Can not assign to pair in a map

Question

I have the following pair defined in my go program

type pair struct {
  a float64
  b float64
}

Then I create a map:

dictio         


        

Answer 1

In order to assign a value to a struct field, that struct must be "addressable". Addressability is covered in the "Address Operators" section of the language specification.

Map values aren't addressable to leave map implementations the freedom to move values around in memory as needed. Because map values aren't addressable, you you can't use a selector (.) operator on the value to assign to the struct field. If you use a pointer type as the map value, i.e. map[string]*pair, the pointer indirection satisfies the addressability requirement.

dict := make(map[string]*pair)
dict["xxoo"] = &pair{5.0, 2.0}
dict["xxoo"].b = 5.0

If you are working with values, you need to copy the existing value via assignment, or supply an entirely new value:

dict := make(map[string]pair)
dict["xxoo"] = pair{5.0, 2.0}

// copy the value, change one field, then reassign
p := dict["xxoo"]
p.b = 5.0
dict["xxoo"] = p

// or replace the value in full
dict["xxoo"] = pair{5.0, 5.0}