Slice declaration

Learn to declare dynamic arrays with slices.

Defining slices

A slice is a flexible-length view onto part of an array. Its type has no length: []int.

a := []int{1, 2, 3}      // with a literal
b := make([]int, 5)      // [0 0 0 0 0]
var c []int              // nil slice, len 0
fmt.Println(len(a), len(b), c == nil)  // 3 5 true

A slice holds three things: a pointer to the underlying array, a length (len) and a capacity (cap).

nil and empty slices

var s []int is a nil slice; s := []int{} is an empty but non-nil slice. Both have len 0, and range and append work fine on either. The difference shows in JSON: a nil slice encodes as null, an empty one as []. Slices can't be compared with == (only to nil); use slices.Equal to compare contents.

Exercise Independent copy

Declare a := []int{1, 2, 3}. Make a copy with b := make([]int, len(a)) and copy(b, a), set b[0] = 99 and print a and b on one line.

Show example solution

package main

import "fmt"

func main() {
    a := []int{1, 2, 3}
    b := make([]int, len(a))
    copy(b, a)
    b[0] = 99
    fmt.Println(a, b)
}

Test yourself

After <code>b := a</code> (a is a slice), if <code>b[0] = 99</code>, what is <code>a[0]</code>?
- A panic
- 99; both share the same array
- Unchanged
- 0
Show answer

99; both share the same array — Assigning a slice copies only its header; the underlying array is shared. For an independent copy use copy or slices.Clone.

Preset length

puanlar := ____([]int, 3)  // [0 0 0]

Show answer

puanlar := make([]int, 3)  // [0 0 0]

Definition and result

[]int{1, 2}

len 2
make([]int, 3)

[0 0 0]
var s []int

nil, len 0
[]int{}

Empty, not nil

Next lesson: Appending with append

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