How to Efficiently Find the Difference Between Two String Slices in Go?

Finding the Difference Between Two String Slices

In programming, comparing the content of two slices is a common task. This question focuses on identifying the elements present in one slice but not in another, effectively finding the "difference" between them.

The desired outcome is to receive a new slice containing the unique elements from the first slice that are not present in the second. For instance, if slice1 contains ["foo", "bar", "hello"] and slice2 contains ["foo", "bar"], the difference would be ["hello"].

A Straightforward Solution Using a Map

A reliable approach to solving this problem involves utilizing a map. Maps in Go offer efficient lookup operations, with an approximately constant time complexity of O(1). Here's how we can implement this approach:

// difference returns the elements in `a` that aren't in `b`.
func difference(a, b []string) []string {
    mb := make(map[string]struct{}, len(b))
    for _, x := range b {
        mb[x] = struct{}{}
    }

    var diff []string
    for _, x := range a {
        if _, found := mb[x]; !found {
            diff = append(diff, x)
        }
    }

    return diff
}

Explanation

This function takes two string slices, a and b, as input. It constructs a map, mb, that maps each element of b to an empty struct. This map is used to efficiently determine the presence of elements in b with O(1) lookups.

The function then iterates through a, checking each element against the elements present in mb. If an element of a is not found in mb, it is added to the diff slice.

Finally, the diff slice containing the unique elements from a that are not present in b is returned.

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