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Cut Along The Table Lines (HTML::TableExtract) - Perl

It is common knowledge that the Internet is a great data source. It is alsocommon knowledge that it is difficult to get the information you want in the format you need. No longer.

  1. Web Mining with Perl
  2. Accessing The Net (LWP)
  3. Cut Along The Table Lines (HTML::TableExtract)
  4. Learning From Links (HTML::LinkExtor)
  5. Checking For Sameness (String::CRC)
  6. Bringing It All Together
  7. Conclusion
By: Tommie Jones
Rating: starstarstarstarstar / 54
March 05, 2002

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HTML Tables not only help visually segregate data on a web page but they also provide helpful landmarks when parsing web pages. Tables are used to align information on web pages. Tables can force information to be in one location or to take up a certain width of a screen.

Tables become even more important on dynamic data driven web sites. This is because on most websites content such as articles are stored separately from the page's visual aspects. When generating the HTML pages the content is separated from other features of the web page with a table. In other words the main page might change but the layout defined by tables rarely changes. This is important because when processing a web page the developer will often want to ignore a lot of the static or template data but want to access the dynamic data. The developer of a web crawler will want to identify what tables/rows/cells the data you are interested in is located and pull his information from there.

Fortunately there exists a Perl Module designed to parse HTML tables. The following example script shows how a particular table can be parsed out of an HTML page.

#!/usr/bin/Perl use lib qw( ..); use HTML::TableExtract; use LWP::Simple; use Data::Dumper; my $te = new HTML::TableExtract( depth=>3, count=>0, gridmap=>0); my $content = get("http://www.computerjobs.com"); $te->parse($content); foreach $ts ($te->table_states) { foreach $row ($ts->rows) { print Dumper $row; # print Dumper $row if (scalar(@$row) == 2); } }
Now to explain the highlights of the code.

my $te = new HTML::TableExtract( depth=>3, count=>0, gridmap=>0);
This is where we create/initialize the TableExtract object. We pass three parameters to the page. depth => 3 - this is the depth of the table we want to work with. This suggest that this table is inside a table (depth=2) which is inside another table (depth = 1) which is at last in another table (depth=>0) count => 0 - More than one table can exists at the level three. count=>0 suggest that it is the first table that is at depth 3. gridmap => 0 - represents tables as a tree instead of a map.

The combination of these two parameters uniquely identify any table in an html page. Note that the table identified by (depth=>3, count=>1) is not necessarily the neighbor to the (depth=>3, count=>0) table. For instance

<table> <tr><td> /*Table depth=>0 count=>0 */ <table><tr><td> /* Table depth=>1 count=>0 */ <table><tr><td> /* Table depth=>2 count=>0 */ </td></tr></table> </td></tr></table> <table><tr><td> /*Table depth=>1 count =>1 */ <table><tr><td> /* Table depth=>2 count=>1 */ </td></tr></table> <table><tr><td> /* Table depth=>2 count=>2 */ </td></tr></table> </td></tr></table> </table><tr><td>
In the example shown above there are three tables at depth 2 . For the tables (depth=>2 count=>0) and (depth=>2 count=>1) notice that they do not share the same parent table. The count does not reset to zero when the html backs out of the depth. The table identified as (depth=>2 count=>1) is literally the second table(count = 1) at the third depth (both numbers start at zero.).

The gridmap option tells whether to logically represent data as a grid or a tree. Consider the following example.

<table> <tr> <td> location [1,1] </td> <td> location [1,2] </td> </tr> <tr colspan=2> <td> location [2,1] </td> </tr> <table>
If gridmap=1 (as is by default) then the cell [2,2] will be defined but empty. This is because gridmap=1 forces the table to look like a grid. If gridmap=0 the map table would look like a tree where each row could have a different number of cells. Trying to access position[2,2] will not be defined.

After the table is identified, the object representing the table can be accessed. These verbs include table_states and table_state. Table_state takes a depth and a count as an identifier to return a table state object. Table_states returns an array of table_states to represent our code.

A TableExtract object can represent multiple tables. This can be accomplished by only specifying depth or count (not both). This will return an object representing multiple tables.

In the first for loop we are going through the list of tables. This is done with the table_states object. The inner loop loops through the rows inside each table (represented by the tr tag.) The results of the rows tag is an array of arrays that represent the two-dimensional table.

>>> More Perl Programming Articles          >>> More By Tommie Jones

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