Detecting if struct exists for key in map

Question

According to the Golang documentation on maps,

If the requested key doesn't exist, we get the value type's zero value. In this case the value type is int, so the zero value is 0:
j := m["root"] // j == 0

So I'm trying to determine if a struct exists with a given string, how would I determine this? Would I just check for an empty struct with zerod values? What would the comparison here look like?

type Hello struct{}
structMap := map[string]Hello{}
j := structMap["example"]
if(j==?) {
 ...
}

Community · Accepted Answer · 2020-06-20 09:12:55Z

11

Use the special "comma, ok" form which tells if the key was found in the map. Go Spec: Index Expressions:

An index expression on a map a of type map[K]V used in an assignment or initialization of the special form
v, ok = a[x]
v, ok := a[x]
var v, ok = a[x]
yields an additional untyped boolean value. The value of ok is true if the key x is present in the map, and false otherwise.

So in your code:

type Hello struct{}
structMap := map[string]Hello{}
if j, ok := structMap["example"]; !ok {
    // "example" is not in the map
}

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered May 13, 2015 at 11:45

icza

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