How to check if a map contains a key in Go?
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) andok
will receive a bool that will be set totrue
if "foo" was actually present in the mapevaluates
ok
, which will betrue
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.
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.
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 return the zero value for the type of the entries in the map. For instance, if the map contains integers, looking up a non-existent key will return 0. A set can be implemented as a map with value type bool. Set the map entry to true to put the value in the set, and then test it by simple indexing.
attended := map[string]bool{ "Ann": true, "Joe": true, ... } if attended[person] { // will be false if person is not in the map fmt.Println(person, "was at the meeting") }
Sometimes you need to distinguish a missing entry from a zero value. Is there an entry for "UTC" or is that 0 because it's not in the map at all? You can discriminate with a form of multiple assignment.
var seconds int var ok bool seconds, ok = timeZone[tz]
For obvious reasons this is called the “comma ok” idiom. In this example, if tz is present, seconds will be set appropriately and ok will be true; if not, seconds will be set to zero and ok will be false. Here's a function that puts it together with a nice error report:
func offset(tz string) int { if seconds, ok := timeZone[tz]; ok { return seconds } log.Println("unknown time zone:", tz) return 0 }
To test for presence in the map without worrying about the actual value, you can use the blank identifier (_) in place of the usual variable for the value.
_, present := timeZone[tz]
Short Answer
_, exists := timeZone[tz] // Just checks for key existence
val, exists := timeZone[tz] // Checks for key existence and retrieves the value
Example
Here's an example at the Go Playground.
Longer Answer
Per the Maps section of Effective Go:
An attempt to fetch a map value with a key that is not present in the map will return the zero value for the type of the entries in the map. For instance, if the map contains integers, looking up a non-existent key will return 0.
Sometimes you need to distinguish a missing entry from a zero value. Is there an entry for "UTC" or is that the empty string because it's not in the map at all? You can discriminate with a form of multiple assignment.
var seconds int var ok bool seconds, ok = timeZone[tz]
For obvious reasons this is called the “comma ok” idiom. In this example, if tz is present, seconds will be set appropriately and ok will be true; if not, seconds will be set to zero and ok will be false. Here's a function that puts it together with a nice error report:
func offset(tz string) int { if seconds, ok := timeZone[tz]; ok { return seconds } log.Println("unknown time zone:", tz) return 0 }
To test for presence in the map without worrying about the actual value, you can use the blank identifier (_) in place of the usual variable for the value.
_, present := timeZone[tz]