How to count items in a Go map?

map go
shirley picture shirley · Sep 22, 2012 · Viewed 81.5k times · Source

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

zzzz picture zzzz · Sep 22, 2012

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.

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