function entropy()

in code/decision-tree/js/EntropyBubble.js [16:41]

• 19 lines of code
• 6 McCabe index (conditional complexity)

function entropy(data) {
  // track data length (used a couple times)
  const n = data.length;
  // empty data has entropy of zero
  if (n <= 1) {
    return 0;
  }
  // init counter
  const counts = {};
  // count occurrences of elements in data
  for (const d of data) {
    counts[d] = counts[d] ? counts[d] + 1 : 1;
  }
  // start entropy at 0
  let entropyValue = 0;
  // loop through data and calculate entropy
  Object.keys(counts).forEach((c) => {
    // get relative frequency of item
    const prob = counts[c] / n;
    // if > 0, tally entropy
    if (prob > 0) {
      entropyValue -= prob * Math.log2(prob);
    }
  });
  return entropyValue;
}