This C Program Converts the given Numbers to Roman Numerals. Roman Number System of representing numbers devised by the ancient Romans. The numbers are formed by combinations of the symbols I, V, X, L, C, D, and M, standing, respectively, for 1, 5, 10, 50, 100, 500, and 1,000 in the Hindu-Arabic numeral system. Decimal is a term that describes the base-10 number system commonly used by lay people in the developed world.
Here is source code of the C program to Convert Numbers to Roman Numerals. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

/*
* C Program to Convert Numbers to Roman Numerals
*/
#include <stdio.h>

void predigit(char num1, char num2);
void postdigit(char c, int n);

char romanval[1000];
int i = 0;
int main()
{
int j;
long number;

printf("Enter the number: ");
scanf("%d", &number);
if (number <= 0)
{
printf("Invalid number");
return 0;
}
while (number != 0)
{
if (number >= 1000)
{
postdigit('M', number / 1000);
number = number - (number / 1000) * 1000;
}
else if (number >= 500)
{
if (number < (500 + 4 * 100))
{
postdigit('D', number / 500);
number = number - (number / 500) * 500;
}
else
{
predigit('C','M');
number = number - (1000-100);
}
}
else if (number >= 100)
{
if (number < (100 + 3 * 100))
{
postdigit('C', number / 100);
number = number - (number / 100) * 100;
}
else
{
predigit('L', 'D');
number = number - (500 - 100);
}
}
else if (number >= 50 )
{
if (number < (50 + 4 * 10))
{
postdigit('L', number / 50);
number = number - (number / 50) * 50;
}
else
{
predigit('X','C');
number = number - (100-10);
}
}
else if (number >= 10)
{
if (number < (10 + 3 * 10))
{
postdigit('X', number / 10);
number = number - (number / 10) * 10;
}
else
{
predigit('X','L');
number = number - (50 - 10);
}
}
else if (number >= 5)
{
if (number < (5 + 4 * 1))
{
postdigit('V', number / 5);
number = number - (number / 5) * 5;
}
else
{
predigit('I', 'X');
number = number - (10 - 1);
}
}
else if (number >= 1)
{
if (number < 4)
{
postdigit('I', number / 1);
number = number - (number / 1) * 1;
}
else
{
predigit('I', 'V');
number = number - (5 - 1);
}
}
}
printf("Roman number is: ");
for(j = 0; j < i; j++)
printf("%c", romanval[j]);
return 0;
}

void predigit(char num1, char num2)
{
romanval[i++] = num1;
romanval[i++] = num2;
}

void postdigit(char c, int n)
{
int j;
for (j = 0; j < n; j++)
romanval[i++] = c;
}

Output:
$ cc pgm8.c
$ a.out
Enter the number: 500
Roman number is be: D