Higher-Order Functions

Higher-Order Functions

Swift arrays have powerful built-in higher-order functions:

map — transform each element

let nums = [1, 2, 3, 4]
let doubled = nums.map { $0 * 2 }  // [2, 4, 6, 8]

filter — keep elements matching a predicate

let evens = nums.filter { $0 % 2 == 0 }  // [2, 4]

reduce — combine all elements into one value

let total = nums.reduce(0) { $0 + $1 }  // 10

Chaining

let result = nums.filter { $0 % 2 == 0 }.map { $0 * $0 }  // [4, 16]

The shorthand $0, $1, etc. refer to the first and second closure arguments.

Your Task

Write a function sumOfSquaresOfEvens that takes an array of integers and returns the sum of the squares of all even numbers. Use filter, map, and reduce.