Lesson 6: Arrays & Slices¶

Big Picture¶

flowchart TD
    A["Array [5]int<br/>fixed size · value type"] 
    S["Slice []int<br/>dynamic · descriptor"]
    S --> H["Slice header"]
    H --> P["ptr → underlying array"]
    H --> L["len"]
    H --> C["cap"]
    P --> UA[("shared backing array")]
    OP["append() may reallocate<br/>when len > cap"]
    OP -.->|"new array"| UA2[("new backing array")]

Slices are lightweight views over arrays — two slices can share the same backing array. append may silently switch you to a new array when capacity is exceeded, which is a classic source of bugs.

Concepts¶

Arrays (Fixed Size)¶

var arr [5]int              // Zero initialized
arr := [3]string{"a", "b", "c"}
arr := [...]int{1, 2, 3}    // Compiler counts

Slices (Dynamic)¶

slice := []int{1, 2, 3}     // Slice literal
slice := make([]int, 5)     // Length 5
slice := make([]int, 3, 10) // Length 3, capacity 10

Slice Operations¶

slice = append(slice, 4)    // Add element
slice = append(s1, s2...)   // Append slice
copy(dest, src)             // Copy slices

Slicing¶

arr[1:3]  // Elements 1, 2, excluding index 3
arr[:3]   // First 3 elements (0,1,2), excluding the index 3
arr[2:]   // From index 2 to end
arr[:]    // All elements

Code Walkthrough¶

Runnable example

package main

import "fmt"

func main() {
    arr := [3]int{1, 2, 3}          // fixed size
    s := []int{1, 2, 3}             // dynamic view
    s = append(s, 4, 5)             // may reallocate
    fmt.Println(arr, s, len(s), cap(s))
    fmt.Println(s[1:3], s[:2])      // slicing
}

Full program: code/06_arrays_slices.go.

Try it yourself

cd code
go run . slices

Exercises

Create a slice with make([]int, 3, 10), then print len and cap before and after appends.
Slice a slice (s[1:3]), mutate an element, and observe the original — why did it change?
Write a function that removes element i from a slice using append.

Check Your Understanding¶

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What happens when append exceeds a slice's capacity?

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s := arr[1:3] — which elements does s contain?