Spring Semester 2007

Lab Notes Two: Our first meaningful CGI scripts.
What does the simplest script look like? There's more than one answer to this question, but here's a pretty simple script: 
#!/usr/bin/perl

use CGI;

$q = new CGI; 

print $q->header, $q->start_html; 

$time = localtime; 

print $time; 

print $q->end_html;
This script uses the Perl library CGI.pm It does. Let's develop it further: 
#!/usr/bin/perl

use CGI;

$q = new CGI;

print $q->header, $q->start_html;

print $q->Dump;

print $q->end_html;
Call this script by adding this to its URL: 
?one=10&two=23
 What does it print?
It prints a list of entries, as in a hashtable. That's how parameters are being passed to the script.
With this we can write a script that counts the number of times you've called it. How?
By constantly sending itself a bigger value: 
#!/usr/bin/perl

use CGI;

$q = new CGI;

print $q->header, $q->start_html;

$number = $q->param('number');

$number += 1;       

print qq{
  You have called me $number times. <p>
  
  <a href="?number=$number">Click here</a> to call me again. 
};

print $q->end_html;
 Very good. Can you add a link to reset the counter?
Sure. I'll let you make that change.
No problem. Can you write this program now?
Just by looking at the HTML interface I think I can. I thought so; you just have to pass a bigger state.
What do you mean by that? More variables: the user score, computer score, and the (already selected) computer choice.
In addition you need to allow the user to enter a choice.
In this case, the user selection is restricted to three possibilities.
That's why we can encode it as part of the link.
What happens if the user needs to enter a number? Like in this program?
Yes. Well, in that case we need to understand HTML forms.
Let's work out an example. Good idea; let's do the calculator.
Here's a program that only prints the interface: Note however that the browser is already set to let you use that interface. 
#!/usr/bin/perl

use CGI;

$q = new CGI;

print $q->header, $q->start_html;

print qq{
  <form>
    Your current balance is: ... <p>
    Please enter an amount: <input type="text" name="arg"> <p>
    Then select a type of operation: <select name="fun">
      <option value="non"> Click me!
      <option value="add"> Deposit
      <option value="sub"> Withdraw
    </select> <p>
    When ready, please press <input type="submit" value="Proceed">
  </form>
};

print $q->end_html;
Interesting: the browser is collecting the values we type, and it does it in the format we discussed earlier. Precisely. Typing a 45 and choosing Deposit turns the URL into 
?fun=45&arg=add
All of this is user input, though. Where could we store our state (the current balance for example)>
We could set up a text field, initialize it and tell the user to never touch it.
Too complicated.
You can hide a textfield that you initialize, too? How do you do that?
Just turn the textfield attribute into hidden... Well, than that settles it. Here's the code: 
#!/usr/bin/perl

use CGI;

$q = new CGI;

print $q->header, $q->start_html;

$balance = $q->param('balance'); # state
$arg     = $q->param('arg');     # input

$balance += $arg;

print qq{
  <form>
    Your current balance is: $balance <p>
    Please enter an amount: <input type="text" name="arg"> <p>
    Then select a type of operation: <select name="fun">
      <option value="non"> Click me!
      <option value="add"> Deposit 
      <option value="sub"> Withdraw
    </select> <p>
    When ready, please press <input type="submit" value="Proceed">
    
    <input type="hidden" name="balance" value="$balance">
  </form>
};

print $q->end_html;
This always adds the input to the balance, but it's a start. I am sure you can finish it.
Same here. Now we can look forward to better, bigger programs.
Updated by Adrian German for A202/A598