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Transforming data in Perl using lists of lists and hashes of hashes
In Perl, you don't have conventional two dimensional arrays - you have lists which are single dimensional only. However, you can have a list that contains a reference to another list, in effect a list of lists. Very powerful stuff as it leads to a very flexible way of handling data structures - we cover it briefly on our Perl Programming Course
and in much more depth on Perl for Larger Projects
As you write code using lists of lists (and hashes of lists and the other alternatives too), you are very strongly advised to draw a diagram of what you are trying to achieve. It will do wonders for you. That way you can see the difference between a list which is a concatenation of other lists, and a list of lists. Say, for example, I had three options for breakfast, 4 for lunch and 4 for dinner, a concatanated list would be 11 elements long, but a list of lists would contain three elements. There's a source code example here
from today's course. The diagram alongside shows how this code works.
One of the big uses of hashes of lists (as an example of a collection of collections) is in inverting data. Let's say I have a file in which each line starts with a person's name, and the person's skills are then listed along the line. But I want to turn that around so that I can see a list of skills, with the people holding each skill alongside the skill. It turns out to be a handful of lines of code - see here
. What works for people and skills is equally good in transforming a web access log file, listing by visiting client, into a report on each of the web pages on your website and telling you how many people have visited each of them!
Three further examples: reading the people / skills file into a list of hashes
and the data inversion with a web (CGI) interface
. Finally, the data inversion with a search facility
. (written 2009-06-12, updated 2009-06-14)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articlesP207 - Perl - File Handling 
Spraying data from one incoming to series of outgoing files in Perl - (2012-08-15) 
Traversing a directory in Perl - (2012-08-08) 
Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10) 
Finding your big files in Perl - design considerations beyond the course environment - (2011-06-14) 
Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27) 
Chancellor George Osborne inspires Perl Program - (2010-06-22) 
File open and read in Perl - modernisation - (2010-06-19) 
But I am reading from a file - no need to prompt (Perl) - (2009-09-14) 
Reactive (dynamic) formatting in Perl - (2008-10-31) 
Seven new intermediate Perl examples - (2008-10-30) 
Formatting with a leading + / Lua and Perl - (2008-10-15) 
There is more that one way - Perl - (2008-07-14) 
stdout v stderr (Tcl, Perl, Shell) - (2007-12-10) 
Reading a file multiple times - file pointers - (2007-11-23) 
Good, steady, simple example - Perl file handling - (2007-10-30) 
Some one line Perl tips and techniques - (2007-08-21) 
Being sure to be positive in Perl - (2006-09-15) 
Iterators - expressions tha change each time you call them - (2006-04-27) 
Perl - its up to YOU to check your file opened - (2006-02-23) 
printf - a flawed but useful function - (2006-02-22) 
STDIN, STDOUT, STDERR and DATA - Perl file handles - (2005-03-23) 
Relative or absolute milkman - (2004-11-10) 
How many people in a room? - (2004-08-12)P409 - Perl - Searching 
Improving searches - from OR to AND? - (2007-05-11)
Some other Articles
Taking a pride in the communityAlumni - revisiting and supporting the old UniversitySending awkward characters by email in PerlLoading external code into Perl from a nonstandard directoryTransforming data in Perl using lists of lists and hashes of hashesWhy sendmail one way, and pop3 the other?What is CGI.pm / A dozen new examplesRunning a piece of code is like drinking a pint of beerDo not re-invent the wheel - use a Perl moduleWhere do I start when writing a program?
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