Summary - Map

Combine map concepts to write a dictionary.

Maps summary

Creating: map[K]V{...} or make(map[K]V) (a nil map can't be written). Reading: m[k] (zero value if missing), v, ok := m[k] (existence). Writing: m[k] = v, m[k]++. Deleting: delete(m, k), clear(m). Iterating: range (random order). Maps behave like references and aren't safe for concurrent writes.

Grouping

One of the most common uses of a map is grouping data by a property:

urunler := []struct{ Ad, Kategori string }{
    {"Kalem", "kırtasiye"}, {"Elma", "meyve"}, {"Defter", "kırtasiye"},
}
grup := map[string][]string{}
for _, u := range urunler {
    grup[u.Kategori] = append(grup[u.Kategori], u.Ad)
}
fmt.Println(grup["kırtasiye"])  // [Kalem Defter]

Exercise Group by category

Group the product list above by category with a map[string][]string. Print len(grup) and grup["kırtasiye"] on one line.

Show example solution

package main

import "fmt"

func main() {
    urunler := []struct{ Ad, Kategori string }{
        {"Kalem", "kırtasiye"}, {"Elma", "meyve"}, {"Defter", "kırtasiye"},
    }
    grup := map[string][]string{}
    for _, u := range urunler {
        grup[u.Kategori] = append(grup[u.Kategori], u.Ad)
    }
    fmt.Println(len(grup), grup["kırtasiye"])
}

Test yourself

Which type fits storing several grades per student?
- map[[]int]string
- map[string]int
- []map[int]
- map[string][]int
Show answer

map[string][]int — The value can be a slice; m[ad] = append(m[ad], not) appends even to a nil slice. A slice can't be a key.

Add to the group

grup[k] = ____(grup[k], ad)

Show answer

grup[k] = append(grup[k], ad)

Group summary

make(map[K]V)

Creating
v, ok := m[k]

Existence check
delete(m, k)

Deleting
map[K][]V

Grouping

Next lesson: String and byte

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