Mean

Requirements

• Implement mean(numbers) as the sum of the array divided by its length.
• Return NaN for an empty array. Support positive, negative, zero, and fractional values.
• The input is a dense array of finite numbers. Validation, arbitrary-precision arithmetic, and numeric overflow are out of scope.
• Leave the input array unchanged. Do not sort it or use an external library.

Examples

Input: mean([2, 4, 6])
Output: 4

Input: mean([])
Output: NaN