Skip to main content

Arrays and Slices in Go

Hey there! In this guide, we'll explore Arrays and Slices in Go. These are the primary ways to group multiple values of the same type together. Let's dive in!

1. Arrays in Go​

An array is a numbered sequence of elements of a specific length. In Go, the size of an array is part of its type, which means arrays cannot be resized once they are created.

Declaring an Array​

You declare an array by specifying its length inside square brackets [], followed by the type of elements it will hold.

var a [5]int // An array of 5 integers, initialized to 0
a[0] = 10    // Set the first element to 10
a[4] = 50    // Set the last element to 50

Initializing an Array​

You can initialize an array with values at the time of declaration using an array literal.

primes := [5]int{2, 3, 5, 7, 11}

If you use ... instead of a length, the compiler will count the elements for you:

names := [...]string{"Alice", "Bob", "Charlie"} // Length is automatically 3

2. Slices in Go​

Because arrays have a fixed size, they can be restrictive. Slices are much more common in Go. A slice is a dynamically-sized, flexible view into the elements of an array.

Declaring a Slice​

You declare a slice just like an array, but without specifying the length in the brackets.

var s []int // A slice of integers

Creating a Slice from an Array​

You can create a slice by specifying a range [low:high] on an existing array. This creates a view from low up to (but not including) high.

var primes = [5]int11 var s []int = primes[1:4] // s will contain [3, 5, 7] var s []int = primes[1:4] // s will contain [3, 5, 7]


### Creating a Slice with `make`

The built-in `make` function is the standard way to create dynamically-sized arrays (slices).

```go
// Creates a slice of ints with length 5 and capacity 5
a := make([]int, 5)

// Creates a slice with length 0 but capacity 5
b := make([]int, 0, 5)

3. Slice Length and Capacity​

A slice has both a length and a capacity.

  • Length (len): The number of elements the slice currently contains.
  • Capacity (cap): The maximum number of elements the underlying array can hold before it needs to be resized.
s := []int{2, 3, 5, 7, 11, 13}
fmt.Printf("len=%d cap=%d\n", len(s), cap(s)) // len=6 cap=6

4. Appending to a Slice​

You can easily add new elements to the end of a slice using the built-in append function. If the underlying array is full, Go will automatically allocate a larger array and copy the elements over.

var s []int
s = append(s, 1)      // [1]
s = append(s, 2, 3)   // [1, 2, 3]

5. Iterating over Slices​

The range form of the for loop iterates over a slice or map. It returns two values: the index and the copy of the element at that index.

fruits := []string{"Apple", "Banana", "Cherry"}

for index, value := range fruits {
    fmt.Printf("Index: %d, Value: %s\n", index, value)
}

Tip: If you don't need the index, use the blank identifier _ to ignore it!

for _, value := range fruits {
    fmt.Println(value)
}

6. Best Practices​

  • Prefer Slices over Arrays: In 99% of Go code, you will use slices rather than arrays because of their flexibility.
  • Preallocate if you know the size: If you know you are going to append 100 elements to a slice, create it with make([]int, 0, 100). This saves Go from having to resize the slice repeatedly behind the scenes, greatly improving performance.