Looking up in a map

Learn to get a value by key and check existence.

Map lookup: comma-ok

To know whether a key really exists, use the two-value read:

fiyat := map[string]float64{"kalem": 12.5, "hediye": 0}

if f, ok := fiyat["hediye"]; ok {
    fmt.Println("var:", f)   // var: 0
}
if _, ok := fiyat["silgi"]; !ok {
    fmt.Println("silgi yok")
}

Reading only fiyat["hediye"], you couldn't tell whether 0 means "free" or "missing".

A map as a set

Go has no separate set type; map[T]bool or the more memory-friendly map[T]struct{} is used:

gorulen := map[string]bool{}
for _, ad := range []string{"ada", "can", "ada"} {
    if gorulen[ad] {
        fmt.Println("tekrar:", ad)
        continue
    }
    gorulen[ad] = true
}

With map[T]bool, a missing key returns false, so comma-ok isn't needed.

Exercise Price query

Write fiyatSor(m map[string]float64, ad string) string: if the product exists return fmt.Sprintf("%s: %.2f", ad, f), otherwise ad + " bulunamadı". Print the results for "kalem" and "silgi".

Show example solution

package main

import "fmt"

func fiyatSor(m map[string]float64, ad string) string {
    if f, ok := m[ad]; ok {
        return fmt.Sprintf("%s: %.2f", ad, f)
    }
    return ad + " bulunamadı"
}

func main() {
    m := map[string]float64{"kalem": 12.5}
    fmt.Println(fiyatSor(m, "kalem"))
    fmt.Println(fiyatSor(m, "silgi"))
}

Test yourself

In <code>v, ok := m[k]</code>, what is returned if the key is missing?
- A panic
- The last value and true
- The zero value and false
- nil and an error
Show answer

The zero value and false — ok is true only if the key exists; v is the value type's zero value when it doesn't.

Existence check

if _, ____ := stok["kalem"]; !ok {
    fmt.Println("stokta yok")
}

Show answer

if _, ok := stok["kalem"]; !ok {
    fmt.Println("stokta yok")
}

Read and result

m["var"]

The value
m["yok"]

The zero value
_, ok := m["yok"]

ok = false
map[string]bool

A set

Next lesson: Deleting from a map

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