decimal to binary, version B. Uses printf's "%x" modifier. Doesn't support floats.

dec2bin_sprintf.c

#include<stdio.h>
#include<string.h>
#define HEX_SIZE 7

/*
 * converts hex to bin
 * bin MUST be able to accomodate hex_size*4 elements
 */
int hex2bin(char* bin, char* hex, size_t hex_size ){
	int i;
	for(i=0; hex[i] != 0 && i<hex_size; i++)
		switch(hex[i]){
			case '0':
				strcpy(bin+i*4, "0000");
				break;
			case '1':
				strcpy(bin+i*4, "0001");
				break;
			case '2':
				strcpy(bin+i*4, "0010");
				break;
			case '3':
				strcpy(bin+i*4, "0011");
				break;
			case '4':
				strcpy(bin+i*4, "0100");
				break;
			case '5':
				strcpy(bin+i*4, "0101");
				break;
			case '6':
				strcpy(bin+i*4, "0110");
				break;
			case '7':
				strcpy(bin+i*4, "0111");
				break;
			case '8':
				strcpy(bin+i*4, "1000");
				break;
			case '9':
				strcpy(bin+i*4, "1001");
				break;
			case 'A':
				strcpy(bin+i*4, "1010");
				break;
			case 'B':
				strcpy(bin+i*4, "1011");
				break;
			case 'C':
				strcpy(bin+i*4, "1100");
				break;
			case 'D':
				strcpy(bin+i*4, "1101");
				break;
			case 'E':
				strcpy(bin+i*4, "1110");
				break;
			case 'F':
				strcpy(bin+i*4, "1111");
				break;
		}

	bin[i*4] = '\0';
	return 0;
}

int main(){
	int n;
	char opt;
	char buf[100];
	char hexbuf[HEX_SIZE];

	scanf("%d%c", &n, &opt);

	if (opt == 'f'){
		puts("Cannot convert float");
		return -1;
	}

	snprintf(hexbuf, 7, "%X", n);
	hex2bin(buf, hexbuf, HEX_SIZE);

	puts(buf);
}