Lesson 16 of 20

6 mins read

Vectors & Growable Collections (Vec<T>)

In Plain English

Vec<T> (vector) is a growable, heap-allocated array that stores elements contiguously. Vectors allow dynamic resizing with .push() and .pop(), safe indexing with .get(), and iteration over elements.

The Mental Model

"If a fixed array [T; N] is a rigid metal filing cabinet with fixed slots, a Vec<T> is an expandable accordion folder that automatically stretches to hold as many documents as you insert."

Deep Dive: How It Works

Creation: Vec::new() or convenient vec![1, 2, 3] macro.

Safe Indexing: v[index] panics on out of bounds. v.get(index) returns Option<&T> safely.

Iteration: for item in &v borrows elements immutably; for item in &mut v allows modification in-place.

Syntax Blueprint

let mut v = vec![10, 20]; v.push(30); let second = v.get(1); // Some(&20)

Vectors manage their own heap buffer, growing capacity automatically as needed.

Core Rules to Remember

Growable Heap Array: Dynamically resizes as elements are added or removed.

.push() & .pop(): O(1) amortized insertions and removals at the end.

.get() Method: Returns Option<&T> to safely prevent index out-of-bounds panics.

Common Beginner Traps & How to Fix Them

Mistake:Pushing to a vector while holding a reference to an element.

Why it happens: Pushing can cause reallocation of the entire underlying buffer, invalidating existing pointers. Rust compiler prevents this!

How to fix: Finish using element references before performing mutating pushes.

Pro Tip: Use Vec::with_capacity(n) when the number of elements is known upfront to avoid intermediate reallocations.

Live Interactive Example

Hit Run Code to see it live

Vector Creation, Mutation and Iteration

Rust 1.78 / rustc

Output Console

Click "Run Code" to view the rendered output.

How it works: Demonstrates vec! macro initialization, pushing elements, and safe .get() lookup.

Your Turn: Micro Challenge

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

Micro Exercise

Push items to vector and sum

Initialize let mut scores = Vec::new();
Push 40, 50, 60 to scores.
Calculate sum with let sum: i32 = scores.iter().sum();
Print "Total Score: 150".

Sandbox Output

Click "Run & Check" to test your solution.

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