Go

Slices

Introduction#

A slice is a data structure that encapsulates an array so that the programmer can add as many elements as needed without having to worry about memory management. Slices can be cut into sub-slices very efficiently, since the resulting slices all point to the same internal array. Go programmers often take advantage of this to avoid copying arrays, which would typically be done in many other programming languages.

Syntax#

  • slice := make([]type, len, cap) // create a new slice
  • slice = append(slice, item) // append a item to a slice
  • slice = append(slice, items…) // append slice of items to a slice
  • len := len(slice) // get the length of a slice
  • cap := cap(slice) // get the capacity of a slice
  • elNum := copy(dst, slice) // copy a the contents of a slice to an other slice

Appending to slice

slice = append(slice, "hello", "world")

Adding Two slices together

slice1 := []string{"!"}
slice2 := []string{"Hello", "world"}
slice  := append(slice1, slice2...)

Run in the Go Playground

Removing elements / “Slicing” slices

If you need to remove one or more elements from a slice, or if you need to work with a sub slice of another existing one; you can use the following method.

Following examples uses slice of int, but that works with all type of slice.

So for that, we need a slice, from witch we will remove some elements:

slice := []int{1, 2, 3, 4, 5, 6} 
// > [1 2 3 4 5 6]

We need also the indexes of elements to remove:

// index of first element to remove (corresponding to the '3' in the slice)
var first = 2

// index of last element to remove (corresponding to the '5' in the slice)
var last = 4

And so we can “slice” the slice, removing undesired elements:

// keeping elements from start to 'first element to remove' (not keeping first to remove), 
// removing elements from 'first element to remove' to 'last element to remove'
// and keeping all others elements to the end of the slice
newSlice1 := append(slice[:first], slice[last+1:]...)
// > [1 2 6]

// you can do using directly numbers instead of variables
newSlice2 := append(slice[:2], slice[5:]...)
// > [1 2 6]

// Another way to do the same
newSlice3 := slice[:first + copy(slice[first:], slice[last+1:])]
// > [1 2 6]

// same that newSlice3 with hard coded indexes (without use of variables)
newSlice4 := slice[:2 + copy(slice[2:], slice[5:])]
// > [1 2 6]

To remove only one element, just have to put the index of this element as the first AND as the last index to remove, just like that:

var indexToRemove = 3
newSlice5 := append(slice[:indexToRemove], slice[indexToRemove+1:]...)
// > [1 2 3 5 6]

// hard-coded version:
newSlice5 := append(slice[:3], slice[4:]...)
// > [1 2 3 5 6]

And you can also remove elements from the beginning of the slice:

newSlice6 := append(slice[:0], slice[last+1:]...)
// > [6]

// That can be simplified into
newSlice6 := slice[last+1:]
// > [6]

You can also removing some elements from the end of the slice:

newSlice7 := append(slice[:first], slice[first+1:len(slice)-1]...)
// > [1 2]

// That can be simplified into
newSlice7 := slice[:first]
// > [1 2]

If the new slice have to contains exactly the same elements than the first one, you can use the same thing but with last := first-1.

(This can be useful in case of your indexes are previously computed)

Length and Capacity

Slices have both length and capacity. The length of a slice is the number of elements currently in the slice, while the capacity is the number of elements the slice can hold before needing to be reallocated.

When creating a slice using the built-in make() function, you can specify its length, and optionally its capacity. If the capacity is not explicitly specified, it will be the specified length.

var s = make([]int, 3, 5) // length 3, capacity 5

You can check the length of a slice with the built-in len() function:

var n = len(s) // n == 3

You can check the capacity with the built-in cap() function:

var c = cap(s) // c == 5

Elements created by make() are set to the zero value for the element type of the slice:

for idx, val := range s {
    fmt.Println(idx, val)
}
// output:
// 0 0
// 1 0
// 2 0

Run it on play.golang.org

You cannot access elements beyond the length of a slice, even if the index is within capacity:

var x = s[3] // panic: runtime error: index out of range

However, as long as the capacity exceeds the length, you can append new elements without reallocating:

var t = []int{3, 4}
s = append(s, t) // s is now []int{0, 0, 0, 3, 4}
n = len(s) // n == 5
c = cap(s) // c == 5

If you append to a slice which lacks the capacity to accept the new elements, the underlying array will be reallocated for you with sufficient capacity:

var u = []int{5, 6}
s = append(s, u) // s is now []int{0, 0, 0, 3, 4, 5, 6}
n = len(s) // n == 7
c = cap(s) // c > 5

It is, therefore, generally good practice to allocate sufficient capacity when first creating a slice, if you know how much space you’ll need, to avoid unnecessary reallocations.

Copying contents from one slice to another slice

If you wish to copy the contents of a slice into an initially empty slice, following steps can be taken to accomplish it-

  1. Create the source slice:

    var sourceSlice []interface{} = []interface{}{“Hello”,5.10,“World”,true}

  2. Create the destination slice, with:

  • Length = Length of sourceSlice

    var destinationSlice []interface{} = make([]interface{},len(sourceSlice))

  1. Now that the destination slice’s underlying array is big enough to accomodate all the elements of the source slice, we can proceed to copy the elements using the builtin copy:

    copy(destinationSlice,sourceSlice)

Creating Slices

Slices are the typical way go programmers store lists of data.

To declare a slice variable use the []Type syntax.

var a []int

To declare and initialize a slice variable in one line use the []Type{values} syntax.

var a []int = []int{3, 1, 4, 1, 5, 9}

Another way to initialize a slice is with the make function. It three arguments: the Type of the slice (or map), the length, and the capacity.

a := make([]int, 0, 5)

You can add elements to your new slice using append.

a = append(a, 5)

Check the number of elements in your slice using len.

length := len(a)

Check the capacity of your slice using cap. The capacity is the number of elements currently allocated to be in memory for the slice. You can always append to a slice at capacity as Go will automatically create a bigger slice for you.

capacity := cap(a)

You can access elements in a slice using typical indexing syntax.

a[0]  // Gets the first member of `a`

You can also use a for loop over slices with range. The first variable is the index in the specified array, and the second variable is the value for the index.

for index, value := range a {
    fmt.Println("Index: " + index + " Value: " + value)  // Prints "Index: 0 Value: 5" (and continues until end of slice)
}

Go Playground

Filtering a slice

To filter a slice without allocating a new underlying array:

// Our base slice
slice := []int{ 1, 2, 3, 4 }
// Create a zero-length slice with the same underlying array
tmp := slice[:0]

for _, v := range slice {
  if v % 2 == 0 {
    // Append desired values to slice
    tmp = append(tmp, v)
  }
}

// (Optional) Reassign the slice
slice = tmp // [2, 4]

Zero value of slice

The zero value of slice is nil, which has the length and capacity 0. A nil slice has no underlying array. But there are also non-nil slices of length and capacity 0, like []int{} or make([]int, 5)[5:].

Any type that have nil values can be converted to nil slice:

s = []int(nil)

To test whether a slice is empty, use:

if len(s) == 0　{
    fmt.Ptintf("s is empty.")
}
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