Convert from Roman to Hindu-Arabic Numerals with the Rust Standard Library
November 30, 2020
As a way to better learn Rust, I've been challenging myself to write small programs using just the Rust Standard Library. This is one of those programs. It takes a movie or game title containing a Roman numeral as a command-line argument and returns the same title with Hindu-Arabic numerals instead.
Below is the completed Rust program.
use std::env;
use std::collections::HashMap;
use std::convert::TryInto;
fn main() {
// Check for Roman numerals in command-line arguments
let args: Vec<String> = env::args().collect();
let mut arg_step = 1;
let mut args_to_convert: Vec<i32> = Vec::new();
// Note: the 0 arg is the command that ran the program
for arg in &args[1..] {
let mut roman_numeral = true;
for character in arg.chars() {
match character {
'I' | 'V' | 'X' | 'L' | 'C' | 'D' | 'M' => (),
_ => roman_numeral = false,
}
};
if roman_numeral == true {
args_to_convert.push(arg_step)
};
arg_step += 1
};
// Setup a key/value map for roman numeral values
let roman_value: HashMap<char, i32> =
[('I', 1), ('V', 5), ('X', 10), ('L', 50), ('C', 100), ('D', 500), ('M', 1000)]
.iter().cloned().collect();
// Convert Roman numerals found in the previous process
let mut title = args.to_vec();
for index_number in args_to_convert {
let arg_number: usize = index_number.try_into().unwrap();
let arg_string = &title[arg_number];
let arg_char_vec: Vec<char> = arg_string.chars().collect();
let mut char_step = 0;
let mut old_converted_number = 0;
let mut new_converted_number = 0;
let mut integer_value = 0;
for roman_char in arg_char_vec {
for (key, value) in roman_value.iter() {
if roman_char == *key {
old_converted_number = new_converted_number;
new_converted_number = *value;
}
}
if char_step > 0 && new_converted_number > old_converted_number {
// Special case for the one-less-than strings like "IX" and "IV"
// Note that with the string "IV", integer_value will be 1
// going into the second loop, hence: 5 - (2 * 1) -> 5 - 1 -> 4
integer_value += new_converted_number - 2 * old_converted_number
} else {
// Most conversions steps will use this
integer_value += new_converted_number
}
char_step += 1;
}
let integer_value_string = integer_value.to_string();
if integer_value_string == "1" {
();
} else {
title[arg_number] = integer_value_string;
}
}
// Print the new title
title.remove(0);
let mut joined_title = title.join(" ");
joined_title = joined_title.to_string();
println!("{}", joined_title);
}
From a UNIX environment, run:
rustc roman-to-hindu-arabic.rs && ./roman-to-hindu-arabic Final Fantasy VIIFinal Fantasy 7