Part 11: Arrays and Slices

Welcome to the part 11 of Golang tutorial series. In this tutorial we deal with Arrays and Slices in Go.

Arrays

An array is a collection of elements that belong to the same type. For example the collection of integers 5, 8, 9, 79, 76 form an array. Mixing values of different types, for example an array that contains both strings and integers is not allowed in Go.

Declaration

An array belongs to type n[T]. n denotes 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.

There are different ways to declare arrays. Lets look at them one by one.

package main

import (  
    "fmt"
)


func main() {  
    var a [3]int //int array with length 3
    fmt.Println(a)
}

var a [3]int declares a 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 output [0 0 0].

The index of an array starts from 0 and ends at length - 1. Lets assign some values to the above array.

package main

import (  
    "fmt"
)


func main() {  
    var a [3]int //int array with length 3
    a[0] = 12 // array index starts at 0
    a[1] = 78
    a[2] = 50
    fmt.Println(a)
}

a[0] assigns value to the first element of the array. The program will output [12 78 50]

Lets create the same array using the short hand declaration.

package main 

import (  
    "fmt"
)

func main() {  
    a := [3]int{12, 78, 50} // short hand declaration to create array
    fmt.Println(a)
}

The program above program 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.

package main

import (  
    "fmt"
)

func main() {  
    a := [3]int{12} 
    fmt.Println(a)
}

In the above program in line no. 8 a := [3]int{12} declares an array of length 3 but is provided with only one value 12. The remaining 2 elements are assigned 0 automatically. The program will output [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.

package main

import (  
    "fmt"
)

func main() {  
    a := [...]int{12, 78, 50} // ... makes the compiler determine the length
    fmt.Println(a)
}



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.

package main

func main() {  
    a := [3]int{5, 78, 8}
    var b [5]int
    b = a //not possible since [3]int and [5]int are distinct types
}

The above program in line no. 6, 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 throw error main.go:6: 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 variable, it will not be reflected in the original array.

package main

import "fmt"

func main() {  
    a := [...]string{"USA", "China", "India", "Germany", "France"}
    b := a // a copy of a is assigned to b
    b[0] = "Singapore"
    fmt.Println("a is ", a)
    fmt.Println("b is ", b) 
}

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 output,

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 in unchanged.

package main

import "fmt"

func changeLocal(num [5]int) {  
    num[0] = 55
    fmt.Println("inside function ", num)

}
func main() {  
    num := [...]int{5, 6, 7, 8, 8}
    fmt.Println("before passing to function ", num)
    changeLocal(num) //num is passed by value
    fmt.Println("after passing to function ", num)
}

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 output,

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]  
Length of an array

The length of the array is found by passing the array as parameter to the len function.

package main

import "fmt"

func main() {  
    a := [...]float64{67.7, 89.8, 21, 78}
    fmt.Println("length of a is",len(a))

}

The output of the above program is length of a is 4

Iterating arrays using range

The for loop can be used to iterate over elements of an array.

package main

import "fmt"

func main() {  
    a := [...]float64{67.7, 89.8, 21, 78}
    for i := 0; i < len(a); i++ { //looping from 0 to the length of the array
        fmt.Printf("%d th element of a is %.2f\n", i, a[i])
    }
}

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.

package main

import "fmt"

func main() {  
    a := [...]float64{67.7, 89.8, 21, 78}
    sum := float64(0)
    for i, v := range a {//range returns both the index and value
        fmt.Printf("%d the element of a is %.2f\n", i, v)
        sum += v
    }
    fmt.Println("\nsum of all elements of a",sum)
}

line no. 8 for i, v := range a of the above program is 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. Its possible to create multidimensional arrays.

package main

import (  
    "fmt"
)

func printarray(a [3][2]string) {  
    for _, v1 := range a {
        for _, v2 := range v1 {
            fmt.Printf("%s ", v2)
        }
        fmt.Printf("\n")
    }
}

func main() {  
    a := [3][2]string{
        {"lion", "tiger"},
        {"cat", "dog"},
        {"pigeon", "peacock"}, //this comma is necessary. The compiler will complain if you omit this comma
    }
    printarray(a)
    var b [3][2]string
    b[0][0] = "apple"
    b[0][1] = "samsung"
    b[1][0] = "microsoft"
    b[1][1] = "google"
    b[2][0] = "AT&T"
    b[2][1] = "T-Mobile"
    fmt.Printf("\n")
    printarray(b)
}

In the above program in line no. 17, a two dimensional string array a has been declared 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://golang.org/doc/effective_go.html#semicolons if you are interested to know more as to why this 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 initialising a 2d array.

The printarray function in line no. 7 uses two for range loops to print the contents of 2d arrays. The output of the above program is

lion tiger  
cat dog  
pigeon peacock 

apple samsung  
microsoft google  
AT&T T-Mobile  

Thats it for arrays. Although arrays seem to be flexible enough, they come with the restriction that they are of fixed length. Its not possible to increase the length of an array. This is were slices come into picture. In fact in Go, slices are more common than conventional arrays.

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 the just references to existing arrays.

Creating a slice

A slice with elements of type T is represented by []T

package main

import (  
    "fmt"
)

func main() {  
    a := [5]int{76, 77, 78, 79, 80}
    var b []int = a[1:4] //creates a slice from a[1] to a[3]
    fmt.Println(b)
}

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].

Lets look at another way to create a slice.

package main

import (  
    "fmt"
)

func main() {  
    c := []int{6, 7, 8} //creates and array and returns a slice reference
    fmt.Println(c)
}

In the above program in line no. 9, c := []int{6, 7, 8} creates an array with 3 integers and returns a slice reference which is stored in 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.

package main

import (  
    "fmt"
)

func main() {  
    darr := [...]int{57, 89, 90, 82, 100, 78, 67, 69, 59}
    dslice := darr[2:5]
    fmt.Println("array before",darr)
    for i := range dslice {
        dslice[i]++
    }
    fmt.Println("array after",darr) 
}

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.

package main

import (  
    "fmt"
)

func main() {  
    numa := [3]int{78, 79 ,80}
    nums1 := numa[:] //creates a slice which contains all elements of the array
    nums2 := numa[:]
    fmt.Println("array before change 1",numa)
    nums1[0] = 100
    fmt.Println("array after modification to slice nums1", numa)
    nums2[1] = 101
    fmt.Println("array after modification to slice nums2", numa)
}

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 its clear that when slices share the same array, the modifications which each one makes 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.

Lets write some code to understand this better.

package main

import (  
    "fmt"
)

func main() {  
    fruitarray := [...]string{"apple", "orange", "grape", "mango", "water melon", "pine apple", "chikoo"}
    fruitslice := fruitarray[1:3]
    fmt.Printf("length of slice %d capacity %d", len(fruitslice), cap(fruitslice)) //length of is 2 and capacity is 6
}

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 outputs 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.

package main

import (  
    "fmt"
)

func main() {  
    fruitarray := [...]string{"apple", "orange", "grape", "mango", "water melon", "pine apple", "chikoo"}
    fruitslice := fruitarray[1:3]
    fmt.Printf("length of slice %d capacity %d\n", len(fruitslice), cap(fruitslice)) //length of is 2 and capacity is 6
    fruitslice = fruitslice[:cap(fruitslice)] //re-slicing furitslice till its capacity
    fmt.Println("After re-slicing length is",len(fruitslice), "and capacity is",cap(fruitslice))
}

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.

package main

import (  
    "fmt"
)

func main() {  
    i := make([]int, 5, 5)
    fmt.Println(i)
}

The values are zeroed by default when a slice is created using make. The above program will output [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 hoods 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.

package main

import (  
    "fmt"
)

func main() {  
    cars := []string{"Ferrari", "Honda", "Ford"}
    fmt.Println("cars:", cars, "has old length", len(cars), "and capacity", cap(cars)) //capacity of cars is 3
    cars = append(cars, "Toyota")
    fmt.Println("cars:", cars, "has new length", len(cars), "and capacity", cap(cars)) //capacity of cars is doubled to 6
}

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.

package main

import (  
    "fmt"
)

func main() {  
    var names []string //zero value of a slice is nil
    if names == nil {
        fmt.Println("slice is nil going to append")
        names = append(names, "John", "Sebastian", "Vinay")
        fmt.Println("names contents:",names)
    }
}

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.

package main

import (  
    "fmt"
)

func main() {  
    veggies := []string{"potatoes","tomatoes","brinjal"}
    fruits := []string{"oranges","apples"}
    food := append(veggies, fruits...)
    fmt.Println("food:",food)
}

In line no. 10 of the above program food is created by appending fruits to veggies. The ... operator expands the slice. 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 its 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.

package main

import (  
    "fmt"
)

func subtactOne(numbers []int) {  
    for i := range numbers {
        numbers[i] -= 2
    }

}
func main() {

    nos := []int{8, 7, 6}
    fmt.Println("array before function call", nos)
    subtactOne(nos)                               //function modifies the slice
    fmt.Println("array after function call", nos) //modifications are visible outside

}

The function call in line number 17 of the above program decrements each element of the slice by 2. When the array 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. Output of the above program is,

array before function call [8 7 6]  
array after function call [6 5 4]  
Multidimensional slices

Similar to arrays, slices can have multiple dimensions.

package main

import (  
    "fmt"
)


func main() {  
     pls := [][]string {
            {"C", "C++"},
            {"JavaScript"},
            {"Go", "Rust"},
            }
    for _, v1 := range pls {
        for _, v2 := range v1 {
            fmt.Printf("%s ", v2)
        }
        fmt.Printf("\n")
    }
}

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. Lets 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.

package main

import (  
    "fmt"
)

func countries() []string {  
    countries := []string{"USA", "Singapore", "Germany", "India", "Australia"}
    neededCountries := countries[:len(countries)-2]
    countriesCpy := make([]string, len(neededCountries))
    copy(countriesCpy, neededCountries) //copies neededCountries to countriesCpy
    return countriesCpy
}
func main() {  
    countriesNeeded := countries()
    fmt.Println(countriesNeeded)
}

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.

Thats it for arrays and slices. Thanks for reading. Please leave your valuable feedback and comments.