with_capacity

Reserve capacity up front with with_capacity for better performance.

with_capacity

A vector's length (len) is the number of elements in it; its capacity is how many elements it can hold without allocating again. When capacity runs out, the vector allocates a larger area and moves the elements. If you know how many elements are coming, allocating up front avoids these moves:

let mut v = Vec::with_capacity(10);
for i in 0..5 { v.push(i); }
println!("{} {}", v.len(), v.capacity() >= 10);   // 5 true

reserve and shrink_to_fit

Capacity can be managed on an existing vector too:

let mut v: Vec<u8> = Vec::new();
v.reserve(1000);       // room for at least 1000 more elements
v.extend([1, 2, 3]);
v.shrink_to_fit();     // brings capacity close to the length

Growth usually doubles the capacity, which makes push O(1) on average. Exact capacity values are an implementation detail; your code should rely only on the "at least" guarantee.

Exercise Vector with capacity

Create a vector with Vec::with_capacity(10) and push i * i for each i in 0..5. Print the length, the result of capacity() >= 10 and the vector on one line.

Show example solution

fn main() {
    let mut v = Vec::with_capacity(10);
    for i in 0..5 {
        v.push(i * i);
    }
    println!("{} {} {:?}", v.len(), v.capacity() >= 10, v);
}

Test yourself

What is <code>len()</code> after <code>Vec::with_capacity(100)</code>?
- 1
- Undefined
- 0
- 100
Show answer

0 — with_capacity only reserves space, it doesn't add elements; v[0] still panics.

Reserve space

let mut v = Vec::____(10);

Show answer

let mut v = Vec::with_capacity(10);

Concept and meaning

len()

Number of elements
capacity()

Reserved space
reserve(n)

At least n more
shrink_to_fit()

Releases extra space

Next lesson: Summary - Vectors

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