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o2sh

Posted on Nov 6, 2018

add one

#rust #algortihm

Here is a little program I wrote in Rust that allows you to add one to any number

o2sh / add-one

adds one to a number

add-one

Returns n + 1.

Example

use add_one::add_one;

fn main() {
    let mut bytes = Vec::new();

    match add_one("123".as_bytes(), &mut bytes) {
        Ok(()) => println!("{}", std::str::from_utf8(&bytes).unwrap()),
        Err(e) => {
            eprintln!("Error: {}", e);
        }
    }
}

cargo install add-one
add-one 12
13

View on GitHub

The idea is pretty simple. Indeed, since incrementing a number leaves most of it unchanged, it's much faster to work on the decimal digits itself.

As a consequence, after breaking the input into a list of digits, we just have to increment or decrement (in case of negative input) the last number.

Not so fast...

What if the number has trailing nines for positive inputs or trailing zeros for negative numbers ?

Well, if the number has trailing nines (e.g.: 7561325999) or trailing zeros (e.g.: -1645000), we just need to increment the last "non-nine" digit (or decrement the last "non-zero" digit) and switch the nines into zeros and zeros into nines (for negative inputs).

And, in the case of floating point inputs (e.g.: 15691.12313) we simply ignore the decimal part and apply the previous rule to the integer part.