Question

I am learning C and this might be a trival question to some.

My IDE complaints that there is an error: expected expression before ‘!=’ token in the while loop.

I can’t see any problem with that, any suggestions?

  /* Print a file to the console, line by line. */
  FILE *fp_infile;
  char linebuffer[512];

  if (( fp_infile=fopen("input.dat", "r")) == NULL )
    {
      fprintf(stderr, "Couldn't open input file.\n");
      return -1;
    }

  while ( fgets( linebuffer, sizeof(linebuffer), fp_infile )) != NULL ) // ERROR
    fputs( linebuffer, stdout );

    if ( ! feof(fp_infile) )   // This means "if not end of file"
      fprintf( stderr, "Error reading from input file.\n" );

      if ( fclose(fp_infile) != 0 )
        {
          fprintf(stderr, "Error closing input file.\n");
          return -2;
        }

Answer 1

You have a parenthesis mismatch error. Count the number of ( and compare to the number of ).

The line should probably be:

while (fgets(linebuffer, sizeof linebuffer, fp_infile) != NULL)

Note that you only need parens with sizeof when required by the argument’s syntax; sizeof itself is an operator, not a function.

My manual and very kindergarten-like way of matching parenthesis, is to simply scan the line from left to right, while maintaining a count:

• Start at 0
• When you find a (, increase the count
• When you find a ), decrease the count

When you reach the end of the line, if the count isn’t back to 0, you have a mismatch. Very obvious, but quite efficient to quickly determine the existence of a mismatch.