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

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+---
+id: 5e4ce2bbac708cc68c1df25f
+title: Letter frequency
+challengeType: 5
+forumTopicId: 385263
+---
+
+## Description
+<section id='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.
+</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');
+  - 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")));
+  - 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]]);
+  - 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]]);
+  - 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]]);
+  - 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]]);
+  - 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]]);
+  - 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]]);
+```
+
+</section>
+
+## Challenge Seed
+<section id='challengeSeed'>
+
+<div id='js-seed'>
+
+```js
+function letterFrequency(txt) {
+
+}
+```
+
+</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>