Iterating a map with range

Learn to loop over a map with range.

range over a map

for k, v := range m walks every key-value pair in the map. The order is undefined and Go deliberately shuffles it on every run; code must not rely on the order.

notlar := map[string]int{"Ada": 90, "Can": 75}
for ad, not := range notlar {
    fmt.Println(ad, not)  // order may vary
}

If only the keys are needed, write for ad := range notlar.

Ordered iteration

When ordered output is needed, the keys are separated and sorted:

anahtarlar := make([]string, 0, len(notlar))
for ad := range notlar {
    anahtarlar = append(anahtarlar, ad)
}
slices.Sort(anahtarlar)
for _, ad := range anahtarlar {
    fmt.Println(ad, notlar[ad])
}

fmt.Println(m), on the other hand, always prints a map in key order; it is a convenience just for printing.

Exercise Sorted grade list

Declare notlar := map[string]int{"Can": 75, "Ada": 90, "Ece": 82}. Collect the keys into a slice, sort them with slices.Sort and print name grade on each line.

Show example solution

package main

import (
    "fmt"
    "slices"
)

func main() {
    notlar := map[string]int{"Can": 75, "Ada": 90, "Ece": 82}
    anahtarlar := make([]string, 0, len(notlar))
    for ad := range notlar {
        anahtarlar = append(anahtarlar, ad)
    }
    slices.Sort(anahtarlar)
    for _, ad := range anahtarlar {
        fmt.Println(ad, notlar[ad])
    }
}

Test yourself

What do you do to print a map sorted by key?
- Collect the keys into a slice, sort it, then iterate the slice
- range is already sorted
- Call sort.Map(m)
- It isn't possible
Show answer

Collect the keys into a slice, sort it, then iterate the slice — With Go 1.21+, slices.Sorted(maps.Keys(m)) does it briefly; in older versions the keys are collected by hand and sorted with sort.Strings.

Key and value

for ad, puan := ____ notlar {
    fmt.Println(ad, puan)
}

Show answer

for ad, puan := range notlar {
    fmt.Println(ad, puan)
}

Code and behavior

for k, v := range m

Random order
for k := range m

Keys only
slices.Sort(anahtarlar)

Sorted keys
fmt.Println(m)

Prints in key order

Next lesson: Summary - Map

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