numbers_filter.go

package main

import "fmt"

func main() {
	nums := []int{1, 3, 4, 8, 9, 12, 15, 20, 25, 31}
	result := filterNumbers(nums, func(x int) bool {
		return x%2 == 0
	})

	fmt.Println(result)
}

func filterNumbers(s []int, fn func(int) bool) []int {
	result, n := make([]int, cap(s)), 0
	for i := 0; i < cap(s); i++ {
		if fn(s[i]) {
			result[n] = s[i]
			n += 1
		}
	}

	return result[0:n] // => [4 8 12 20 0 0 0 0 0 0] => [4 8 12 20]
}