Open Nth File

I have a program that has to output some data to
different files each having a unique filename like :

file01.dat , file02.dat, file03.dat ,.......

my question is how can you use fopen to do this job?

FILE *fopenNth(int n) /* 0 lessThanEqualTo n lessThan 100 ! */
{
char * name = "file00.dat";
name[4]+=n / 10;
name[5]+=n % 10;

return fopen (name, "w");
}