The slice Data Type | Golang basics

Mahdi.Kh

September 21, 2026

In this part, we get familiar with the slice data type, and look at how to use it along with the operations related to it.

The slice data type is an abstraction built on top of the array data type, meaning slice is implemented using an array. So before getting familiar with slice, we need a basic familiarity with arrays.

To define an array, we just need to specify its length and the type of its items:

var x [5]int

x[0] = 10
x[1] = 7
x[2] = 35

fmt.Println(x) // [10 7 35 0]

The values inside an array are placed sequentially in RAM.

article figures — How an array's items are laid out in memory

For a few reasons, arrays aren't commonly used:

Once we define an array, its length is immutable.
An array's length is part of the array's data type; for example, an array of type [4]int can't be assigned to a variable whose type is [5]int. (The types [4]int and [5]int are distinct and incompatible!)

Unlike arrays, defining a slice doesn't require specifying its length! To declare a slice, we use a pair of square brackets [ ] together with the data type we want:

package main

import "fmt"

func main() {	
	// numeric slice
	var y []int
	
	// a numeric array with length 3
	var x [3]int

	fmt.Println(x) // [0 0 0]
	fmt.Println(y) // []
}

When declaring a slice, we do the same thing as with an array, except we leave the square brackets [ ] empty.

In Go, every data type has a default value called the zero value, which is assigned when a variable is declared without an initial value. This feature means variables in Go always have a valid value, which prevents errors related to initialization. A slice's zero value is nil. You can allocate memory to a slice using the make function.

Note: unlike slices, arrays don't need explicit initialization; an array's zero value is a ready-to-use array whose items themselves hold the zero value!

package main

import "fmt"

func main() {
	var y []int

	y = make([]int, 3)

	fmt.Println(y) // [0 0 0]
	
	y[0] = 2
	y[1] = 4
	y[2] = 6
	
	fmt.Println(y) // [2 4 6]
}

Another way to create a slice is using := and the make function:

package main

import "fmt"

func main() {
	y := make([]int, 3)

	fmt.Println(y) // [0 0 0]

	y[0] = 2
	y[1] = 4
	y[2] = 6

	fmt.Println(y) // [2 4 6]
}

If you know, before creating the slice, the values that will go inside it, you can use a slice literal:

#golang #go

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