Maps / Get map size (number of map entries) / Essential Go

package main

import "fmt"

func main() {
    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)
}

Go

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