How can I sort a Map[string]int by its values?

Given this code block

map[string]int {"hello":10, "foo":20, "bar":20}

I would like to print out

foo, 20
bar, 20
hello, 10

In the order of highest to lowest

Found the answer on Golang-nuts by Andrew Gerrand

You can implement the sort interface by writing the len/less/swap functions

func rankByWordCount(wordFrequencies map[string]int) PairList{
  pl := make(PairList, len(wordFrequencies))
  i := 0
  for k, v := range wordFrequencies {
    pl[i] = Pair{k, v}
    i++
  }
  sort.Sort(sort.Reverse(pl))
  return pl
}

type Pair struct {
  Key string
  Value int
}

type PairList []Pair

func (p PairList) Len() int { return len(p) }
func (p PairList) Less(i, j int) bool { return p[i].Value < p[j].Value }
func (p PairList) Swap(i, j int){ p[i], p[j] = p[j], p[i] }

For the original post, please find it here https://groups.google.com/forum/#!topic/golang-nuts/FT7cjmcL7gw

There's a new sort.Slice function in go 1.8, so now this is simpler.

package main

import (
    "fmt"
    "sort"
)

func main() {
    m := map[string]int{
	    "something": 10,
	    "yo":        20,
	    "blah":      20,
    }

    type kv struct {
    	Key   string
    	Value int
    }

	var ss []kv
	for k, v := range m {
		ss = append(ss, kv{k, v})
	}

	sort.Slice(ss, func(i, j int) bool {
		return ss[i].Value > ss[j].Value
	})

    for _, kv := range ss {
	    fmt.Printf("%s, %d\n", kv.Key, kv.Value)
    }
}

https://play.golang.org/p/y1_WBENH4N

Sort keys first by value and then iterate map:

package main

import (
    "fmt"
    "sort"
)

func main() {
    counts := map[string]int{"hello": 10, "foo": 20, "bar": 20}

    keys := make([]string, 0, len(counts))
    for key := range counts {
        keys = append(keys, key)
    }
    sort.Slice(keys, func(i, j int) bool { return counts[keys[i]] > counts[keys[j]] })

    for _, key := range keys {
        fmt.Printf("%s, %d\n", key, counts[key])
    }
}

For example:

package main

import (
        "fmt"
        "sort"
)

func main() {
        m := map[string]int{"hello": 10, "foo": 20, "bar": 20}
        n := map[int][]string{}
        var a []int
        for k, v := range m {
                n[v] = append(n[v], k)
        }
        for k := range n {
                a = append(a, k)
        }
        sort.Sort(sort.Reverse(sort.IntSlice(a)))
        for _, k := range a {
                for _, s := range n[k] {
                        fmt.Printf("%s, %d\n", s, k)
                }
        }
}

Playground

Output:

foo, 20
bar, 20
hello, 10

I often need to sort a map[string]int of something I’m counting and have been using the following.

func rankMapStringInt(values map[string]int) []string {
	type kv struct {
		Key   string
		Value int
	}
	var ss []kv
	for k, v := range values {
		ss = append(ss, kv{k, v})
	}
	sort.Slice(ss, func(i, j int) bool {
		return ss[i].Value > ss[j].Value
	})
	ranked := make([]string, len(values))
	for i, kv := range ss {
		ranked[i] = kv.Key
	}
	return ranked
}

Use it to iterate over the keys in order of value

values := map[string]int{"foo": 10, "bar": 20, "baz": 1}

for i, index := range rankMapStringInt(values) {
    fmt.Printf("%3d: %s -> %d", i, index, values[index])
}

In my case, I was dealing with a program that I created. In this program, I created a Map just like you, with string and int. Then I discovered like you that Go doesn't really have a built-in way to sort something like this. I read the other answers and didn't really like what I read.

So I tried to think about the problem differently. Go can use sort.Ints with a slice. Also, Go can use sort.Slice with a custom comparator. So instead of creating a Map of string and int, I created a struct of string and int. Then you can sort:

package main

import (
   "fmt"
   "sort"
)

type file struct {
   name string
   size int
}

func main() {
   a := []file{
      {"april.txt", 9}, {"may.txt", 7},
   }
   sort.Slice(a, func (d, e int) bool {
      return a[d].size < a[e].size
   })
   fmt.Println(a)
}

This will not work for everyone, because maybe you will be forced to deal with a map someone else created. But it was useful for me. Good part is, unlike all the other answers, this one uses no loops.