Why Does Changing the Formatting of a Go Map\'s Value Affect the Output Order of its Keys?

Why Does the Assignment Order of Map Keys Affect Output in Go?

Given the Go code:

package main

import "fmt"

type Vertex struct {
    Lat, Long float64
}

var m map[string]Vertex

func main() {
    m = make(map[string]Vertex)
    m["Bell Labs"] = Vertex{
        40.68433, 74.39967,
    }
    m["test"] = Vertex{
        12.0, 100,
    }
    fmt.Println(m["Bell Labs"])
    fmt.Println(m)
}

When we run this code, it will output:

{40.68433 74.39967}
map[Bell Labs:{40.68433 74.39967} test:{12 100}]

However, if we move the right curly brace in the test vertex declaration four spaces to the left, the output will change to:

{40.68433 74.39967}
map[test:{12 100} Bell Labs:{40.68433 74.39967}]

This seemingly minor modification affects the order of the map keys in the output because the map "order" is dependent on the hash function used. Go uses a randomized hash function to prevent denial of service attacks based on hash collisions. Consequently, the map's order is not guaranteed, and modifications to the key-value pairs can alter that order.

Go's specification defines a map as an unordered group of elements with unique keys. It provides no guarantee of a specific order. This design allows for optimizations that could potentially change the order of map keys without any modification from the user's code. As a result, it is unwise to rely on the order of map keys in your applications.

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