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Published at Aug 26 2019
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Instructions

Test suite

Solution

Write a function to convert from normal numbers to Roman Numerals.

The Romans were a clever bunch. They conquered most of Europe and ruled it for hundreds of years. They invented concrete and straight roads and even bikinis. One thing they never discovered though was the number zero. This made writing and dating extensive histories of their exploits slightly more challenging, but the system of numbers they came up with is still in use today. For example the BBC uses Roman numerals to date their programmes.

The Romans wrote numbers using letters - I, V, X, L, C, D, M. (notice these letters have lots of straight lines and are hence easy to hack into stone tablets).

```
1 => I
10 => X
7 => VII
```

There is no need to be able to convert numbers larger than about 3000. (The Romans themselves didn't tend to go any higher)

Wikipedia says: Modern Roman numerals ... are written by expressing each digit separately starting with the left most digit and skipping any digit with a value of zero.

To see this in practice, consider the example of 1990.

In Roman numerals 1990 is MCMXC:

1000=M 900=CM 90=XC

2008 is written as MMVIII:

2000=MM 8=VIII

See also: http://www.novaroma.org/via_romana/numbers.html

Make sure you have read D page on exercism.io. This covers the basic information on setting up the development environment expected by the exercises.

Get the first test compiling, linking and passing by following the three rules of test-driven development. Create just enough structure by declaring namespaces, functions, classes, etc., to satisfy any compiler errors and get the test to fail. Then write just enough code to get the test to pass. Once you've done that, uncomment the next test by moving the following line past the next test.

```
static if (all_tests_enabled)
```

This may result in compile errors as new constructs may be invoked that you haven't yet declared or defined. Again, fix the compile errors minimally to get a failing test, then change the code minimally to pass the test, refactor your implementation for readability and expressiveness and then go on to the next test.

Try to use standard D facilities in preference to writing your own low-level algorithms or facilities by hand. DRefLanguage and DReference are references to the D language and D standard library.

The Roman Numeral Kata http://codingdojo.org/cgi-bin/index.pl?KataRomanNumerals

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

```
module roman_numerals;
import std.stdio;
unittest
{
immutable int allTestsEnabled = 0;
// one_yields_I
{
writefln("Conversion of 1: %s", convert(1));
assert("I" == convert(1));
}
static if (allTestsEnabled)
{
// two_yields_II
{
writefln("Conversion of 2: %s", convert(2));
assert("II" == convert(2));
}
// three_yields_III
{
writefln("Conversion of 3: %s", convert(3));
assert("III" == convert(3));
}
// four_yields_IV
{
writefln("Conversion of 4: %s", convert(4));
assert("IV" == convert(4));
}
// five_yields_V
{
writefln("Conversion of 5: %s", convert(5));
assert("V" == convert(5));
}
// six_yields_VI
{
writefln("Conversion of 6: %s", convert(6));
assert("VI" == convert(6));
}
// nine_yields_IX
{
writefln("Conversion of 9: %s", convert(9));
assert("IX" == convert(9));
}
// twenty_seven_yields_XXVII
{
writefln("Conversion of 27: %s", convert(27));
assert("XXVII" == convert(27));
}
// forty_eight_yields_XLVIII
{
writefln("Conversion of 48: %s", convert(48));
assert("XLVIII" == convert(48));
}
// fifty_nine_yields_LIX
{
writefln("Conversion of 59: %s", convert(59));
assert("LIX" == convert(59));
}
// ninety_three_yields_XCIII
{
writefln("Conversion of 93: %s", convert(93));
assert("XCIII" == convert(93));
}
// one_hundred_forty_one_yields_CXLI
{
writefln("Conversion of 141: %s", convert(141));
assert("CXLI" == convert(141));
}
// one_hundred_sixty_three_yields_CLXIII
{
writefln("Conversion of 163: %s", convert(163));
assert("CLXIII" == convert(163));
}
// four_hundred_two_yields_CDII
{
writefln("Conversion of 402: %s", convert(402));
assert("CDII" == convert(402));
}
// five_hundred_seventy_five_yields_DLXXV
{
writefln("Conversion of 575: %s", convert(575));
assert("DLXXV" == convert(575));
}
// nine_hundred_eleven_yields_CMXI
{
writefln("Conversion of 911: %s", convert(911));
assert("CMXI" == convert(911));
}
// one_thousand_twenty_four_yields_MXXIV
{
writefln("Conversion of 1024: %s", convert(1024));
assert("MXXIV" == convert(1024));
}
// three_thousand_yields_MMM)
{
writefln("Conversion of 3000: %s", convert(3000));
assert("MMM" == convert(3000));
}
}
}
```

```
module roman_numerals;
import std.typecons : Tuple;
alias DigitTuple = Tuple!(char, "character", int, "value");
enum Digit : DigitTuple
{
I = DigitTuple('I', 1),
V = DigitTuple('V', 5),
X = DigitTuple('X', 10),
L = DigitTuple('L', 50),
C = DigitTuple('C', 100),
D = DigitTuple('D', 500),
M = DigitTuple('M', 1000),
}
string convert(int n)
{
char[] result;
int remainder = convertAux([Digit.M, Digit.D, Digit.C], result, n);
remainder = convertAux([Digit.C, Digit.L, Digit.X], result, remainder);
convertAux([Digit.X, Digit.V, Digit.I], result, remainder);
return cast(string) result;
}
private int convertAux(Digit[3] digits, ref char[] result, int remainder)
{
while (remainder >= digits[0].value)
{
result ~= digits[0].character;
remainder -= digits[0].value;
}
if (remainder >= digits[0].value - digits[2].value)
{
result ~= digits[2].character;
result ~= digits[0].character;
remainder -= digits[0].value - digits[2].value;
}
if (remainder >= digits[1].value)
{
result ~= digits[1].character;
remainder -= digits[1].value;
}
if (remainder >= digits[1].value - digits[2].value)
{
result ~= digits[2].character;
result ~= digits[1].character;
remainder -= digits[1].value - digits[2].value;
}
while (remainder >= digits[2].value)
{
result ~= digits[2].character;
remainder -= digits[2].value;
}
return remainder;
}
unittest
{
// one_yields_I
assert("I" == convert(1));
// two_yields_II
assert("II" == convert(2));
// three_yields_III
assert("III" == convert(3));
// four_yields_IV
assert("IV" == convert(4));
// five_yields_V
assert("V" == convert(5));
// six_yields_VI
assert("VI" == convert(6));
// nine_yields_IX
assert("IX" == convert(9));
// twenty_seven_yields_XXVII
assert("XXVII" == convert(27));
// forty_eight_yields_XLVIII
assert("XLVIII" == convert(48));
// fifty_nine_yields_LIX
assert("LIX" == convert(59));
// ninety_three_yields_XCIII
assert("XCIII" == convert(93));
// one_hundred_forty_one_yields_CXLI
assert("CXLI" == convert(141));
// one_hundred_sixty_three_yields_CLXIII
assert("CLXIII" == convert(163));
// four_hundred_two_yields_CDII
assert("CDII" == convert(402));
// five_hundred_seventy_five_yields_DLXXV
assert("DLXXV" == convert(575));
// nine_hundred_eleven_yields_CMXI
assert("CMXI" == convert(911));
// one_thousand_twenty_four_yields_MXXIV
assert("MXXIV" == convert(1024));
// three_thousand_yields_MMM)
assert("MMM" == convert(3000));
}
```

A huge amount can be learned from reading other peopleâ€™s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

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