Assignment order

Question

Let's look at the following 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)
}

It outputs this:

{40.68433 74.39967}

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

However, if I change one minor part of the test vertex declaration, by moving the right "}" 4 spaces, like so:

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

.. then the output changes to this:

{40.68433 74.39967}

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

Why does that little modification affect the order of my map?

Answer 1

Map "order" depends on the hash function used. The hash function is randomized to prevent denial of service attacks that use hash collisions. See the issue tracker for details:

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

Map order is not guaranteed according to the specification. Although not done in current go implementations, a future implementation could do some compacting during GC or other operation that changes the order of a map without the map being modified by your code. It is unwise to assume a property not defined in the specification.

A map is an unordered group of elements of one type, called the element type, indexed by a set of unique keys of another type, called the key type.