How to iterate through a map in Golang in order?

Question

Please see below my map

var romanNumeralDict map[int]string = map[int]string{   1000: "M",   900 : "CM",   500 : "D",   400 : "CD",   100 : "C",   90  : "XC",   50  : "L",   40  : "XL",   10  : "X",   9   : "IX",   5   : "V",   4   : "IV",   1   : "I", } 

I am looking to loop through this map in the order of the size of the key

  for k, v := range romanNumeralDict {     fmt.Println("k:", k, "v:", v)   } 

However, this prints out

k: 1000 v: M k: 40 v: XL k: 5 v: V k: 4 v: IV k: 900 v: CM k: 500 v: D k: 400 v: CD k: 100 v: C k: 90 v: XC k: 50 v: L k: 10 v: X k: 9 v: IX k: 1 v: I 

Is there a way that I can print them out in the order of the size of the key so, I would like to loop through this map like this

k:1 K:4 K:5 K:9 k:10 

etc...

Thank you very much for your help!

Answer 1

Collect all keys, sort them and iterate your map by key, like the following:

keys := make([]int, 0) for k, _ := range romanNumeralDict {     keys = append(keys, k) } sort.Ints(keys) for _, k := range keys {     fmt.Println(k, romanNumeralDict[k]) } 

Answer 2

You can make it a little faster by preallocating keys because you know its length:

func sortedKeys(m map[Key]Value) ([]Key) {         keys := make([]Key, len(m))         i := 0         for k := range m {             keys[i] = k             i++         }         sort.Keys(keys)         return keys } 

Replace Key and Value with your key and value types (including the sort line). cough generics cough

Edit: Go 1.18 is finally getting generics! Here's the generic version:

// Ordered is a type constraint that matches any ordered type. // An ordered type is one that supports the <, <=, >, and >= operators. // // Note the generics proposal suggests this type will be available from // a standard "constraints" package in future. type Ordered interface {     type int, int8, int16, int32, int64,         uint, uint8, uint16, uint32, uint64, uintptr,         float32, float64,         string }  func sortedKeys[K Ordered, V any](m map[K]V) ([]K) {         keys := make([]K, len(m))         i := 0         for k := range m {             keys[i] = k             i++         }         sort.Slice(keys, func(i, j int) bool { return keys[i] < keys[j] })         return keys } 

Playground example