The following is a C program to convert a decimal number to Roman numerals:
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/****************************************************** Program to convert a decimal number to roman numerals * * Enter a number: 1996 * Roman numerals: mmxii * ******************************************************/ #include <stdio.h> int main(void) { int num, rem; printf("Enter a number: "); scanf("%d", &num); printf("Roman numerals: "); while(num != 0) { if (num >= 1000) // 1000 - m { printf("m"); num -= 1000; } else if (num >= 900) // 900 - cm { printf("cm"); num -= 900; } else if (num >= 500) // 500 - d { printf("d"); num -= 500; } else if (num >= 400) // 400 - cd { printf("cd"); num -= 400; } else if (num >= 100) // 100 - c { printf("c"); num -= 100; } else if (num >= 90) // 90 - xc { printf("xc"); num -= 90; } else if (num >= 50) // 50 - l { printf("l"); num -= 50; } else if (num >= 40) // 40 - xl { printf("xl"); num -= 40; } else if (num >= 10) // 10 - x { printf("x"); num -= 10; } else if (num >= 9) // 9 - ix { printf("ix"); num -= 9; } else if (num >= 5) // 5 - v { printf("v"); num -= 5; } else if (num >= 4) // 4 - iv { printf("iv"); num -= 4; } else if (num >= 1) // 1 - i { printf("i"); num -= 1; } } return 0; } |
Expected Output:
1st run:
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1 2 |
Enter a number: 99 Roman numerals: xcix |
2nd run:
|
1 2 |
Enter a number: 2020 Roman numerals: mmxx |
How it works
The following table lists some decimal numbers and their corresponding Roman numerals:
| Decimal Number | Roman Numeral |
|---|---|
| 1 | I |
| 4 | IV |
| 5 | V |
| 9 | IX |
| 10 | X |
| 40 | XL |
| 50 | L |
| 90 | XC |
| 100 | C |
| 400 | CD |
| 500 | D |
| 900 | CM |
| 1000 | M |
To convert a decimal number num to roman numerals, we do the following:
- Find the largest decimal number
rin the above table which is less than or equal to the decimal numbernum. - Write the Roman numeral corresponding to the decimal number
r. - Subtract the
rfromnumand assign it back tonumi.enum = num - r. - Repeat steps 1, 2 and 3, until the
numis reduced to0.
Let’s take an example:
Example: Convert decimal 123 to Roman numerals
| Iteration | num | r | Roman Numeral | New Value of num |
|---|---|---|---|---|
| After 1st iteration | 123 |
100 |
c |
num=123-100=23 |
| After 2nd iteration | 23 |
10 |
x |
num=23-10=13 |
| After 3rd iteration | 13 |
10 |
x |
num=13-10=3 |
| After 4th iteration | 3 |
1 |
i |
num=3-1=2 |
| After 5th iteration | 2 |
1 |
i |
num=2-1=1 |
| After 6th iteration | 1 |
1 |
i |
num=1-1=0 |
Hence, 123 = cxiii.
