CodeBlanks Logo Icon CodeBlanks
Login Register

format! macro

Build a new String with the format! macro.

The format! macro

format! uses the same format string as println! but returns a String instead of printing:

let urun = "Kalem";
let adet = 3;
let fiyat = 12.5;
let line = format!("{urun} x{adet} = {:.2} TL", fiyat * adet as f64);
println!("{line}");   // Kalem x3 = 37.50 TL

The format string is checked at compile time and the arguments are borrowed, so the values can still be used afterwards.

Positional and named arguments

Arguments can be used in order, by number or by name:

format!("{0} {1} {0}", "a", "b");    // "a b a"
format!("{ad} {ad}", ad = "Ada");     // "Ada Ada"
format!("{:^7}", "ok");                // "  ok   "
format!("{:08.3}", 3.14159);           // "0003.142"
format!("{:x} {:b}", 255, 5);         // "ff 101"

Practice Receipt line

Write fn line(urun: &str, adet: u32, fiyat: f64) -> String returning format!("{urun:<8}{adet:>3} x {fiyat:6.2}"). Print the result of line("Kalem", 3, 12.5) inside square brackets: println!("[{}]", ...).

Starter code

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

Sample solution

main.rs
fn line(urun: &str, adet: u32, fiyat: f64) -> String {
    format!("{urun:<8}{adet:>3} x {fiyat:6.2}")
}

fn main() {
    println!("[{}]", line("Kalem", 3, 12.5));
}

Expected output

[Kalem     3 x  12.50]

Questions and answers

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

1 What does <code>format!("{:&gt;5}", "ab")</code> produce?

  • "ab "
  • " ab "
  • "ab"
  • " ab" Correct answer

Explanation: > right-aligns and pads the 5-character field with spaces on the left; < left-aligns and ^ centers.

2 Produce text

Question 
let dosya = ____!("rapor-{}.csv", 2026);
Answer 
let dosya = format!("rapor-{}.csv", 2026);

Explanation: format! returns the formatted text as a String.

3 Format and output

Term Meaning
{:>5} "ab" " ab"
{:05} 42 "00042"
{:x} 255 "ff"
{0}{0} "a" "aa"

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