Published at Aug 17 2018
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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.stdio;
/**
Convert (Arabic) numbers to Roman numerals as a String.
Params:
num = number to be converted.
Returns: the Roman numeral of the number
Bugs: Doesn't handle numbers > 3000 (contract-enforced currently)
*/
string convert(uint num)
in {
assert(num <= 3000);
assert(num > 0);
} do {
string[int] ones = [
1: "I",
4: "IV",
5: "V",
9: "IX",
];
if (num in ones)
{
return ones[num];
}
// this should probably get refactored into a nested function or mutually
// recursive functions (needs to be investigated), but not sure about that yet
if (num < 10)
{
for (int i = 9; i >= 1; i--)
{
if (num <= i)
{
continue;
}
return convert(i) ~ convert(num - i);
}
}
string[int] tens = [
10: "X",
40: "XL",
50: "L",
90: "XC",
];
if (num in tens)
{
return tens[num];
}
if (num < 100)
{
for (int i = 90; i >= 10; i-=10)
{
if (i in tens && num > i)
{
if (num <= i)
{
continue;
}
return convert(i) ~ convert(num - i);
}
}
}
string[int] hundreds = [
900: "CM",
500: "D",
400: "CD",
100: "C",
];
if (num in hundreds)
{
return hundreds[num];
}
if (num < 1000)
{
for (int i = 900; i >= 100; i-=100)
{
if (i in hundreds && num > i)
{
if (num <= i)
{
continue;
}
return convert(i) ~ convert(num - i);
}
}
}
string[int] thousands = [
1000: "M",
];
if (num in thousands)
{
return thousands[num];
}
if (num < 10000)
{
for (int i = 10000; i >= 1000; i-=1000)
{
if (i in thousands && num > i)
{
if (num <= i)
{
continue;
}
return convert(i) ~ convert(num - i);
}
}
}
assert(0);
}
///
unittest
{
assert("MCMLXXVI" == convert(1976));
}
unittest
{
immutable int allTestsEnabled = 1;
// 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));
}
}
}
```

A huge amount can be learned from reading other people’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?

## Community comments