Indexing files to write to?

I'm writing a program and I want to put a written file inside a for loop. For each run through the loop, I want to write to a different file. How do I do this? Here's the code:

Code:

for(f=0;f<files;f++) {

   srand(f);

   output= fopen("sequence.in","w");             //now open/create file for writing

   for(t=0;t<number;t++) {                     //bases are generated with the given frequencies
      rand=rnum();

      if(rand<freqa) {
      	base='A';
      }
      if(rand>=freqa && rand< (freqa + freqt)) {
      	base='T';
      }
      if(rand>=(freqa + freqt) && rand< (freqa + freqt + freqg)) {
      	base='G';
      }
      if(rand>= (1 - freqc) && rand<1.00) {
      	base='C';
      }

      fprintf(output, "%c",base);
   }

   fprintf(output, "%c",'Q');           //add 'Q' to end of file (quit command in program that will read the file)

   fclose(output);

   }

I tried putting a '%d' inside the fopen command where I specify the name of the file, but that obviously didn't work. Ideally, I'd like it to write files with the names sequence0.in, sequence1.in, etc. up to whatever value I feed it for the variable 'files.' Is there any way to do what I'm trying to accomplish?

Read the manual for fopen. It takes a filename, but it never mentions it takes a variable number of arguments or can format a string.
Then read the manual for sprintf and you should understand.

Thanks, that did it. I figured fscanf wouldn't do it, but I didn't know about sprintf.