Arrays and Slices
Welcome to the part 11 of Golang tutorial series. In this tutorial, we will learn about Arrays and Slices in Go.
What is an Array?
An array is a collection of elements. For example, the collection of integers 5, 8, 9, 79, 76 constitute an array.
Declaration
An array belongs to the type [n]T. n represents the number of elements in an array and T represents the type of each element. The number of elements n is also a part of the type(We will discuss this in more detail shortly.)
There are different ways to declare arrays. Let’s look at them one by one.
1package main
2
3import (
4 "fmt"
5)
6
7
8func main() {
9 var a [3]int //int array with length 3
10 fmt.Println(a)
11}
var a [3]int declares an integer array of length 3. All elements in an array are automatically assigned the zero value of the array type. In this case a is an integer array and hence all elements of a are assigned to 0, the zero value of int. Running the above program will print
[0 0 0]
The index of an array starts from 0 and ends at length - 1. Let’s assign some values to the above array.
1package main
2
3import (
4 "fmt"
5)
6
7
8func main() {
9 var a [3]int //int array with length 3
10 a[0] = 12 // array index starts at 0
11 a[1] = 78
12 a[2] = 50
13 fmt.Println(a)
14}
a[0] assigns value to the first element of the array. The program will print
[12 78 50]
Let’s create the same array using the short hand declaration.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 a := [3]int{12, 78, 50} // short hand declaration to create array
9 fmt.Println(a)
10}
The program above will print the same output
[12 78 50]
It is not necessary that all elements in an array have to be assigned a value during short hand declaration.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 a := [3]int{12}
9 fmt.Println(a)
10}
Line no. 8 of the above program declares an array of length 3 but adds one element 12 to it. The remaining 2 elements are assigned 0 automatically. This program will print
[12 0 0]
You can even ignore the length of the array in the declaration and replace it with ... and let the compiler find the length for you. This is done in the following program.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 a := [...]int{12, 78, 50} // ... makes the compiler determine the length
9 fmt.Println(a)
10}
The size of the array is a part of the type. Hence [5]int and [25]int are distinct types. Because of this, arrays cannot be resized. Don’t worry about this restriction since slices exist to overcome this.
1package main
2
3func main() {
4 a := [3]int{5, 78, 8}
5 var b [5]int
6 b = a //not possible since [3]int and [5]int are distinct types
7}
In line no. 6 of the program above, we are trying to assign a variable of type [3]int to a variable of type [5]int which is not allowed and hence the compiler will print the following error
./prog.go:6:7: cannot use a (type [3]int) as type [5]int in assignment
Arrays are value types
Arrays in Go are value types and not reference types. This means that when they are assigned to a new variable, a copy of the original array is assigned to the new variable. If changes are made to the new array, it will not be reflected in the original array.
1package main
2
3import "fmt"
4
5func main() {
6 a := [...]string{"USA", "China", "India", "Germany", "France"}
7 b := a // a copy of a is assigned to b
8 b[0] = "Singapore"
9 fmt.Println("a is ", a)
10 fmt.Println("b is ", b)
11}
In the above program in line no. 7, a copy of a is assigned to b. In line no. 8, the first element of b is changed to Singapore. This will not reflect in the original array a. The program will print,
a is [USA China India Germany France]
b is [Singapore China India Germany France]
Similarly when arrays are passed to functions as parameters, they are passed by value and the original array is unchanged.
1package main
2
3import "fmt"
4
5func changeLocal(num [5]int) {
6 num[0] = 55
7 fmt.Println("inside function ", num)
8
9}
10func main() {
11 num := [...]int{5, 6, 7, 8, 8}
12 fmt.Println("before passing to function ", num)
13 changeLocal(num) //num is passed by value
14 fmt.Println("after passing to function ", num)
15}
In the above program in line no. 13, the array num is actually passed by value to the function changeLocal and hence will not change because of the function call. This program will print,
before passing to function [5 6 7 8 8]
inside function [55 6 7 8 8]
after passing to function [5 6 7 8 8]
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Length of an array
The length of the array is found by passing the array as parameter to the built-in len function.
1package main
2
3import "fmt"
4
5func main() {
6 a := [...]float64{67.7, 89.8, 21, 78}
7 fmt.Println("length of a is",len(a))
8
9}
The output of the above program is
length of a is 4
Iterating arrays using for loop
The for loop can be used to iterate over the elements of an array.
1package main
2
3import "fmt"
4
5func main() {
6 a := [...]float64{67.7, 89.8, 21, 78}
7 for i := 0; i < len(a); i++ { //looping from 0 to the length of the array
8 fmt.Printf("%d th element of a is %.2f\n", i, a[i])
9 }
10}
The above program uses a for loop to iterate over the elements of the array starting from index 0 to length of the array - 1. This program works and will print,
0 th element of a is 67.70
1 th element of a is 89.80
2 th element of a is 21.00
3 th element of a is 78.00
Go provides a better and concise way to iterate over an array by using the range form of the for loop. range returns both the index and the value at that index. Let’s rewrite the above code using range. We will also find the sum of all elements of the array.
1package main
2
3import "fmt"
4
5func main() {
6 a := [...]float64{67.7, 89.8, 21, 78}
7 sum := float64(0)
8 for i, v := range a {//range returns both the index and value
9 fmt.Printf("%d the element of a is %.2f\n", i, v)
10 sum += v
11 }
12 fmt.Println("\nsum of all elements of a",sum)
13}
Line no. 8 of the above program uses the range form of the for loop. It will return both the index and the value at that index. We print the values and also calculate the sum of all elements of the array a. The output of the program is,
0 the element of a is 67.70
1 the element of a is 89.80
2 the element of a is 21.00
3 the element of a is 78.00
sum of all elements of a 256.5
In case you want only the value and want to ignore the index, you can do this by replacing the index with the _ blank identifier.
for _, v := range a { //ignores index
}
The above for loop ignores the index. Similarly, the value can also be ignored.
Multidimensional arrays
The arrays we created so far are all single dimension. It is possible to create multidimensional arrays.
1package main
2
3import (
4 "fmt"
5)
6
7func printarray(a [3][2]string) {
8 for _, v1 := range a {
9 for _, v2 := range v1 {
10 fmt.Printf("%s ", v2)
11 }
12 fmt.Printf("\n")
13 }
14}
15
16func main() {
17 a := [3][2]string{
18 {"lion", "tiger"},
19 {"cat", "dog"},
20 {"pigeon", "peacock"}, //this comma is necessary. The compiler will complain if you omit this comma
21 }
22 printarray(a)
23 var b [3][2]string
24 b[0][0] = "apple"
25 b[0][1] = "samsung"
26 b[1][0] = "microsoft"
27 b[1][1] = "google"
28 b[2][0] = "AT&T"
29 b[2][1] = "T-Mobile"
30 fmt.Printf("\n")
31 printarray(b)
32}
In the above program in line no. 17, a two dimensional string array a has been declared and defined using short hand syntax. The comma at the end of line no. 20 is necessary. This is because of the fact that the lexer automatically inserts semicolons according to simple rules. Please read https://go.dev/doc/effective_go#semicolons if you are interested to know more about why a semicolon is needed.
Another 2d array b is declared in line no. 23 and strings are added to it one by one for each index. This is another way of initializing a 2d array.
The printarray function in line no. 7 uses two for range loops to print the contents of 2d arrays. The above program will print
lion tiger
cat dog
pigeon peacock
apple samsung
microsoft google
AT&T T-Mobile
That is it for arrays. Although arrays seem to be flexible enough, they come with the restriction that they are of fixed length. It is not possible to increase the length of an array. This is where slices come into the picture. In fact in Go, slices are more commonly used than arrays.
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Slices
A slice is a convenient, flexible and powerful wrapper on top of an array. Slices do not own any data on their own. They are just references to existing arrays.
Creating a slice
A slice with elements of type T is represented by []T
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 a := [5]int{76, 77, 78, 79, 80}
9 var b []int = a[1:4] //creates a slice from a[1] to a[3]
10 fmt.Println(b)
11}
The syntax a[start:end] creates a slice from array a starting from index start to index end - 1. So in line no. 9 of the above program a[1:4] creates a slice representation of the array a starting from indexes 1 through 3. Hence the slice b has values [77 78 79].
Let’s look at another way to create a slice.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 c := []int{6, 7, 8} //creates an array and returns a slice reference
9 fmt.Println(c)
10}
In the above program, line no. 8 creates an array with 3 integers and returns a slice reference which is stored in the variable c.
modifying a slice
A slice does not own any data of its own. It is just a representation of the underlying array. Any modifications done to the slice will be reflected in the underlying array and vice versa.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 darr := [...]int{57, 89, 90, 82, 100, 78, 67, 69, 59}
9 dslice := darr[2:5]
10 fmt.Println("array before",darr)
11 for i := range dslice {
12 dslice[i]++
13 }
14 fmt.Println("array after",darr)
15}
In line number 9 of the above program, we create dslice from indexes 2, 3, 4 of the array. The for loop increments the value in these indexes by one. When we print the array after the for loop, we can see that the changes to the slice are reflected in the array. The output of the program is
array before [57 89 90 82 100 78 67 69 59]
array after [57 89 91 83 101 78 67 69 59]
When a number of slices share the same underlying array, the changes that each one makes will be reflected in the array.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 numa := [3]int{78, 79 ,80}
9 nums1 := numa[:] //creates a slice which contains all elements of the array
10 nums2 := numa[:]
11 fmt.Println("array before change 1",numa)
12 nums1[0] = 100
13 fmt.Println("array after modification to slice nums1", numa)
14 nums2[1] = 101
15 fmt.Println("array after modification to slice nums2", numa)
16}
In line no. 9, in numa[:] the start and end values are missing. The default values for start and end are 0 and len(numa) respectively. Both slices nums1 and nums2 share the same array. The output of the program is
array before change 1 [78 79 80]
array after modification to slice nums1 [100 79 80]
array after modification to slice nums2 [100 101 80]
From the output, it’s clear that when slices share the same array. The modifications made to the slice are reflected in the array.
length and capacity of a slice
The length of the slice is the number of elements in the slice. The capacity of the slice is the number of elements in the underlying array starting from the index from which the slice is created.
Let’s write some code to understand this better.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 fruitarray := [...]string{"apple", "orange", "grape", "mango", "water melon", "pine apple", "chikoo"}
9 fruitslice := fruitarray[1:3]
10 fmt.Printf("length of slice %d capacity %d", len(fruitslice), cap(fruitslice)) //length of fruitslice is 2 and capacity is 6
11}
In the above program, fruitslice is created from indexes 1 and 2 of the fruitarray. Hence the length of fruitslice is 2.
The length of the fruitarray is 7. fruiteslice is created from index 1 of fruitarray. Hence the capacity of fruitslice is the no of elements in fruitarray starting from index 1 i.e from orange and that value is 6. Hence the capacity of fruitslice is 6. The program prints length of slice 2 capacity 6.
A slice can be re-sliced upto its capacity. Anything beyond that will cause the program to throw a run time error.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 fruitarray := [...]string{"apple", "orange", "grape", "mango", "water melon", "pine apple", "chikoo"}
9 fruitslice := fruitarray[1:3]
10 fmt.Printf("length of slice %d capacity %d\n", len(fruitslice), cap(fruitslice)) //length of is 2 and capacity is 6
11 fruitslice = fruitslice[:cap(fruitslice)] //re-slicing fruitslice till its capacity
12 fmt.Println("After re-slicing length is",len(fruitslice), "and capacity is",cap(fruitslice))
13}
In line no. 11 of the above program,fruitslice is re-sliced to its capacity. The above program outputs,
length of slice 2 capacity 6
After re-slicing length is 6 and capacity is 6
creating a slice using make
func make([]T, len, cap) []T can be used to create a slice by passing the type, length and capacity. The capacity parameter is optional and defaults to the length. The make function creates an array and returns a slice reference to it.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 i := make([]int, 5, 5)
9 fmt.Println(i)
10}
The values are zeroed by default when a slice is created using make. The above program will print [0 0 0 0 0].
Appending to a slice
As we already know arrays are restricted to fixed length and their length cannot be increased. Slices are dynamic and new elements can be appended to the slice using append function. The definition of append function is func append(s []T, x ...T) []T.
x …T in the function definition means that the function accepts variable number of arguments for the parameter x. These type of functions are called variadic functions.
One question might be bothering you though. If slices are backed by arrays and arrays themselves are of fixed length then how come a slice is of dynamic length. Well what happens under the hood is, when new elements are appended to the slice, a new array is created. The elements of the existing array are copied to this new array and a new slice reference for this new array is returned. The capacity of the new slice is now twice that of the old slice. Pretty cool right :). The following program will make things clear.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 cars := []string{"Ferrari", "Honda", "Ford"}
9 fmt.Println("cars:", cars, "has old length", len(cars), "and capacity", cap(cars)) //capacity of cars is 3
10 cars = append(cars, "Toyota")
11 fmt.Println("cars:", cars, "has new length", len(cars), "and capacity", cap(cars)) //capacity of cars is doubled to 6
12}
In the above program, the capacity of cars is 3 initially. We append a new element to cars in line no. 10 and assign the slice returned by append(cars, "Toyota") to cars again. Now the capacity of cars is doubled and becomes 6. The output of the above program is
cars: [Ferrari Honda Ford] has old length 3 and capacity 3
cars: [Ferrari Honda Ford Toyota] has new length 4 and capacity 6
The zero value of a slice type is nil. A nil slice has length and capacity 0. It is possible to append values to a nil slice using the append function.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 var names []string //zero value of a slice is nil
9 if names == nil {
10 fmt.Println("slice is nil going to append")
11 names = append(names, "John", "Sebastian", "Vinay")
12 fmt.Println("names contents:",names)
13 }
14}
In the above program names is nil and we have appended 3 strings to names. The output of the program is
slice is nil going to append
names contents: [John Sebastian Vinay]
It is also possible to append one slice to another using the ... operator. You can learn more about this operator in the variadic functions tutorial.
1package main
2
3import (
4 "fmt"
5)
6
7func main() {
8 veggies := []string{"potatoes","tomatoes","brinjal"}
9 fruits := []string{"oranges","apples"}
10 food := append(veggies, fruits...)
11 fmt.Println("food:",food)
12}
In line no. 10 of the above program food is created by appending fruits to veggies. Output of the program is food: [potatoes tomatoes brinjal oranges apples]
Passing a slice to a function
Slices can be thought of as being represented internally by a structure type. This is how it looks,
type slice struct {
Length int
Capacity int
ZerothElement *byte
}
A slice contains the length, capacity and a pointer to the zeroth element of the array. When a slice is passed to a function, even though it’s passed by value, the pointer variable will refer to the same underlying array. Hence when a slice is passed to a function as parameter, changes made inside the function are visible outside the function too. Lets write a program to check this out.
1package main
2
3import (
4 "fmt"
5)
6
7func subtactOne(numbers []int) {
8 for i := range numbers {
9 numbers[i] -= 2
10 }
11
12}
13func main() {
14 nos := []int{8, 7, 6}
15 fmt.Println("slice before function call", nos)
16 subtactOne(nos) //function modifies the slice
17 fmt.Println("slice after function call", nos) //modifications are visible outside
18}
The function call in line number 17 of the above program decrements each element of the slice by 2. When the slice is printed after the function call, these changes are visible. If you can recall, this is different from an array where the changes made to an array inside a function are not visible outside the function. The above program will print
slice before function call [8 7 6]
slice after function call [6 5 4]
Multidimensional slices
Similar to arrays, slices can have multiple dimensions.
1package main
2
3import (
4 "fmt"
5)
6
7
8func main() {
9 pls := [][]string {
10 {"C", "C++"},
11 {"JavaScript"},
12 {"Go", "Rust"},
13 }
14 for _, v1 := range pls {
15 for _, v2 := range v1 {
16 fmt.Printf("%s ", v2)
17 }
18 fmt.Printf("\n")
19 }
20}
The output of the program is,
C C++
JavaScript
Go Rust
Memory Optimisation
Slices hold a reference to the underlying array. As long as the slice is in memory, the array cannot be garbage collected. This might be of concern when it comes to memory management. Let’s assume that we have a very large array and we are interested in processing only a small part of it. Henceforth we create a slice from that array and start processing the slice. The important thing to be noted here is that the array will still be in memory since the slice references it.
One way to solve this problem is to use the copy function func copy(dst, src []T) int to make a copy of that slice. This way we can use the new slice and the original array can be garbage collected.
1package main
2
3import (
4 "fmt"
5)
6
7func countries() []string {
8 countries := []string{"USA", "Singapore", "Germany", "India", "Australia"}
9 neededCountries := countries[:len(countries)-2]
10 countriesCpy := make([]string, len(neededCountries))
11 copy(countriesCpy, neededCountries) //copies neededCountries to countriesCpy
12 return countriesCpy
13}
14func main() {
15 countriesNeeded := countries()
16 fmt.Println(countriesNeeded)
17}
In line no. 9 of the above program, neededCountries := countries[:len(countries)-2] creates a slice of countries barring the last 2 elements. Line no. 11 of the above program copies neededCountries to countriesCpy and also returns it from the function in the next line. Now countries array can be garbage collected since neededCountries is no longer referenced.
I have compiled all the concepts we discussed so far into a single program. You can download it from github.
I hope you liked this tutorial. Please leave your feedback and comments. Please consider sharing this tutorial on twitter and LinkedIn. Have a good day.
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Previous tutorial - Switch Statement
