Why Does Go\'s Map Output Seem Ordered Despite Maps Being Unordered?

Assignment Order in Maps

In Go, maps are unordered collections of key-value pairs. However, the output of the following code snippet appears to suggest an order to the key-value pairs:

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 the above code snippet is executed, it outputs:

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

However, if the whitespace around the right curly brace in the assignment of the "test" key is modified, the output changes:

m["test"] = Vertex{
    12.0, 100,
}

With this modification, the output becomes:

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

Explanation

The observed behavior is due to the randomization of Go's map hash function. This randomization is implemented to prevent denial of service attacks that exploit hash collisions. As stated in the Go issue tracker:

http://code.google.com/p/go/issues/detail?id=2630

Map order is not guaranteed in Go. Although current implementations do not explicitly reorder maps, future implementations may compact maps during GC or other operations, potentially affecting the order. Therefore, it is unwise to rely on map order in your code.

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