6.7. Example: Extracting Temperatures from Weather Underground

The Weather Underground web site (http://www.wunderground.com) is a great source of meteorological information. Let's write a program to tell us which of the two O'Reilly offices, Cambridge and Sebastopol, is warmer and by how many degrees.

First, we fetch the pages with the temperatures. A quick look around the Weather Underground site indicates that the best way to get the temperature for a place is to fetch a URL like:

http://www.wunderground.com/cgi-bin/findweather/getForecast?query=95472

95472 is the Zip Code for the Sebastopol office, while 02140 is the Zip Code for the Cambridge office. The program begins by fetching those pages:

#!/usr/bin/perl -w
  
use strict;
use LWP::Simple;
  
my $url = "http://www.wunderground.com/cgi-bin/findweather/getForecast?query=";
my $ca = get("${url}95472"); # Sebastopol, California
my $ma = get("${url}02140"); # Cambridge, Massachusetts

Next, we need to extract the temperature from the HTML. Viewing the source to one of the pages reveals the relevant portion as:

<tr ><td>Temperature</td>
<td><b>52&#176;</b> F</td></tr>

Because we need to extract the temperature from multiple pages, we define a subroutine that takes the HTML string and returns the temperature:

sub current_temp {
  local $_ = shift;
  m{<tr ><td>Temperature</td>\s+<td><b>(\d+)} || die "No temp data?";
  return $1;
}

Now all that's left to do is extract the temperatures and display the message:

my $ca_temp = current_temp($ca);
my $ma_temp = current_temp($ma);
my $diff = $ca_temp - $ma_temp;
  
print $diff > 0 ? "California" : "Massachusetts";
print " is warmer by ", abs($diff), " degrees F.\n";

When you run the program, you see something like:

% ora-temps
California is warmer by 21 degrees F.

The complete program is shown in Example 6-6.

#!/usr/bin/perl -w

use strict;
use LWP::Simple;

my $url = "http://www.wunderground.com/cgi-bin/findweather/getForecast?"
        . "query=";
my $ca = get("${url}95472"); # Sebastopol, California
my $ma = get("${url}02140"); # Cambridge, Massachusetts

my $ca_temp = current_temp($ca);
my $ma_temp = current_temp($ma);
my $diff = $ca_temp - $ma_temp;

print $diff > 0 ? "California" : "Massachusetts";
print " is warmer by ", abs($diff), " degrees F.\n";

sub current_temp {
  local $_ = shift;
  m{<tr ><td>Temperature</td>\s+<td><b>(\d+)} || die "No temp data?";
  return $1;
}