How to count items in a Go map?

If I want to count the items in the map structure, what statement should I use?
I tried to use

for _, _ := range m {...}

but it seems the syntax is false.

Use len(m). From http://golang.org/ref/spec#Length_and_capacity

len(s)    string type      string length in bytes
          [n]T, *[n]T      array length (== n)
          []T              slice length
          map[K]T          map length (number of defined keys)
          chan T           number of elements queued in channel buffer

Here are a couple examples ported from the now-retired SO documentation:

m := map[string]int{}
len(m) // 0

m["foo"] = 1
len(m) // 1

If a variable points to a nil map, then len returns 0.

var m map[string]int
len(m) // 0

Excerpted from Maps - Counting map elements. The original author was Simone Carletti. Attribution details can be found on the contributor page. The source is licenced under CC BY-SA 3.0 and may be found in the Documentation archive. Reference topic ID: 732 and example ID: 2528.

Stack Overflow Q#12544641, score: 242