How to check if a map contains a key in go?
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.
One line answer:
if val, ok := dict["foo"]; ok {
//do something here
}
Explanation:
if statements in Go can include both a condition and an initialization statement. The example above uses both:
initializes two variables - val will receive either the value of "foo" from the map or a "zero value" (in this case the empty string) and ok will receive a bool that will be set to true if "foo" was actually present in the map
evaluates ok , which will be true if "foo" was in the map
If "foo" is indeed present in the map, the body of the if statement will be executed and val will be local to that scope.
Edit: The following answer is prior to Go 1. Starting with Go 1, it is no longer accurate / valid.
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]
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.
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