Subset check with slices in Go

Question

I am looking for a efficient way to check if a slice is a subset of another. I could simply iterate over them to check, but I feel there has to be a better way.

E.g.

{1, 2, 3} is a subset of {1, 2, 3, 4}
{1, 2, 2} is NOT a subset of {1, 2, 3, 4}

What is the best way to do this efficiently?

Thanks!

Answer 1

I think the most common way to solve a subset problem is via a map.

package main

import "fmt"

// subset returns true if the first array is completely
// contained in the second array. There must be at least
// the same number of duplicate values in second as there
// are in first.
func subset(first, second []int) bool {
    set := make(map[int]int)
    for _, value := range second {
        set[value] += 1
    }

    for _, value := range first {
        if count, found := set[value]; !found {
            return false
        } else if count < 1 {
            return false
        } else {
            set[value] = count - 1
        }
    }

    return true
}

func main() {
    fmt.Println(subset([]int{1, 2, 3}, []int{1, 2, 3, 4}))
    fmt.Println(subset([]int{1, 2, 2}, []int{1, 2, 3, 4}))
}

The ability to check duplicate values is relatively uncommon. The code above solves the problem as asked (see: http://play.golang.org/p/4_7Oh-fgDQ) though. If you plan on having duplicate values, you'll have to keep a count like the code above does. If there will not be duplicate values, you can solve the problem more compactly by using a boolean for the map value instead of an integer.