Rust: What is the difference between Vec and array?

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

Vec:

• Dynamically sized — can grow or shrink at runtime.
• Heap allocated with a pointer, length, and capacity.
• Cannot be copied (only cloned or moved).
• Has methods like .push(), .pop(), .insert().
• Slightly slower due to heap indirection.

1let mut v = Vec::new();
2v.push(1);
3v.push(2);
4v.push(3);
5println!("{:?}", v);  // [1, 2, 3]
6let v2 = vec![4, 5, 6]; // Vec from macro

Array:

• Fixed size, known at compile time.
• Stack allocated — fast, no heap overhead.
• Implements Copy if elements are Copy.
• Cannot grow or shrink.
• Bounds checked at runtime.

1let a: [i32; 3] = [1, 2, 3];   // Explicit type
2let a = [0; 5];                 // [0,0,0,0,0]
3let first = a[0];               // 0

When to use:

• Use Vec when the number of elements varies (most common).
• Use arrays when the size is fixed and small (e.g., RGB color [u8; 3]).
• Use slices (&[T]) to work with either generically.

Common mistakes:

• Trying to push to an array: a.push(4) — not allowed.
• Not realizing Vec<T> and [T] are different types.
• Forgetting that vec! macro returns Vec, not an array.