Question

How to count the number of digits in an integer with Rust

Answer 1

fn count_digits_string(value: i32) -> usize {
    // Convert the number to a string
    let text = value.to_string();

    // If negative, ignore the leading '-'
    if text.starts_with('-') {
        return text.len() - 1;
    }

    text.len()
}

fn main() {
    let number: i32 = -12345;
    let digits: usize = count_digits_string(number);

    println!("Number: {}", number);
    println!("Digit count (String method): {}", digits);
}



/*
run:

Number: -12345
Digit count (String method): 5

*/

 

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Answer 2

fn count_digits_log10(value: i32) -> u32 {
    let num = value.abs();

    // Zero must be handled explicitly
    if num == 0 {
        return 1;
    }

    // Use floor(log10(n)) + 1
    (num as f64).log10().floor() as u32 + 1
}

fn main() {
    let number: i32 = 987_654_321;
    let digits: u32 = count_digits_log10(number);

    println!("Number: {}", number);
    println!("Digit count (log10 method): {}", digits);
}


/*
run:

Number: 987654321
Digit count (log10 method): 9

*/

 

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