slice-basics

Go Slice

This example highlights common operations, including tips and pitfalls, on Go slice.

Tips and Gotchas

You can append to a nil or empty (or initialized) slice

Go append function works for both nil and empty slices. Here's an example for nil slice.

var nilslice []State

// Append to a nil slice.
nilslice = append(nilslice, State{name: "California", population: 39512223})
fmt.Println(nilslice)

Works with an empty slice.

var emptyslice = []State{}

// Append to an empty slice.
emptyslice = append(emptyslice, State{name: "California", population: 39512223})
fmt.Println(emptyslice)

Note

We can think of the append function to be a 2-step operation that create a new slice and copy content form one slice to the new slice.

b := append([]T{}, a)

is the same as:

b := make([]T{}, len(a))
copy(b, a)

Subslice

In this blog, Rob Pike describes it best:

"A slice is a data structure describing a contiguous section of an array stored separately from the slice variable itself. A slice is not an array. A slice describes a piece of an array."

In other words, we can create slice out of a slice. And the new subslice isn't a copy of the original slice, it's merely a slice of the original slice - all data stays.

We describe the boundaries of subslice by using the [:] operator. Let's use the following slice as a reference:

s := []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

Operator	Output	Description
s[:4]	[0 1 2 3]	slice with elements from start thru index (4-1)
s[4:]	[4 5 6 7 8 9]	slice with elements from index 4 thru end
s[2:6]	[2 3 4 5]	slice with elements from index 2 thru index (6-1)
s[0:0]	[]	slice with 0 elements, an empty slice
s[:]	[0 1 2 3 4 5 6 7 8 9]	full slice inclusive of all elements

Use [:] to remove an element from a slice. For example:

numbers := []int{1, 2, 3, 4}
numbers = numbers[1:]
fmt.Println(numbers)  // Output 2, 3, 4

Setup

1. Run the program

   $ make run

Reference

• The Go Blog: Arrays, slices (and strings): The mechanics of 'append'
• The Go Blog: Go maps in action