Map declaration
- Go course
- Beginner
- Lesson 51/90
- 4 min
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 2The 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)) // 1Maps aren't safe for concurrent writes; if several goroutines will write, a lock is needed.
Practice 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.
Starter code
package main
import "fmt"
func main() {
// buraya yaz
}Sample 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"])
}Expected output
3 4
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What happens when <code>var m map[string]int; m["a"] = 1</code> runs?
- A panic: you can't write to a nil map Correct answer
- m["a"] becomes 1
- A compile error
- Nothing happens
Explanation: A map declared with var is nil; it can be read but not written. Create it first with make or a literal.
2 Create a map
stok := ____(map[string]int)
stok["kalem"] = 3stok := make(map[string]int)
stok["kalem"] = 3Explanation: make creates an empty, writable map.
3 Code and result
| Term | Meaning |
|---|---|
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 |