Go: slices y arrays, characteristicas and basic uses

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In go or golang slices, arrays and maps are structures for handling data collections. In this entry I am going to talk about the first two: slices and arrays.

In this entry I am going to use data types, zero values, and other very basic aspects of go. If you don't know what I'm talking about, visit my entry Golang: introduction to the language, variables and data types.

Arrays in go

Arrays are immutable data collections, to create an array we need to define its size and the type of data it will contain, once declared it cannot be modified.

var array [4]int

In the above example, we have an array with space for 4 integers. The values that we do not assign will be assigned as zero values.

Assign values to an array

To assign values to an array, previously declared, we use its index.

array[1] = 1
// [0, 1, 0, 0]

We can also create an array directly inside a function by placing each element of the array between braces, separated by commas.

array := [4]int{0, 0, 0, 0}

Slices in go

Slices are mutable collections of data types. Internally it is an abstraction of an array, with a difference, they can be modified.

However, when declaring a slice, and then trying to modify one of its indexes, just as we would do with an array, we will have an error. Why? Because a slice is a reference, and when it is created empty, we are pointing to nothing, to nil.

var slice []int
slice[0] = 1
// panic: runtime error: index out of range [0] with length 0

Internally a slice is a struct with a pointer to the real array. In addition to the pointer, it has a property called cap and len, which refer to the capacity and length of the array, respectively.

A slice pointer points to the data it contains
A slice pointer points to the data it contains

To create a non-empty slice we have two ways to do it:

  • Allocate memory with the make function.
  • Declare it directly by passing the content after the slice data type.

Create a slice with make

The make function allocates memory and initializes an object of type slice, map or chan and returns it. If we use make, it is necessary to pass the length of the slice as a second argument.

var slice = make([]int, 4)
slice[0] = 1
// [1,0,0,0]

If we try to add an element beyond the capacity we defined we will get an error.

slice[4] = 4
// panic: runtime error: index out of range [4] with length 4

For more details check the documentation of the make function.

Creating slices with values

We can create a slice in a single step, passing the content directly to it, placing the elements of the slice between braces, separated by commas.

Notice how we do not specify the size of the slice.

var slice = []int{0, 1, 2, 3, 4}

It is also possible to let go automatically detect that it is a slice using the walrus operator ":=". Only possible within a function

slice := []int{0, 1, 2, 3, 4}

Partitioning slices

The slices can be split in a Python-like style, specifying an inclusive position for the first digit and exclusive for the second digit.

slice[2:] // {2, 3, 4}
slice[:2] // {0, 1}
slice[2:3]// { 2 }

If we do not specify one of the two, it will take the first position for the first digit and the last position for the second digit.

Extending a slice

Slices are mutable, they can be extended using the append function, which receives any number of elements, separated by commas.

sliceExtendido := append(slice, 5)
// [1 2 3 4 6 5]
otroSliceExtendido := append(sliceExtendido, 6, 7, 8)
// [1 2 3 4 6 5 6 7 8]

It is possible to create a new slice from the destructuring of a slice. The destructuring is carried out by putting three dots (...) at the end of the slice.

nuevoSlice :=[]int{9, 10}
// [8 9]
sliceHastaDiez = append(nuevoSlice, otroSliceExtendido...)
// [1 2 3 4 6 5 6 7 8 9 10]

Traverse array and slices with range

Similar to Python syntax, we can traverse an array, a slice or a map (I'll talk about maps in the next post) using range. Each iteration will return the index and element of the array or slice, or the key and value in the case of maps.

Here we iterate over an array, notice how we are declaring the size beforehand.

var array [2]string
    array[0] = "Nier"
    array[1] = "Hollow knight"
    for index, videojuego := range array {
    	fmt.Println(index, videojuego)

In the case of a slice, notice how we do not specify a size between the square brackets.

list := []int{0, 1, 2, 3, 4, 5}
    for i, n := range list {
    	fmt.Println(i, n)

How to convert an array into an slice in Go?

To convert an array into a slice in Go, we can use the following syntax [:]

newArray := mySlice[:]

And that's the basics of slices and arrays, for the next post I'm going to talk about maps in go.

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