*********  primes.cgi PERL code  **********   

#!/usr/bin/perl -w

use CGI ':standard';

my $limit;
my @primes;

print header;
print start_html('Prime numbers via Perl/CGI'),
      h1('Prime numbers calculated by PERL/CGI script');

print start_form,
      textfield('upperLimit', '300', '5', '5'),
      "This program prints out all the prime numbers <= n ", 
      br,
      "Enter an integer n as an upper limit: ",
      p,
      submit('Show Primes'),
      end_form,
      hr,
      p;

$limit = param('upperLimit');

@primes = (0, 0, 2 .. $limit);

for (my $index = 2; $index <= sqrt($limit); $index++)
{
   if ( $primes[$index] )
   {
      my $cross_out = $index * 2;
      while ( $cross_out <= $limit )
      {
         $primes[$cross_out] = 0;
         $cross_out += $index;
      }
   }
}

my $per_line = ($limit > 1000)? 15 : 18;

my   $adjust = ($limit > 10000)? " " : "";

my $n = 0;

if (param())
{
   print "\n<BR><PRE>\n";
   foreach my $prime_number ( @primes )
   {
      if ( $prime_number )
      {
         $n++;
         my $spaces = "";

         if ($prime_number < 10)
         {
            $spaces .= "   " . $adjust;
         }
         elsif ($prime_number < 100)
         {
            $spaces .= "  " . $adjust;
         }
         elsif ($prime_number < 1000)
         {
            $spaces .= " " . $adjust;
         }
         elsif ($prime_number < 10000)
         {
            $spaces .=  $adjust;
         }

         print "$spaces" . "$prime_number ";
         if ($n % $per_line == 0)
         {
            print "\n";
         }
      }  
   }

   print "\n\nThere are $n primes between 2 and $limit\n</PRE>";
}

print hr,
      p,
      "See the script of the ",
      a( {-href=>'showPrimesCode.cgi'},
         "primes.cgi PERL/CGI statements"),
      ", which illustrates many techniques and strategies.",
      p,
      hr;

($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                               localtime(time);   
$year = $year + 1900;
$nextMonth = $mon + 2;

print "\n<PRE>\n" . `cal` . "\n\n" . `cal $nextMonth $year` .
      "\n</PRE>\n";                        # back tic for Unix command
     
print end_html;