Zoro - 6 months ago 21

C# Question

I am trying to figure out how to convert roman numerals to integers. This is a portion of my code. When I prompt the user to enter M it shows 1000, but when I prompt the user to enter a roman numeral such as VM, it does not give me 995 but instead 1005. This is because I am telling my program to do just that.

What I am trying to figure out is how I can look ahead and get it to know when it is adding or subtracting roman numerals.

How do I begin to go about doing this?

`class Roman`

{

public int inprogress = 0;

public Roman(string roman)

{

char temp = 'Z';

int length;

length = roman.Length;

for (int i = 0; i < length; i++)

{

temp = roman[i];

if (temp == 'M')

{

inprogress = inprogress + 1000;

}

if (temp == 'D')

{

inprogress = inprogress + 500;

}

if (temp == 'C')

{

inprogress = inprogress + 100;

}

if (temp == 'L')

{

inprogress = inprogress + 50;

}

if (temp == 'X')

{

inprogress = inprogress + 10;

}

if (temp == 'V')

{

inprogress = inprogress + 5;

}

if (temp == 'I')

{

inprogress = inprogress + 1;

}

}

}

}

}

Answer

the trick to converting roman numerals is to work backwards (from the end of the string) not forwards, makes it a lot easier.

eg, if you have IX

- you start with X, = 10
- move back 1.... now its I, I is less than X so now subtract off 1 = 9

A reference solution....

```
public class RomanNumeral
{
public static int ToInt(string s)
{
var last = 0;
return s.Reverse().Select(NumeralValue).Sum(v =>
{
var r = (v >= last)? v : -v;
last = v;
return r;
});
}
private static int NumeralValue(char c)
{
switch (c)
{
case 'I': return 1;
case 'V': return 5;
case 'X': return 10;
case 'L': return 50;
case 'C': return 100;
case 'D': return 500;
case 'M': return 1000;
}
return 0;
}
}
```

NOTE: this doesn't validate roman numerals, just convert ones that are already valid.