How To Count Elements

Author : Doe Hoon LEE

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How Do We Count Number of Items in an Array?

const givenAlphabets =  ["A", "B", "A", "C", "B", "B", "A", "S", "C", "B", "C", "A", "D", "S"];
const givenNums = [1,8,4,6,3,5,2,6,3,8,9,10,10,2,2,2,4,5];

There are several ways you can count occurrences or frequency of elements

Let’s try one way with givenAlphabets array

We will first sort the array

function freq(arr) {
    arr.sort();
    // arr => ["A", "A", "A", "A", "B", "B", "B", "B", "C", "C", "C", "D", "S", "S"];
}

Then we loop through the array and start counting

When we hit an element that is different from the previous one, we will push [currentElement, count] to the answer array

function freq(arr) {
    let count = 1;
    const answer = [];
    arr.sort();
    // arr => ["A", "A", "A", "A", "B", "B", "B", "B", "C", "C", "C", "D", "S", "S"];

    for (let i=0; i < arr.length; i++) {
        if (arr[i] !== arr[i+1]) {
            answer.push([arr[i], count]);
            count = 1;
        }
        else {
            count++;
        }
    }
    return answer;
}

Let’s move onto Dev Tool and see if it works

How about counting num frequency though?

const givenNums = [1,8,4,6,3,5,2,6,3,8,9,10,10,2,2,2,4,5];
givenNums.sort() // => [1, 10, 10, 2, 2, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 8, 8, 9];

The thing is that you cannot sort numbers this way

So we will have to add something to it

givenNums.sort((a,b) => a - b) // => [1, 2, 2, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 8, 8, 9, 10, 10];

Now, we apply the same logic to this sorted array

function freq(arr) {
    let count = 1;
    const answer = [];
    arr.sort((a,b) => a - b);
    // arr => [1, 2, 2, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 8, 8, 9, 10, 10];

    for (let i=0; i < arr.length; i++) {
        if (arr[i] !== arr[i+1]) {
            answer.push([arr[i], count]);
            count = 1;
        }
        else {
            count++;
        }
    }

    return answer;
}

Of course, though, this is one of the ways you can count items in an array, but this isn’t really my thing.. Yes. I am lazy :)

Also, sort() messes up the original array unless you use the copy of the original (see post on sort()).

This is my personal way of counting items.

function occurence(arr) {
    const counter = {};

    for (let i=0; i < arr.length; i++) {
        counter[arr[i]] = counter[arr[i]] + 1 || 1;
    }

    return counter;
}

What does it do?

First, we create an empty array

Then we go through the given array with a for loop

If object counter does not have the key arr[i], we give assign it with value of 1

If it does have the key arr[i], then we will add 1 to its value

So this is what you get !

Thanks for studying with me! Have fun developing!! :D