How to count items in a Go map?

Question

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.

Answer 1

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.

Answer 2

For anyone wanting to count the number of elements in a nested map:

  var count int   m := map[string][]int{}   for _, t := range m {     count += len(t)   }