Maps:
*Get map size (number of map entries)
The built-in function len returns the number of elements in a map:
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)
Size of empty map: 0
Size of map: 1
A nil map has size 0:
var m map[string]int
size := len(m)
fmt.Printf("Size of nil map: %d\n\n", size)
Size of nil map: 0