How to Modify Struct Fields Within Go Map Values?

Addressing Map Values

In Go, attempting to modify a struct field directly within a map value, as shown in the following example, will result in a compilation error:

import (
    "fmt"
)

type pair struct {
    a float64
    b float64
}

func main() {
    // Create a map where values are of the "pair" type.
    dictionary := make(map[string]pair)

    // Add an element to the map.
    dictionary["xxoo"] = pair{5.0, 2.0}

    fmt.Println(dictionary["xxoo"]) // Output: {5 2}

    // Attempt to modify a field within the map value.
    dictionary["xxoo"].b = 5.0 // Error: cannot assign to dictionary["xxoo"].b
}

This error message is encountered because map values are not addressable. Addressability is a fundamental concept in Go, and it refers to the ability to locate a variable's memory address. Non-addressable values cannot be modified indirectly, as attempting to access a struct field of a non-addressable value results in a compilation error.

To resolve this issue, there are two main approaches:

Using Pointer Values

One approach is to use a pointer value as the map value. This indirection makes the value addressable, allowing field modifications. Here's an example:

import (
    "fmt"
)

type pair struct {
    a float64
    b float64
}

func main() {
    // Create a map where values are pointers to "pair" structs.
    dictionary := make(map[string]*pair)

    // Add an element to the map.
    dictionary["xxoo"] = &pair{5.0, 2.0}

    fmt.Println(dictionary["xxoo"]) // Output: &{5 2}

    // Modify a field within the pointed-to struct.
    dictionary["xxoo"].b = 5.0

    fmt.Println(dictionary["xxoo"].b) // Output: 5
}

Value Copying or Replacement

Alternatively, you can work with non-addressable values by copying the value or replacing it entirely. Here are two examples:

// Value Copying
dictionary["xxoo"] = pair{5.0, 5.0}

// Value Replacement
p := dictionary["xxoo"]
p.b = 5.0
dictionary["xxoo"] = p

Both of these approaches allow you to modify the "pair" struct within the map.

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