Adding elements
- Go course
- Beginner
- Lesson 52/90
- 4 min
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"] = uAlternatively the map holds pointers: map[string]*Urun; then stok["kalem"].Adet += 2 works.
Practice 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.
Starter code
package main
import (
"fmt"
"strings"
)
func main() {
// buraya yaz
}Sample 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"])
}Expected output
3 1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 For <code>m := map[string]int{}</code>, what does <code>m["yok"]</code> return?
- nil
- A panic
- -1
- 0 Correct answer
Explanation: For a missing key the int zero value is returned; to know whether the key really exists use the comma-ok form.
2 Increment the counter
for _, kelime := range kelimeler {
say[kelime]____
}for _, kelime := range kelimeler {
say[kelime]++
}Explanation: say[kelime]++ starts a missing key at 0 and increments it.
3 Operation and result
| Term | Meaning |
|---|---|
m["a"] = 1 (new) |
Adds |
m["a"] = 2 (existing) |
Updates |
m["yok"] |
Zero value |
m["a"]++ |
Increments a counter |