Perl Exercises 4
			   ----------------
These exercises use eval.

1. pcalc.pl given below is a Perl calculator.  It accepts any Perl
   expression and evaluates it.  It is very powerful.  However, it is
   very dangerous as it will accept 'system("rm -rf ~)' that will wipe
   out the user's entire home directory.  Modify pcalc.pl to make it
   safe.  It should only accept arithmetic and logical expressions.
   Hint: use regexp for validation of the input.
   	  while (<>)
	    {
	      print eval;
	      print "\n";
	    }

2. Write a simple trignometric calculator, tcalc.pl.  It should accept
   any trignometric function, such as sin, cos, tan, etc, and an
   argument, and print the result of applying the function to the
   argument.  Before execution, verify that the function is in the
   list of trig functions.  Catch errors and print suitable messages.
   Hint: hash tables may be useful.

3. The function FileCountLines(inf) accepts a filename, opens the file
   and counts the number of lines in the file.  Complete the body of
   the function.  Write Perl code that accepts a filename from the
   user, calls FileCountLines() and prints the resulting line count.
   In case the file cannot be opened, it prompts the user for another
   filename.
	   sub FileCountLines()
	     {
	       my ($fname) = @_;		# argument filename
	       open (INF, $fname) || die "Could not open $fname";
                    <<< insert code here to count the number of lines >>>

	       return $linecount;
	     }

			*	*	*	*	*