#roman #numeral #septem


A library for parsing and working with Roman numerals

5 releases (3 stable)

1.1.0 Jan 26, 2021
1.0.1 Nov 29, 2018
0.9.1 Nov 20, 2018
0.9.0 Nov 20, 2018

#17 in Value formatting

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MIT license

401 lines

Build Status Crate API


A library for parsing and working with Roman numerals.

Supports easy conversion from strings or numbers to roman numerals, and easy conversion back again.


extern crate septem;
use septem::{Roman};

let sept: Roman = "vii".parse().unwrap();
assert_eq!(7, *sept);
assert_eq!("VII", sept.to_string());
assert_eq!("vii", sept.to_lowercase());

The use septem::prelude::* is required to support the std::str::{FromStr} trait and the Roman::from_str function.

extern crate septem;
use septem::prelude::*;
use septem::{Roman};

let roman = Roman::from_str("dxxxii").unwrap();
assert_eq!(532, *roman);

To Roman Numerals

Parsing from string is provided using Rust's FromStr trait.

let num: Roman = "XLII".parse().unwrap();

Parsing from an integer is done using Roman::from.

let num: Roman = Roman::from(42).unwrap();

From Roman Numerals

A string representation of the roman numeral can be gotten using Rust's Display trait.

println!("Roman number: {}", Roman::from(42).unwrap());

There are also functions to get the string without going through the formatter; to_string, to_lowercase and to_uppercase.

let dis = Roman::from(42).unwrap().to_string();

The numerical value of the roman numeral is available through Rust's Deref trait.

let roman = Roman::from(42).unwrap();
assert_eq!(42, *roman);


If you need to work with the digits that make up the Roman numeral you can get those with the to_digits function.

let roman = Roman::from(42).unwrap();
roman.to_digits().iter().for_each(|i| {
  println!("digit: {}", i);


Benchmarks for converting from a Roman numeral in string form to an integer, and the other way around are supplied. Testing against a few other Roman numeral libraries shows that this crate is performing on the same levels, or slightly faster than the alternatives. It is after all very important to have fast roman numeral conversion, can't have such an important part of a program be slow!

The benchmarks cannot be run on stable Rust at the moment, so they should be run with the following command:

$ cargo +nightly bench


Septem functions can return three kinds of errors

  • InvalidDigit(char), when a char could not be parsed as a roman numeral
  • InvalidNumber(u64), when a number could not be parsed as a single roman numeral
  • OutOfRange(u32), when trying to convert a number less than, or equal to, 0 or larger than 3999

No runtime deps