Get map size (number of map entries)

The built-in function len returns the number of elements in a map:

main.go

    m := map[string]int{}
    size := len(m)
    fmt.Printf("Size of empty map: %d\n\n", size)

    
    m["foo"] = 1
    size = len(m)
    fmt.Printf("Size of map: %d\n\n", size)

Output:

Size of empty map: 0

Size of map: 1

A nil map has size 0:

main.go

    var m map[string]int
    size := len(m)
    fmt.Printf("Size of nil map: %d\n\n", size)

Output:

Size of nil map: 0

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