Go — Slices

Creating slices

package main

import "fmt"

func main() {
    // Literal
    nums := []int{1, 2, 3, 4, 5}

    // Make
    s := make([]int, 5)       // len=5, cap=5
    s2 := make([]int, 0, 10)  // len=0, cap=10

    // From array
    arr := [5]int{10, 20, 30, 40, 50}
    slice := arr[1:4] // [20, 30, 40]

    fmt.Println(nums, s, s2, slice)
}

Slice internals

A slice is a struct with three fields:

type slice struct {
    array unsafe.Pointer // pointer to underlying array
    len   int
    cap   int
}

Append

s := []int{1, 2, 3}
s = append(s, 4, 5)
fmt.Println(s) // [1 2 3 4 5]

// Append slice
s2 := []int{6, 7}
s = append(s, s2...)
fmt.Println(s) // [1 2 3 4 5 6 7]

Copy

src := []int{1, 2, 3, 4, 5}
dst := make([]int, 3)
n := copy(dst, src)
fmt.Println(dst, n) // [1 2 3] 3

Delete elements

s := []int{1, 2, 3, 4, 5}

// Delete index 2
s = append(s[:2], s[3:]...)
fmt.Println(s) // [1 2 4 5]

// Delete range
s = append(s[:1], s[3:]...)
fmt.Println(s) // [1 4 5]

Common patterns

// Filter
nums := []int{1, 2, 3, 4, 5, 6}
var evens []int
for _, n := range nums {
    if n%2 == 0 {
        evens = append(evens, n)
    }
}

// Map
doubled := make([]int, len(nums))
for i, n := range nums {
    doubled[i] = n * 2
}

// Reverse
for i, j := 0, len(nums)-1; i < j; i, j = i+1, j-1 {
    nums[i], nums[j] = nums[j], nums[i]
}

Mini Practice

Write Go code that:

1. Creates a slice with make and append
2. Uses copy to duplicate a slice
3. Deletes an element without losing other elements
4. Implements filter and map operations

Up Next

In the next lesson, you'll learn about Maps — key-value collections.