Map

Maps in Go can be created via the make function or via the map syntax.

The following creates a map where the keys are strings and the values are integers:

map.go

var m1 map[string]int = make(map[string]int)
var m2 map[string]int = map[string]int{
    "one": 1,
    "two": 2,
}

Caution

Unlike JavaScript/TypeScript, iteration order of maps is not guaranteed in Go!

Accessing values

You can access values in maps using array brackets:

map_access.go

m := map[string]int{
    "one": 1,
    "two": 2,
}
fmt.Println(m["one"]) // prints 1
fmt.Println(m["two"]) // prints 2

If the key does not exist, it will return the zero value of the value type:

map_access_zero.go

m := map[string]int{
    "one": 1,
    "two": 2,
}
fmt.Println(m["three"]) // prints 0

Luckily the lookup also returns a boolean indicating if the key exists in the map:

map_access_exists.go

m := map[string]int{
    "one": 1,
    "two": 2,
}

v, exists := m["three"]
fmt.Println(v) // prints 0
fmt.Println(exists) // prints false

Adding values

You can add values to a map using the same syntax as accessing values:

map_add.go

m := make(map[string]int)
m["one"] = 1
m["two"] = 2
fmt.Println(m) // prints map[one:1 two:2]

Deleting values

You can delete values from a map using the delete function:

map_delete.go

m := map[string]int{
    "one": 1,
    "two": 2,
}
delete(m, "one")
fmt.Println(m) // prints map[two:2]

Iterating over maps

You can use the range keyword to iterate over maps.

As said, iteration order is not guaranteed!

map_iterate.go

m := map[string]int{
    "one": 1,
    "two": 2,
}
for k, v := range m {
    fmt.Printf("Key: %s, Value: %d\n", k, v)
}

This will likely print:

Key: one, Value: 1
Key: two, Value: 2

The order will vary between different runs of the program.