Rust: What is the difference between Vec and array?

Vec — dynamic, heap-allocated, growable. Array — fixed-size, stack-allocated, Copy.

Vec:

• Dynamically sized at runtime.
• Heap allocated — stores pointer, length, and capacity.
• Can grow or shrink with .push(), .pop(), .resize().
• Cannot implement Copy (owns heap data).
• Slightly slower due to heap allocation and indirection.

1let mut v = Vec::new();
2v.push(10);
3v.push(20);
4v.push(30);
5println!("{:?}", v);     // [10, 20, 30]
6println!("{}", v.len()); // 3
7
8let v2 = vec![1, 2, 3];  // Macro syntax
9let v3: Vec<i32> = (0..5).collect();

Array:

• Fixed size, known at compile time.
• Stack allocated — fast, no heap overhead.
• Implements Copy if elements do.
• Cannot grow or shrink.
• Useful for fixed-size data like coordinates or buffers.

1let a: [i32; 4] = [1, 2, 3, 4];
2let zeros = [0; 10];      // [0; 10]
3println!("{}", a[2]);     // 3
4println!("{}", a.len());  // 4

When to use:

• Use Vec for most collection needs (default choice).
• Use arrays for small, fixed-size data (RGB, coordinates).
• Use slices (&[T]) to abstract over both.

Common mistakes:

• Trying to push to an array — not allowed.
• Confusing vec! return type with arrays.
• Not using .as_slice() when a function expects &[T].