Mean

Prompt

Write a function mean(array) that calculates the arithmetic average of the numbers in an array. Add everything up, divide by how many there are.

If the array is empty, return NaN.

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mean([1, 2, 3]); // 2
mean([1, 2, 3, 4, 5]); // 3
mean([10]); // 10
mean([]); // NaN

Playground

Solution

Explanation

The idea is simple: add up all the numbers, then divide by how many there are.

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function mean(array) {
  return (
    array.reduce((sum, num) => sum + num, 0) / array.length
  );
}

reduce() walks through the array and accumulates a running total. We start with 0 and add each number to it. Once we have the sum, we divide by array.length to get the average.

For mean([1, 2, 3]): the sum is 1 + 2 + 3 = 6, and there are 3 numbers, so 6 / 3 = 2.

For mean([10]): the sum is 10, there's 1 number, so 10 / 1 = 10.

For mean([]): the sum is 0 (reduce starts at 0 with nothing to add), and the length is 0. Dividing 0 / 0 gives NaN in JavaScript, which is exactly what we want for an empty array.

For loop alternative

If reduce doesn't come to mind, a plain loop works just as well:

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function mean(array) {
  let sum = 0;
  for (let i = 0; i < array.length; i++) {
    sum += array[i];
  }
  return sum / array.length;
}

Same logic, just more explicit. Add everything up, divide by the count.

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