Thread: Changing integer input into string, wrong value

I'm creating a code to transform integers into roman numerals. I thought about attempting to use getchar() to read each number but then decided to try to use scanf and use algebraic means to break the number into components for a string. Below is the code, which seems to work fine except for this error. I commented in the code several values that return wrong.

Code:

#include <stdio.h>
#include <math.h>

#define MAXNUM 4

int main() {

	double input, fraction, integer;
	int i, j;
	int b10_num[MAXNUM];
	char *N_1000[] = {"", "M", "MM"};
	char *N_100[] = {"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
	char *N_10[] = {"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
	char *N_1[] = {"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};
	
	while(1) {
		printf("\nEnter a number (between 1 and 3999): ");
		scanf("%lf", &input);
		if (input < 4000 && input > 0 && modf(input, &integer) == 0) 
			break;
		else {
			printf("Error: number is invalid");
			continue;
		}	
	}
	//transforming into string
	/* ERROR LIST
	  1551 returns 1550
	  1552 returns 1551
	  1556 returns 1555
	  1557 returns 1556
	  1560 returns 1550
	*/
	for(i = 0; i < MAXNUM; i++) {
		input = input / 10;
		printf("input: %g\t", input);
		fraction = modf(input, &integer) * 10;
		b10_num[i] =  (int)fraction;
		printf("fraction: %lf\t b10_num[%d]: %d\n", fraction, i, b10_num[i]);
	}
	input *= pow(10,MAXNUM);
	//result
	printf("Digits of the number: ");
		for(j = 3; j >= 0; j--) {
			printf("%d,", b10_num[j]);
		}
	printf("\nThe number %g as Roman numeral: %s%s%s%s\n", input, 
		N_1000[b10_num[3]], N_100[b10_num[2]], N_10[b10_num[1]], N_1[b10_num[0]]);

return 0;
}

Thank you for the help. It didn't occur to me to use the remainder operator. My first thought was modf. I ended up reducing the transformation part to:

Code:

for(i = 0; i < MAXNUM; i++) {
	b10_num[i] =  (input % (int)pow(10, i + 1)) / (int)pow(10, i);
}