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Slices, maps and strings · 3 MIN READ

Map iteration order is not specified

Produce stable output from a map without hoping.

The language does not specify map iteration order, and the order may change from one iteration to the next. A small example that repeatedly uses one order does not establish a guarantee.

When output must be deterministic, collect the keys, sort them and read values in that sorted order. When order is irrelevant, test the entries without requiring a particular traversal sequence.

SEE THE IDEA

Sort keys when output order matters

  1. 01

    Map

    go
    map[string]int

    Stores associations, not an order

  2. 02

    Collect

    go
    keys = append(keys, key)

    Extract the keys

  3. 03

    Sort then read

    go
    slices.Sort(keys)

    Produce deterministic output

The language does not specify map iteration order.

In practice

Use sorted keys for reports, snapshots, stable tests and serialized text. Test membership independently of order when order is irrelevant. An iteration may use a different order on the next pass; seeing one repeated order in a small example does not establish a guarantee.

Watch out

Do not describe map traversal as insertion order or rely on a particular random distribution.

Official reference ↗

Work through the exercise

go
package main

// SortedKeys returns the map's keys in ascending order.
func SortedKeys(m map[string]int) []string {
	return nil
}

func main() {}
Try it yourself

Trace each value and write down your reasoning. Copy the source into GopherForge or your local Go environment to compile and check it. This web page does not execute your code or send it to a compiler service.

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