mr_eclair - 1 year ago 61

C Question

How can I convert an integer to its String representation in Roman numerals in C ?

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Answer Source

The easiest way is probably to set up three arrays for the complex cases and use a simple function like:

```
// convertToRoman:
// In: val: value to convert.
// res: buffer to hold result.
// Out: n/a
// Cav: caller responsible for buffer size.
void convertToRoman (unsigned int val, char *res) {
char *huns[] = {"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
char *tens[] = {"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
char *ones[] = {"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};
int size[] = { 0, 1, 2, 3, 2, 1, 2, 3, 4, 2};
// Add 'M' until we drop below 1000.
while (val >= 1000) {
*res++ = 'M';
val -= 1000;
}
// Add each of the correct elements, adjusting as we go.
strcpy (res, huns[val/100]); res += size[val/100]; val = val % 100;
strcpy (res, tens[val/10]); res += size[val/10]; val = val % 10;
strcpy (res, ones[val]); res += size[val];
// Finish string off.
*res = '\0';
}
```

This will handle any unsigned integer although large numbers will have an awful lot of `M`

characters at the front and the caller has to ensure their buffer is large enough.

Once the number has been reduced below 1000, it's a simple 3-table lookup, one each for the hundreds, tens and units. For example, take the case where `val`

is `314`

.

`val/100`

will be `3`

in that case so the `huns`

array lookup will give `CCC`

, then `val = val % 100`

gives you `14`

for the `tens`

lookup.

Then `val/10`

will be `1`

in that case so the `tens`

array lookup will give `X`

, then `val = val % 10`

gives you `4`

for the `ones`

lookup.

Then `val`

will be `4`

in that case so the `ones`

array lookup will give `IV`

.

That gives you `CCCXIV`

for `314`

.

A buffer-overflow-checking version is a simple step up from there:

```
// convertToRoman:
// In: val: value to convert.
// res: buffer to hold result.
// Out: returns 0 if not enough space, else 1.
// Cav: n/a
int convertToRoman (unsigned int val, char *res, size_t sz) {
char *huns[] = {"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
char *tens[] = {"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
char *ones[] = {"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};
int size[] = { 0, 1, 2, 3, 2, 1, 2, 3, 4, 2};
// Add 'M' until we drop below 1000.
while (val >= 1000) {
if (sz-- < 1) return 0;
*res++ = 'M';
val -= 1000;
}
// Add each of the correct elements, adjusting as we go.
if (sz < size[val/100]) return 0;
sz -= size[val/100];
strcpy (res, huns[val/100]);
res += size[val/100];
val = val % 100;
if (sz < size[val/10]) return 0;
sz -= size[val/10];
strcpy (res, tens[val/10]);
res += size[val/10];
val = val % 10;
if (sz < size[val) return 0;
sz -= size[val];
strcpy (res, ones[val]);
res += size[val];
// Finish string off.
if (sz < 1) return 0;
*res = '\0';
return 1;
}
```

although, at that point, you could think of refactoring the processing of hundreds, tens and units into a separate function since they're so similar. I'll leave that as an extra exercise.

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