Map declaration

Learn to declare a key-value map with make.

Defining maps

A map is a data structure that maps keys to values; lookup by key is very fast. Its type is written map[KeyType]ValueType:

fiyat := map[string]float64{
    "kalem":  12.5,
    "defter": 20,
}
stok := make(map[string]int)
fmt.Println(fiyat["kalem"], len(fiyat))  // 12.5 2

The key type must be comparable with == (string, numbers, structs...); slices and maps can't be keys.

A map is a reference

A map variable carries a reference to the underlying data structure. Passing a map to a function or assigning it to another variable doesn't copy it; changes are visible everywhere:

func ekle(m map[string]int) {
    m["yeni"] = 1
}

m := map[string]int{}
ekle(m)
fmt.Println(len(m))  // 1

Maps aren't safe for concurrent writes; if several goroutines will write, a lock is needed.

Exercise Price table

Declare fiyat := map[string]float64{"kalem": 12.5, "defter": 20}. Add fiyat["silgi"] = 4 and print the number of keys and the eraser's price on one line.

Show example solution

package main

import "fmt"

func main() {
    fiyat := map[string]float64{"kalem": 12.5, "defter": 20}
    fiyat["silgi"] = 4
    fmt.Println(len(fiyat), fiyat["silgi"])
}

Test yourself

What happens when <code>var m map[string]int; m["a"] = 1</code> runs?
- A panic: you can't write to a nil map
- m["a"] becomes 1
- A compile error
- Nothing happens
Show answer

A panic: you can't write to a nil map — A map declared with var is nil; it can be read but not written. Create it first with make or a literal.

Create a map

stok := ____(map[string]int)
stok["kalem"] = 3

Show answer

stok := make(map[string]int)
stok["kalem"] = 3

Code and result

map[string]int{"a": 1}

Map from a literal
make(map[string]int)

Empty map
var m map[string]int

nil map
len(m)

Number of keys

Next lesson: Adding elements

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