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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.

Practice 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.

Starter code

main.rs
fn main() {
    // buraya yaz
}

Sample solution

main.rs
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);
}

Expected output

5 true [0, 1, 4, 9, 16]

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What is <code>len()</code> after <code>Vec::with_capacity(100)</code>?

  • 1
  • Undefined
  • 0 Correct answer
  • 100

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

2 Reserve space

Question 
let mut v = Vec::____(10);
Answer 
let mut v = Vec::with_capacity(10);

Explanation: with_capacity reserves room for the given number of elements up front.

3 Concept and meaning

Term Meaning
len() Number of elements
capacity() Reserved space
reserve(n) At least n more
shrink_to_fit() Releases extra space

All lessons in Beginner

1 Rust basics

  1. What is Rust?
  2. Project with Cargo
  3. Your first program
  4. println! and formatting
  5. Comments
  6. Summary - Rust basics

2 Variables

  1. Variables with let
  2. Mutability with mut
  3. Constants with const
  4. Static variables
  5. Shadowing
  6. Summary - Variables

3 Data types

  1. Integers
  2. Floating point
  3. Bool and char
  4. Tuple
  5. Array
  6. Summary - Data types

4 Strings

  1. &str vs String
  2. Creating a String
  3. Concatenating strings
  4. format! macro
  5. String length
  6. Summary - Strings

5 Project: Calculator

  1. Project skeleton
  2. Reading input
  3. Computing
  4. Error handling
  5. Loop and exit

6 Functions

  1. Defining with fn
  2. Parameters
  3. Return value
  4. Expressions and statements
  5. Early return
  6. Summary - Functions

7 Control flow

  1. if-else
  2. match basics
  3. loop
  4. while loop
  5. for loop
  6. Summary - Control flow

8 Ownership

  1. What is ownership?
  2. Move semantics
  3. Copy trait
  4. Functions and ownership
  5. Return and ownership
  6. Summary - Ownership

9 Borrowing

  1. References
  2. Mutable reference
  3. Borrowing rules
  4. Dangling reference
  5. References and functions
  6. Summary - Borrowing

10 Slices

  1. String slices
  2. Slice syntax
  3. Array slices
  4. Slices and functions
  5. Range types
  6. Summary - Slices

11 Project: Guessing game

  1. Game skeleton
  2. Reading input
  3. Comparing numbers
  4. Random number
  5. Loop and exit

12 Structs

  1. Struct definition
  2. Creating an instance
  3. Tuple struct
  4. Unit-like struct
  5. Field update
  6. Summary - Structs

13 Methods

  1. impl block
  2. self parameter
  3. &mut self
  4. Associated functions
  5. Multiple impl blocks
  6. Summary - Methods

14 Enums

  1. Enum definition
  2. Variants
  3. Variants with data
  4. Option<T>
  5. Option instead of null
  6. Summary - Enums

15 Pattern matching

  1. match patterns
  2. Value binding
  3. if let
  4. while let
  5. _ wildcard
  6. Summary - Pattern matching

16 Project: To-do CLI

  1. Task model
  2. Adding
  3. Listing
  4. Marking done
  5. Reading commands

17 Vectors

  1. Vec<T>
  2. Push and pop
  3. Index and get
  4. Looping a vector
  5. with_capacity
  6. Summary - Vectors

18 Advanced strings

  1. String and &str conversion
  2. chars and bytes
  3. Slicing strings
  4. contains and replace
  5. split and join
  6. Summary - Advanced strings

19 Error handling

  1. Result<T, E>
  2. panic!
  3. ? operator
  4. match and Result
  5. unwrap and expect
  6. Summary - Error handling

20 Beginner final

  1. Vowel counter
  2. Average calculator
  3. Bank account
  4. Temperature converter