strcat
char * strcat ( char * destination, const char * source );
Concatenate strings
Appends a copy of the source string to the destination string. The terminating null character in destination is overwritten by the first character of source, and a null-character is included at the end of the new string formed by the concatenation of both in destination.
Puedes poner esto antes del
for:
num[0] = 0;