Rust
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.
Exercise 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.
Show example solution
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));
}
Test yourself
-
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
Show answer
Compile error: not all possibilities are covered — Rust checks at compile time that a match is exhaustive; since a &str has infinitely many values, _ is needed.
-
Default arm
match op {
"+" => Ok(a + b),
____ => Err("bilinmeyen".to_string()),
}
Show answer
match op {
"+" => Ok(a + b),
_ => Err("bilinmeyen".to_string()),
}
-
Input and result
- (12, "*", 3)
- Ok(36)
- (7, "/", 2)
- Ok(3.5)
- (2, "^", 3)
- Err(unknown operator)
- "x".parse::<f64>()
- Err
Next lesson: Error handling
