Adding elements

Learn to add elements to a map by key.

Adding and updating elements

m[key] = value adds the key if it doesn't exist and updates its value if it does. Reading a missing key gives the value type's zero value; this is very handy for counters:

sayac := map[string]int{}
for _, k := range []string{"go", "sql", "go"} {
    sayac[k]++
}
fmt.Println(sayac)  // map[go:2 sql:1]

sayac[k]++ starts from 0 the first time, then increases.

Updating struct values

If a map value is a struct, its field can't be written directly: m["a"].Adet = 5 is a compile error. Read the value, change it, write it back:

u := stok["kalem"]
u.Adet += 2
stok["kalem"] = u

Alternatively the map holds pointers: map[string]*Urun; then stok["kalem"].Adet += 2 works.

Exercise Word counter

Get the words with strings.Fields("go sql go web go") and count them with a map[string]int. Print the counts of go and web on one line.

Show example solution

package main

import (
    "fmt"
    "strings"
)

func main() {
    say := map[string]int{}
    for _, k := range strings.Fields("go sql go web go") {
        say[k]++
    }
    fmt.Println(say["go"], say["web"])
}

Test yourself

For <code>m := map[string]int{}</code>, what does <code>m["yok"]</code> return?
- nil
- A panic
- -1
- 0
Show answer

0 — For a missing key the int zero value is returned; to know whether the key really exists use the comma-ok form.

Increment the counter

for _, kelime := range kelimeler {
    say[kelime]____
}

Show answer

for _, kelime := range kelimeler {
    say[kelime]++
}

Operation and result

m["a"] = 1 (new)

Adds
m["a"] = 2 (existing)

Updates
m["yok"]

Zero value
m["a"]++

Increments a counter

Next lesson: Looking up in a map

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