Go by Example: Issue: Loop Struct Slice

Challenge:

What is expected output?

How to fix this code?

Reference: Go Wiki: Range Clauses

Reference: Should you use slices of pointers to structs?

Reference: Go Spec: For statements

 
package main

import "fmt"

func main() {
    type player struct {
        name  string
        score int
    }
    players := []player{
        {"Bob", 10},
        {"Alice", 207},
    }

    for _, p := range players {
        p.score += 10
    }

    for _, p := range players {
        fmt.Printf("player %s has score %d\n", p.name, p.score)
    }

(End of Challenge)

Explaination:

  • The player struct is stored in the backing array of the slice players

  • p is declaredin in range clause using a form of short variable declaration (:=). The variables have the types of their respective iteration values.

  • p is only a copy of the original player

  • when updating p.score, it only updates the “copy” of the player in the slice

 
    for _, p := range players {
        p.score += 10
    }

How to fix it?

 
    fmt.Println("---- After fix ------")

Directly access the player in the slice

 
    for i := range players {
        players[i].score += 10
    }

    for _, p := range players {
        fmt.Printf("player %s has score %d\n", p.name, p.score)
    }
}

player Bob has score 10
player Alice has score 207
---- After fix ------
player Bob has score 20
player Alice has score 217

Next example: encoding/json.

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