How to Modify Non-Addressable Map Values in Go?

Modifying Map Values in Go: Addressing the "Can Not Assign to Pair" Error

In Go, when encountering the error message "can not assign to it" while trying to modify a pair within a map, it's important to understand the concept of addressability.

Map values in Go are not addressable, meaning they cannot be assigned values directly using the dot (.) operator. This is a deliberate design choice to give map implementations the flexibility to move values around in memory as needed.

To modify a map value that is not addressable, such as a pair, you have two options:

1. Use a Pointer as the Map Value: By defining the map value as a pointer to the pair, you can modify the value by dereferencing the pointer using the indirection operator (*).

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

2. Copy or Reassign the Value: Instead of directly modifying the map value, you can create a copy, modify it, and then reassign it to the map. You can also replace the value entirely with a new one.

// Copy and modify
dict := make(map[string]pair)
dict["xxoo"] = pair{5.0, 2.0}
p := dict["xxoo"]
p.b = 5.0
dict["xxoo"] = p

// Replace the value
dict["xxoo"] = pair{5.0, 5.0}

By understanding addressability and using these techniques, you can effectively modify map values that are not directly addressable.

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