Date: Mon, 02 Jul 2001 14:02:11 -0500 (CDT)
   From: Mark Jacobson <jacobson@math-cs.cns.uni.edu>
     To: 810-088-05@uni.edu
Subject: Hidden Fields, pages 314-316 of 24 Hours book

Hi Perl/CGI students,

----------------------------------------------------------------------------
Also see Elizabeth Castro book:          <-------- Tue, 10 Jul 2001 addition  

Chapter 16: Remembering What Visitors Tell You.............205
            About Hidden Fields............................206
            Adding Hidden Fields to a Form.................207
            Storing Collected Data in a Hidden Field.......208
----------------------------------------------------------------------------
    On pages 314-316 of the Perl in 24 Hours book, Hidden Fields
are covered.                                       -------------

    "The easiest way to have Web forms "remember" information is to
     embed previous information in them by using hidden fields.
     Hidden fields are part of the HTML forms specification which
     allows fields and values to be part of an HTML form but not
     appear in the form when it's rendered visually.
     In HTML, they're written as follows:

     <INPUT TYPE="hidden" NAME="musicianfullname" value="Carlos Santana">

     With the preceeding HTML in any form, when that form
     is submitted to a PERL CGI program, the param function
     would return a key and a value for the HIDDEN field.

     param('musicianfullname') would return the value "Carlos Santana"

     See today's handout:

     The following HIDDEN FIELD is toward the very end of the FORM.

     <INPUT TYPE="hidden" NAME="identity" VALUE="$studentTriplet">
                  ------        --------         ---------------

     On today's (Monday, July 2nd) handout, there is a copy of the
     form088Q1.cgi Perl script, that displays your 1st online quiz.

     As you will recall, you need to login with your user id and
     password to see and take this quiz.  When you login successfully,
     I look up your 1st name and last name and store your user id
     in a HIDDEN FIELD named
          ------------       identity.  Its value might be:
                             --------       -----
                                                  SMITHJ2468:John:Smith
     for example, for student John Smith with
                  user id of SMITHJ2468

     Note the colons (:) are separating each of the 3 elements.

     $studentTriplet = join(":", $a[0], $a[1], $a[2]);

     <INPUT TYPE="hidden" NAME="identity" VALUE="$studentTriplet">

   Here is the Perl code that created the data for the HIDDEN FIELD
   named "identity" with value "SMITHJ2468:John:Smith".
          --------       -----  ---------------------

   if ($a[0] eq (uc param('userid')))
   {
      $pw = uc param('password');
      if ($a[3] eq (crypt $pw, "AC"))    #    $a[3] is crypted password
      {                                  #                   of SMITHJ2468
          $firstName = $a[1];            #    John
          $lastName = $a[2];             #    Smith
          $userID = $a[0];               #    SMITHJ2468
          $studentTriplet =
                            join(":", $a[0], $a[1], $a[2]);
          $success = 1;
          last;
      }

   We will talk more about this and do some Perl exercise on Tuesday
   that use hidden fields.  For my 9 quotes, the probability that
   there would be a repeated quote for any given user of my random
   quotes page is as follows (using the bc calculator on chaos):

(9*8)/9^2
.88888888888888888888    11% chance of getting a repeated quote
                             if you REFRESH or RELOAD random.cgi ONCE.

(9*8*7)/9^3              31% chance of getting a repeated quote if
.69135802469135802469        you REFRESH or RELOAD random.cgi TWICE.

(9*8*7*6)/9^4            54% chance of getting a repeated quote if
.46090534979423868312        you run random.cgi 4 times.
                             In other words, if you RELOAD only 3 times,
                             there is a better than even chance you
                             will see only 3 quotes of the 9 quotes
                             files!!!!

(9*8*7*6*5)/9^5          74% if you view random.cgi quotes only 5 times,
.25605852766346593507        there is only a 25.6% chance you will
                             see 5 of the 9 quotes!

   We are going to use a hidden field in the FORM to remember
   what quotes have been viewed so far.              --------

   We might generate the random numbers:

       2, then 7, then 8, then 7    <--- for example

   The hidden field would have the following:

       8:7:2

   We would use the Perl split function on the hidden field
   data, splitting on the : symbol that separates the number.

   Then we would look through the @a array of numbers and discover
   the number 7 had ALREADY BEEN USED (RECENTLY), so we would
   decide to:
                1.  NOT SHOW file $quotes[7] as it had already been
                                             recently viewed....
                2.  and we would generate a 2nd random number,
                    and if it wasn't in the list of numbers 8, 7 and 2,
                    we would show that file.....

                For example, the next number might be 9, so
                quotes[9] would be opened and sent to the user's
                browser window.

    See you in class.  On Tuesday and on Thursday, we are scheduled
    for hands-on lab classes in Wright 112 lab.

    Mark

P.S.  Suppose the user of random.cgi random quote displaying
      program at http://www.cns.uni.edu/~jacobson/random.cgi
      runs the random.cgi program 6 times in a row.

      What are the chances he or she will see fewer than 6 different
      quotes?
                                   (9*8*7*6*5*4) / 9^6
                                   .11380379007265152669
      In the bc calculator,
      2 ^ 10 means 2 raised to the 10th power,

      so ^ is to bc and ** is to Perl.  ^ is the exponentiation operator
                                          for bc, just as ** is that
                                          operator for Perl.