How to test key existence in a map?

Question

I know I can iterate over a map m by,

for k, v := range m { ... }

and look for a key but is there a more efficient way of testing a key's existence in a map? Thanks. I couldn't find an answer in the language spec.

Answer 1

Searched on the go-nuts email list and found a solution posted by Peter Froehlich on 11/15/2009.

package main

import "fmt"

func main() {
        dict := map[string]int {"foo" : 1, "bar" : 2}
        value, ok := dict["baz"]
        if ok {
                fmt.Println("value: ", value)
        } else {
                fmt.Println("key not found")
        }
}

Or, more compactly,

if value, ok := dict["baz"]; ok {
    fmt.Println("value: ", value)
} else {
    fmt.Println("key not found")
}

Note, using this form of the if statement, the value and ok variables are only visible inside the if conditions.

Answer 2

One line answer:

if val,ok := dict["foo"]; ok {
    //do something here
}

Answer 3

In addition to The Go Programming Language Specification, you should read Effective Go. In the section on maps, they say, amongst other things:

"An attempt to fetch a map value with a key that is not present in the map will cause the program to crash, but there is a way to do so safely using a multiple assignment."

var seconds int
var ok bool
seconds, ok = timeZone[tz]

"To test for presence in the map without worrying about the actual value, you can use the blank identifier, a simple underscore (_). The blank identifier can be assigned or declared with any value of any type, with the value discarded harmlessly. For testing presence in a map, use the blank identifier in place of the usual variable for the value."

_, present := timeZone[tz]