wiki/content/20200828192034-maps.md

---
date: 2020-08-28
id: 85fe90d9-f7d2-47fb-9471-f2557b1fa95d
title: Golang maps
---

# Basics

A map maps keys to values, ie: associative arrays. Zero value of a map
is `nil`. `make` function returns a new `map`.

``` go
package main

import "fmt"

type Vertex struct {
    Lat, Long float64
}

var m map[string]Vertex

func main() {
    m = make(map[string]Vertex)
    m["Bell Labs"] = Vertex{
        40.68433, -74.39967,
    }
    fmt.Println(m["Bell Labs"])
}
```

# Map literals

These are essentially the same as [struct
literals](20200828181259-structs), keys are required.

``` go
package main

import "fmt"

type Vertex struct {
    Lat, Long float64
}

var m = map[string]Vertex{
    "Bell Labs": Vertex{
        40.68433, -74.39967,
    },
    "Google": Vertex{
        37.42202, -122.08408,
    },
}

func main() {
    fmt.Println(m)
}
```

If top-level type is just a type name, it can be omitted from literal
elements. Saves some typing.

``` go
package main

import "fmt"

type Vertex struct {
    Lat, Long float64
}

var m = map[string]Vertex{
    "Bell Labs": {40.68433, -74.39967},
    "Google":    {37.42202, -122.08408},
}

func main() {
    fmt.Println(m)
}
```

# Mutating maps

Nothing shocking here either. Map elements can be
added/deleted/modified/etc.

``` go
package main

import "fmt"

func main() {
    m := make(map[string]int)

    m["Answer"] = 42
    fmt.Println("The value:", m["Answer"])

    m["Answer"] = 48
    fmt.Println("The value:", m["Answer"])

    delete(m, "Answer")
    fmt.Println("The value:", m["Answer"])

    v, ok := m["Answer"]
    fmt.Println("The value:", v, "Present?", ok)
}
```

# Types

A `type` can be declared as a thin wrapper around a `map`:

``` go
import "fmt"

type Dictionary map[string]string

func (d Dictionary) Search(word string) string {
    return d[word]
}

func main() {
    d := Dictionary{"key": "This is the value"}
    fmt.Println(d.Search("key"))
}
```