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Computing

Compute the result with trim, parse and arithmetic operations.

Computation

Because the pieces are text, they are converted to numbers with parse; the operator symbol is chosen with match:

fn hesapla(a: f64, op: &str, b: f64) -> Result<f64, String> {
    match op {
        "+" => Ok(a + b),
        "-" => Ok(a - b),
        "*" => Ok(a * b),
        "/" => Ok(a / b),
        _ => Err(format!("bilinmeyen işlem: {op}")),
    }
}

The _ arm catches all other cases; Rust requires match to cover every possibility.

Converting to numbers

"12".parse::<f64>() returns a Result<f64, ParseFloatError>. To convert the error message into our own format, map_err is used:

let a: f64 = "12".parse().map_err(|_| "geçersiz sayı".to_string())?;

The ? operator returns the error to the caller immediately if there is one; otherwise it gives the inner value. This avoids writing nested matches.

Practice The calculator function

Write the hesapla function above with the four operations and the _ arm. Print the results of hesapla(12.0, "*", 3.0) and hesapla(2.0, "^", 3.0) with {:?} on separate lines.

Starter code

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

Sample solution

main.rs
fn hesapla(a: f64, op: &str, b: f64) -> Result<f64, String> {
    match op {
        "+" => Ok(a + b),
        "-" => Ok(a - b),
        "*" => Ok(a * b),
        "/" => Ok(a / b),
        _ => Err(format!("bilinmeyen işlem: {op}")),
    }
}

fn main() {
    println!("{:?}", hesapla(12.0, "*", 3.0));
    println!("{:?}", hesapla(2.0, "^", 3.0));
}

Expected output

Ok(36.0)
Err("bilinmeyen işlem: ^")

Questions and answers

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

1 What happens if you remove the <code>_</code> arm from the <code>match</code>?

  • On an unknown operator the program silently continues
  • A run-time panic
  • Nothing happens
  • Compile error: not all possibilities are covered Correct answer

Explanation: Rust checks at compile time that a match is exhaustive; since a &str has infinitely many values, _ is needed.

2 Default arm

Question 
match op {
    "+" => Ok(a + b),
    ____ => Err("bilinmeyen".to_string()),
}
Answer 
match op {
    "+" => Ok(a + b),
    _ => Err("bilinmeyen".to_string()),
}

Explanation: The _ arm catches every value that didn't match.

3 Input and result

Term Meaning
(12, "*", 3) Ok(36)
(7, "/", 2) Ok(3.5)
(2, "^", 3) Err(unknown operator)
"x".parse::<f64>() Err

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