Go Arrays, Slices and Maps

Arrays, slices and maps let one variable hold many values.

Arrays

An array has a fixed length that is part of its type. [3]int is an array of exactly 3 integers.

Example

package main

import "fmt"

func main() {
	var marks [3]int
	marks[0] = 78
	marks[1] = 91
	fmt.Println(marks)

	days := [3]string{"Mon", "Tue", "Wed"}
	fmt.Println(days[1], len(days))
}

Output

[78 91 0]
Tue 3

The third mark is 0, the zero value. Arrays are rarely used directly; slices are much more common.

Slices

A slice is like an array that can grow. Its type has no number: []string.

Example

package main

import "fmt"

func main() {
	cart := []string{"Rice", "Daal"}
	cart = append(cart, "Oil", "Tea")
	fmt.Println(cart)
	fmt.Println(len(cart))

	fmt.Println(cart[1:3])
	fmt.Println(cart[:2])
	fmt.Println(cart[2:])
}

Output

[Rice Daal Oil Tea]
4
[Daal Oil]
[Rice Daal]
[Oil Tea]

append returns a new slice, so always write cart = append(cart, ...).
cart[1:3] takes items from index 1 up to, but not including, index 3.

Expression	Gives
`s[a:b]`	Items from `a` to `b-1`
`s[:b]`	From the start to `b-1`
`s[a:]`	From `a` to the end
`len(s)`	Number of items
`make([]int, 5)`	A new slice of five zeros

A slice made from another slice shares the same data. Changing part[0] changes the original too. Use copy() when you need a separate copy.

Maps

A map stores key: value pairs. The type map[string]int has string keys and int values.

Example

package main

import "fmt"

func main() {
	prices := map[string]int{
		"Samosa": 40,
		"Chai":   60,
	}
	prices["Paratha"] = 80
	fmt.Println(prices["Chai"])
	fmt.Println(len(prices))

	delete(prices, "Samosa")
	fmt.Println(prices)

	price, ok := prices["Burger"]
	fmt.Println(price, ok)

	if p, found := prices["Paratha"]; found {
		fmt.Println("Paratha costs Rs", p)
	}
}

Output

60
3
map[Chai:60 Paratha:80]
0 false
Paratha costs Rs 80

Reading a missing key returns the zero value, here 0.
The comma ok form value, ok := m[key] tells you if the key really exists.
fmt.Println prints maps with keys sorted, but a range loop over a map visits keys in a random order.

Creating an Empty Map

Example

stock := make(map[string]int)
stock["Pens"] = 25

A map must be created with make or a literal before you add keys. Writing to a nil map crashes the program.

Looping over a Slice and a Map

Example

package main

import "fmt"

func main() {
	scores := []int{70, 85, 90}
	total := 0
	for _, s := range scores {
		total += s
	}
	fmt.Println("Average:", total/len(scores))

	ages := map[string]int{"Ali": 20}
	for name, age := range ages {
		fmt.Println(name, "is", age)
	}
}

Output

Average: 81
Ali is 20

Exercise

If s := []int{10, 20, 30, 40}, what is s[1:3]?

[10 20 30][20 30][20 30 40][10 20]