What is the difference between Vec and array?

Question

Accepted Answer

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

**Vec:**
- Dynamically sized at runtime.
- Heap allocated — pointer, length, capacity.
- Can grow or shrink.
- Cannot implement `Copy`.
- Rich API for manipulation.

```rust
let mut v = vec![1, 2, 3];
v.push(4);
v.pop();
v.reserve(10);
println!("{:?}", v);  // [1, 2]
```

**Array:**
- Fixed size, compile-time known.
- Stack allocated — fast.
- Implements `Copy` if elements do.
- Cannot grow or shrink.
- Useful for fixed-size data.

```rust
let a: [i32; 3] = [1, 2, 3];
let zeros = [0; 5];
let first = a[0];
let len = a.len();
```

**When to use:**
- Use `Vec` for most collection needs.
- Use arrays for small, fixed-size data.
- Use slices to abstract over both.

**Common mistakes:**
- Trying to push to an array.
- Confusing `vec!` with arrays.
- Not using `.as_slice()` when needed.

Rust: What is the difference between Vec and array?