

function findIntersection() {

    // turns the list of arguments into an array
    var args = Array.prototype.slice.call(arguments);

    // concatenate all the arrays
    var all = args.reduce(function (a, b) { return a.concat(b); });

    // use the temporary object to store the number of times a
    // number appears
    for (var i = 0, obj = {}, l = all.length; i < l; i++) {
        var key = all[i];
        if (!obj[key]) obj[key] = 0;
        obj[key]++;
    }

    // return those numbers that have a value matching the number
    // of arguments passed into the function
    return Object.keys(obj).filter(function (el) {
        return obj[el] === args.length;
    }).map(Number);
}

findIntersection(a,b,c,d,e); // [ "4", "5" ]

DEMO

Upvotes: 1

thefourtheye
thefourtheye

Reputation: 239443

First find the common elements between first and the second arrays and then find the common elements between the previous set of common elements and the third array and so on.

var listOfArrays = [a, b, c, d, e, ...];
var commons = listOfArrays.slice(1).reduce(function(result, currentArray) {
    return currentArray.filter(function(currentItem) {
        return result.indexOf(currentItem) !== -1;
    });
}, listOfArrays[0]);

Here,

currentArray.filter(function(currentItem) {...});

is the function responsible for finding the common elements between two arrays, result and currentArray.

We use Array.prototype.reduce where the value returned by the function passed to it will be fed back to the same function, in the next iteration. So, we keep on feeding the common elements from the previous iteration to the next iteration.

Upvotes: 5

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