freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/rosetta-code/letter-frequency.english.md

---
id: 5a23c84252665b21eecc7ee1
title: Letter frequency
challengeType: 5
---

## Description
<section id='description'>
You are given a string and your task is to calculate frequency of each character.
All characters should be counted. This includes lower and upper case letters, digits, whitespace, special characters, or any other distinct characters.
</section>

## Instructions
<section id='instructions'>
Write a function to count the occurrences of each character in a given string.
The function should return a 2D array with each of the elements in the following form: <code>['char', freq]</code>. The character should be a string with a length of 1, and frequency is a number denoting the count.
For example, given the string "ab", your function should return <code>[['a', 1], ['b', 1]]</code>.
</section>

## Tests
<section id='tests'>

``` yml
tests:
  - text: <code>letterFrequency</code> should be a function.
    testString: assert(typeof letterFrequency == 'function', '<code>letterFrequency</code> should be a function.');
  - text: <code>letterFrequency("Not all that Mrs. Bennet, however")</code> should return an array.
    testString: assert(Array.isArray(letterFrequency("Not all that Mrs. Bennet, however")), '<code>letterFrequency("Not all that Mrs. Bennet, however")</code> should return an array.');
  - text: <code>letterFrequency("Not all that Mrs. Bennet, however")</code> should return <code>[[" ", 5], [", ", 1], [".", 1], ["B", 1], ["M", 1], ["N", 1], ["a", 2], ["e", 4], ["h", 2], ["l", 2], ["n", 2], ["o", 2], ["r", 2], ["s", 1], ["t", 4], ["v", 1], ["w", 1]]</code>.
    testString: assert.deepEqual(letterFrequency("Not all that Mrs. Bennet, however"), [[' ', 5], [',', 1], ['.', 1], ['B', 1], ['M', 1], ['N', 1], ['a', 2], ['e', 4], ['h', 2], ['l', 2], ['n', 2], ['o', 2], ['r', 2], ['s', 1], ['t', 4], ['v', 1], ['w', 1]], '<code>letterFrequency("Not all that Mrs. Bennet, however")</code> should return <code>[[" ", 5], [", ", 1], [".", 1], ["B", 1], ["M", 1], ["N", 1], ["a", 2], ["e", 4], ["h", 2], ["l", 2], ["n", 2], ["o", 2], ["r", 2], ["s", 1], ["t", 4], ["v", 1], ["w", 1]]</code>.');
  - text: <code>letterFrequency("daughters, could ask on the ")</code> should return <code>[[' ',5],[',',1],['a',2],['c',1],['d',2],['e',2],['g',1],['h',2],['k',1],['l',1],['n',1],['o',2],['r',1],['s',2],['t',2],['u',2]]</code>.
    testString: assert.deepEqual(letterFrequency("daughters, could ask on the "), [[' ', 5], [',', 1], ['a', 2], ['c', 1], ['d', 2], ['e', 2], ['g', 1], ['h', 2], ['k', 1], ['l', 1], ['n', 1], ['o', 2], ['r', 1], ['s', 2], ['t', 2], ['u', 2]], '<code>letterFrequency("daughters, could ask on the ")</code> should return <code>[[" ", 5], [", ", 1], ["a", 2], ["c", 1], ["d", 2], ["e", 2], ["g", 1], ["h", 2], ["k", 1], ["l", 1], ["n", 1], ["o", 2], ["r", 1], ["s", 2], ["t", 2], ["u", 2]]</code>.');
  - text: <code>letterFrequency("husband any satisfactory description")</code> should return <code>[[" ", 3], ["a", 4], ["b", 1], ["c", 2], ["d", 2], ["e", 1], ["f", 1], ["h", 1], ["i", 3], ["n", 3], ["o", 2], ["p", 1], ["r", 2], ["s", 4], ["t", 3], ["u", 1], ["y", 2]]</code>.
    testString: assert.deepEqual(letterFrequency("husband any satisfactory description"), [[' ', 3], ['a', 4], ['b', 1], ['c', 2], ['d', 2], ['e', 1], ['f', 1], ['h', 1], ['i', 3], ['n', 3], ['o', 2], ['p', 1], ['r', 2], ['s', 4], ['t', 3], ['u', 1], ['y', 2]], '<code>letterFrequency("husband any satisfactory description")</code> should return <code>[[" ", 3], ["a", 4], ["b", 1], ["c", 2], ["d", 2], ["e", 1], ["f", 1], ["h", 1], ["i", 3], ["n", 3], ["o", 2], ["p", 1], ["r", 2], ["s", 4], ["t", 3], ["u", 1], ["y", 2]]</code>.');
  - text: <code>letterFrequency("in various ways--with barefaced")</code> should return <code>[[" ", 3], ["-", 2], ["a", 4], ["b", 1], ["c", 1], ["d", 1], ["e", 2], ["f", 1], ["h", 1], ["i", 3], ["n", 1], ["o", 1], ["r", 2], ["s", 2], ["t", 1], ["u", 1], ["v", 1], ["w", 2], ["y", 1]]</code>.
    testString: assert.deepEqual(letterFrequency("in various ways--with barefaced"), [[' ', 3], ['-', 2], ['a', 4], ['b', 1], ['c', 1], ['d', 1], ['e', 2], ['f', 1], ['h', 1], ['i', 3], ['n', 1], ['o', 1], ['r', 2], ['s', 2], ['t', 1], ['u', 1], ['v', 1], ['w', 2], ['y', 1]], '<code>letterFrequency("in various ways--with barefaced")</code> should return <code>[[" ", 3], ["-", 2], ["a", 4], ["b", 1], ["c", 1], ["d", 1], ["e", 2], ["f", 1], ["h", 1], ["i", 3], ["n", 1], ["o", 1], ["r", 2], ["s", 2], ["t", 1], ["u", 1], ["v", 1], ["w", 2], ["y", 1]]</code>.');
  - text: <code>letterFrequency("distant surmises; but he eluded")</code> should return <code>[[" ", 4], ["; ", 1], ["a", 1], ["b", 1], ["d", 3], ["e", 4], ["h", 1], ["i", 2], ["l", 1], ["m", 1], ["n", 1], ["r", 1], ["s", 4], ["t", 3], ["u", 3]]</code>.
    testString: assert.deepEqual(letterFrequency("distant surmises; but he eluded"), [[' ', 4], [';', 1], ['a', 1], ['b', 1], ['d', 3], ['e', 4], ['h', 1], ['i', 2], ['l', 1], ['m', 1], ['n', 1], ['r', 1], ['s', 4], ['t', 3], ['u', 3]], '<code>letterFrequency("distant surmises; but he eluded")</code> should return <code>[[" ", 4], ["; ", 1], ["a", 1], ["b", 1], ["d", 3], ["e", 4], ["h", 1], ["i", 2], ["l", 1], ["m", 1], ["n", 1], ["r", 1], ["s", 4], ["t", 3], ["u", 3]]</code>.');
  - text: <code>letterFrequency("last obliged to accept the second-hand,")</code> should return <code>[[" ", 5], [", ", 1], ["-", 1], ["a", 3], ["b", 1], ["c", 3], ["d", 3], ["e", 4], ["g", 1], ["h", 2], ["i", 1], ["l", 2], ["n", 2], ["o", 3], ["p", 1], ["s", 2], ["t", 4]]</code>.
    testString: assert.deepEqual(letterFrequency("last obliged to accept the second-hand,"), [[' ', 5], [',', 1], ['-', 1], ['a', 3], ['b', 1], ['c', 3], ['d', 3], ['e', 4], ['g', 1], ['h', 2], ['i', 1], ['l', 2], ['n', 2], ['o', 3], ['p', 1], ['s', 2], ['t', 4]], '<code>letterFrequency("last obliged to accept the second-hand,")</code> should return <code>[[" ", 5], [", ", 1], ["-", 1], ["a", 3], ["b", 1], ["c", 3], ["d", 3], ["e", 4], ["g", 1], ["h", 2], ["i", 1], ["l", 2], ["n", 2], ["o", 3], ["p", 1], ["s", 2], ["t", 4]]</code>.');
```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
function letterFrequency (txt) {
  // Good luck!
}
```

</div>

</section>

## Solution
<section id='solution'>

```js
function letterFrequency (txt) {
    var cs = txt.split(''),
        i = cs.length,
        dct =  {},
        c = '',
        keys;

    while (i--) {
        c = cs[i];
        dct[c] = (dct[c] || 0) + 1;
    }

    keys = Object.keys(dct);
    keys.sort();
    return keys.map(function (c) { return [c, dct[c]]; });
}
```

</section>