freeCodeCamp/curriculum/challenges/chinese/10-coding-interview-prep/rosetta-code/letter-frequency.md

id	title	challengeType	forumTopicId	dashedName
5e4ce2bbac708cc68c1df25f	Letter frequency	5	385263	letter-frequency

--description--

Given a string, calculate the 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.

--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: ['char', freq]. 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 [['a', 1], ['b', 1]].

--hints--

letterFrequency should be a function.

assert(typeof letterFrequency == 'function');

letterFrequency("Not all that Mrs. Bennet, however") should return an array.

assert(Array.isArray(letterFrequency('Not all that Mrs. Bennet, however')));

letterFrequency("Not all that Mrs. Bennet, however") should return [[" ", 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]].

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]
]);

letterFrequency("daughters, could ask on the ") should return [[" ", 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]].

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]
]);

letterFrequency("husband any satisfactory description") should return [[" ", 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]].

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]
]);

letterFrequency("in various ways--with barefaced") should return [[" ", 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]].

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]
]);

letterFrequency("distant surmises; but he eluded") should return [[" ", 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]].

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]
]);

letterFrequency("last obliged to accept the second-hand,") should return [[" ", 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]].

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]
]);

--seed--

--seed-contents--

function letterFrequency(txt) {

}

--solutions--

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]]; });
}