External Files

General information

You must be able to:
• Specify a file to work with - this is very much language dependent. C has the programmer define a file pointer and use it, FORTRAN has the programmer specify a file stream number and uses it.
• Open files - usually this is a simple call such as

  file_designation = open("myfilename.dat","read")

• Close files - this also a simple call such as close(file_designation)
• Read from an open file - every language will provide mechanisms to read from files. These mechanisms are often an adaptation of the input mechanisms used with keyboard input.
• Write to an open file - writing to files is usually similar to writing to the screen.
• Append or read/write to a file is language dependent - see your specific language to see what other read/write features such as append are provided.
• Determine that the end of the file has been reached, which indicated there is no more information in the file. - This is also language dependent but every language has some method of doing this.

C Specifics

• Specify a file to work with: The declaration simply sets up a file pointer. At this point in the program, the pointers do NOT point to any file. C requires the open function to set the pointers to a file.

      FILE *ptr, *input_file;
  

• Open a file - a return value of NULL from the fopen() indicates that the file could not be opened for some reason.

      input_file = fopen("inputfile.dat", "r");
      ptr = fopen("output_name.dat", "w");
  

• Close a file

      fclose(input_file);
      fclose(ptr);
  

• Read from a file

      fscanf(input_file, "%d", &value);
  

• Write to a file

      fprintf(ptr, "%d", value); 
  

• Other read/write options for opening a file
• Determine the end of file

       feof(ptr) - function that returns true when the end of the file is reached
       value == EOF - EOF is the end of file marker.  If I'm reading values into
                      value, I can check to see if I have read the end of file
                      marker
  

• Reading and writing structures - you can only use fread() on files that have been created using fwrite(). See Foster, 10.5 for discussion and examples.

      fwrite(&value, size_of(type of value), how_many, file_ptr);
      fread(&value, size_of(type of value), how_many, file_prt);