freeCodeCamp/curriculum/challenges/en/08-coding-interview-prep/rosetta-code/entropy.en.md

title, id, challengeType

title	id	challengeType
Entropy	599d15309e88c813a40baf58	5

Description

Task:

Calculate the Shannon entropy H of a given input string.

Given the discreet random variable $X$ that is a string of $N$ "symbols" (total characters) consisting of $n$ different characters (n=2 for binary), the Shannon entropy of X in bits/symbol is :

$H_2(X) = -\sum_{i=1}^n \frac{count_i}{N} \log_2 \left(\frac{count_i}{N}\right)$

where $count_i$ is the count of character $n_i$.

Instructions

Tests

- text: <code>entropy</code> is a function.
  testString: 'assert(typeof entropy === ''function'', ''<code>entropy</code> is a function.'');'
- text: <code>entropy("0")</code> should return <code>0</code>
  testString: 'assert.equal(entropy(''0''), 0, ''<code>entropy("0")</code> should return <code>0</code>'');'
- text: <code>entropy("01")</code> should return <code>1</code>
  testString: 'assert.equal(entropy(''01''), 1, ''<code>entropy("01")</code> should return <code>1</code>'');'
- text: <code>entropy("0123")</code> should return <code>2</code>
  testString: 'assert.equal(entropy(''0123''), 2, ''<code>entropy("0123")</code> should return <code>2</code>'');'
- text: <code>entropy("01234567")</code> should return <code>3</code>
  testString: 'assert.equal(entropy(''01234567''), 3, ''<code>entropy("01234567")</code> should return <code>3</code>'');'
- text: <code>entropy("0123456789abcdef")</code> should return <code>4</code>
  testString: 'assert.equal(entropy(''0123456789abcdef''), 4, ''<code>entropy("0123456789abcdef")</code> should return <code>4</code>'');'
- text: <code>entropy("1223334444")</code> should return <code>1.8464393446710154</code>
  testString: 'assert.equal(entropy(''1223334444''), 1.8464393446710154, ''<code>entropy("1223334444")</code> should return <code>1.8464393446710154</code>'');'

Challenge Seed

function entropy (s) {
  // Good luck!
}

Solution

function entropy(s) {
	// Create a dictionary of character frequencies and iterate over it.
  function process(s, evaluator) {
    let h = Object.create(null),
      k;
    s.split('').forEach(c => {
      h[c] && h[c]++ || (h[c] = 1); });
    if (evaluator) for (k in h) evaluator(k, h[k]);
    return h;
  }
	// Measure the entropy of a string in bits per symbol.

  let sum = 0,
    len = s.length;
  process(s, (k, f) => {
    const p = f / len;
    sum -= p * Math.log(p) / Math.log(2);
  });
  return sum;
}