Arrays vs Slices: Dynamic Memory & Headers

In Plain English

In Go, fixed arrays are values, while slices are lightweight windows (slice headers containing pointer, len, cap) pointing to backing arrays. Learn append semantics, capacity doubling, and reallocations.

The Mental Model

"Think of an array as a concrete driveway of fixed length. A slice is a portable frame you place over any portion of that driveway. Moving or resizing the frame is instant because the concrete stays in place."

Deep Dive: How It Works

Array vs Slice: An array [5]int has fixed size; a slice []int is a 3-word header (data pointer, length, capacity).

Append Mechanics: When append exceeds capacity, Go allocates a larger backing array, copies data, and updates the header.

Sub-slicing: s[low:high] creates a new slice header referencing the same underlying backing array.

Syntax Blueprint

s := make([]int, 0, 4)
s = append(s, 10, 20)
sub := s[0:1]

make allocates a slice with length 0 and capacity 4; append adds elements; s[low:high] sub-slices.

Core Rules to Remember

Lightweight Header: Slices are 24-byte descriptors (pointer, length, capacity) on 64-bit platforms.

Append Reallocation: Always capture append return value: s = append(s, val) in case a new backing array was allocated.

Shared Storage: Sub-slices share backing array storage until an append triggers reallocation.

Common Beginner Traps & How to Fix Them

Mistake:Assuming modifying a sub-slice will not affect the original slice.

Why it happens: Both slice headers point to the same underlying backing array until reallocation occurs.

How to fix: Use copy(dest, src) to clone elements into an independent slice if isolation is required.

Pro Tip: Pre-allocate slice capacity with make([]T, 0, expectedCap) to eliminate dynamic allocation overhead during bulk appends.

Live Interactive Example

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Slice Header Growth & Sub-slicing

Go 1.22 / Runtime

Output Console

Click "Run Code" to view the rendered output.

How it works: When appending 30, initial cap=2 was exceeded, doubling capacity to 4.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Filter Even Numbers into a Dynamic Slice

Define a function filterEvens(nums []int) []int that takes a slice of integers.
Iterate through the slice and append only even numbers to a new result slice.
In main(), call filterEvens with [1, 2, 3, 4, 5, 6, 7, 8] and print the filtered slice.

Sandbox Output

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