How to check if a map is empty in Golang?

Question

When the following code:

m := make(map[string]string)
if m == nil {
    log.Fatal("map is empty")
}

is run, the log statement is not executed, while fmt.Println(m) indicates that the map is empty:

map[]

Solution 1

You can use len:

if len(m) == 0 {
    ....
}

From https://golang.org/ref/spec#Length_and_capacity

len(s) map[K]T map length (number of defined keys)

Solution 2

The following example demonstrates both the nil check and the length check that can be used for checking if a map is empty

package main

import (
    "fmt"
)

func main() {
    a := new(map[int64]string)
    if *a == nil {
        fmt.Println("empty")
    }
    fmt.Println(len(*a))
}

Prints

empty
0

Comments

[Cirelli94]

This question has a lot of upvote but I think there's a little misunderstanding here: a map can be nil or can be initialized and with 0 value inside that. This are two different situations!

[Slava Bacherikov]

For those who is wondering about complexity it is O(1). Here's implementation.

[super]

new(map[int64]string) return a uninitialized pointer to a map. You can't use that to check if the map is empty.

[Alessandro Argentieri]

@Mradul you don't need a pointer to a map or a slice: they're already pointers!