Representing a Set with a Map (Duplicate-Free Data)

In this lesson you will learn to represent a set with a map in Go and see a data structure put to practical use. This is for anyone searching for "Go map as a set".

Go has no dedicated Set type, but map[T]bool (a map that only records presence) gives you a duplicate-free box. Using the value as the key makes membership tests fast.

The sample code registers each value of the duplicate-containing slice {1, 2, 2, 3, 3, 3} as a map key with seen[n] = true, so the duplicates disappear automatically. Notice how len(seen) then gives the number of unique elements.

A common stumbling block for beginners is that the order of a map is not guaranteed. Unlike a slice, items are not necessarily kept in insertion order, so another structure is a better fit when order matters.

In real-world development a map-based lookup outperforms checking a slice one element at a time once there is a lot of data.

📖 Reference code

package main

import "fmt"

func main() {
    nums := []int{1, 2, 2, 3, 3, 3}
    seen := map[int]bool{}
    for _, n := range nums {
        seen[n] = true
    }
    fmt.Println(len(seen))
    fmt.Println(seen[2])
}

✍️ Your code

Type your code, then press "Run"

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