CGI and Object Oriented Perl: Output
May 10, 1999
We've now completed reading the input from the form
submission. What to do with it? Anything, really. Our Perl
program could process this data in any number of ways,
from performing arithmetic calculations to using the data
as parameters in a database lookup. Of course, executing
these tasks can vary greatly in complexity. Regardless,
it is likely that we ultimately want to output some content
back to the user's web browser. After all, right now
all the poor user sees is the spinning browser logo,
anxiously awaiting some sort of result from our Perl program.
process.cgi
#!/usr/bin/perl
use CGI;
#create an instance of the CGI object
$cgiobject = new CGI;
#grab the values submitted by the user
$name=$cgiobject->param("username");
$age=$cgiobject->param("userage");
#output HTML header to web browser
print $cgiobject->header;
#evaluate data, output appropriate message
print "<HTML><BODY>";
print "<CENTER>Hello, $name</CENTER><BR>";
if ($age<18)
{print "<H3>We\'re sorry, but you are quite young ".
"to use Currency Central. Money is the root ".
"of all evil. Please ask a parent.</H3>"}
else
{print "<H3>Congratulations! You are beyond the age ".
"of corruption. Click <a href=\"next.html\">here</a>".
"to continue.</H3>"}
print "</BODY></HTML>";
Picking up this riveting narrative at the point where we
output the HTML header to the web browser -- the CGI
object possesses a method header, which when called
outputs an appropriate set of header text to the web
browser so that it properly displays the upcoming output
as HTML. We don't need to know anything about the header
itself; that's what makes the CGI object a wonderful tool.
Simply output the header before you output any HTML
and the deed is done.
In the remainder of this Perl program we essentially construct
a new web page on-the-fly. Using familiar print
functions, we output the necessary HTML to produce the desired
page. Notice that we can interweave data from the
Perl program into the output page. For instance, the greeting
"Hello, $name" will substitute the user's
name.
An if...else statement is used to evaluate the user's
age; in this case, the only action that is taken
is to output a different message depending upon the given age.
There are some additional Perl syntactical tricks strewn in
this example which may be worth some notes. Notice
the escaped single quote in "We're". This simply
ensures that Perl outputs a single quote rather than
interpreting it as part of the syntax of our statement.
Typically, the escape sequence notation is used when a
character might be ambiguous to Perl.
Also note the way the output strings have been broken up
in the two long print statements. Although we
could have simply typed one long line, it would have been
difficult to read within this article. To make the line
more legible, the output string has been broken into
portions, with a concatenation operator between each portion.
The result is exactly the same as if we typed one long line.
To summarize, we've seen the two most common methods of the
CGI object: param, which retrieves named
pieces of data submitted from the web page, and
header, which prepares the web browser for
HTML content.
CGI and Object Oriented Perl: Input
The Perl You Need to Know
Form Validation
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