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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Not multidimentional arrays - but lists of lists. Much more flexible. Perl!

In Perl, you don't have multidimensional structures as such - you have list and hashes ... and you can have lists of hashes, hashes of lists and hashed of hases too. Which give you almost all of the benefits of multi dimensional structures, but with few of the limitations. And they can be set up in short pieces of code too.

while (<s;>) {
  my @hit = split;
  push @accesses,\@hit;
  }


Each line from a data file named on the command line is read into $_. (<> defaults to reading from a file named on the command line, and in a while statement it's saved into $_ unless the code specifies somewhere else).

The line is split into a list of fields, the separator between the fields being white spaces. (split uses white space and $_ as its defaults).

Onto a list called @accesses, we add a reference to (i.e. the address of the list @hit). The very first time through the loop, the list is created and it's automatically extended each time.

On reaching the } at the end of the loop, the name @hit is released as this was a "my" variable scoped to the block. However, since the actual data is now referenced within the @accesses list, the memory that it has used will be retained ... resulting in a new location for a new @hit next time around the loop!

So - having set up that data, how do we access it?

  print ${$accesses[100]}[9],"\n";

That's "Look at contents of the 100th element of the @accesses list and within the list that's pointed to there look at the contents of the 9th element" [but counted in each case from 0 not 1, so really the 10th element of the 101st row].

We could make that a bit easier:

  print $accesses[100]->[9],"\n";

The -> operator has replaced ${---} - it's just a syntactic change, but never the less it helps with followability, and indeed I can also write:

  print $accesses[100][9],"\n";

According to the rules of the language, I shouldn't be able to do this ... except that wehn Perl sees two subscripts butted up against each other without any opertor in between them, it assumes the author wanted -> ... which usually he did!
(written 2011-08-26)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P217 - Perl - More than Simple Lists and Hashes!
  [43] Hash of lists in Perl - (2004-09-09)
  [293] Course follow-ups - (2005-04-27)
  [1514] Autovivification - the magic appearance of variables in Perl - (2008-01-21)
  [2241] Perl references - $$var and \$var notations - (2009-06-15)
  [2840] Just pass a pointer - do not duplicate the data - (2010-06-30)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [3007] Setting up a matrix of data (2D array) for processing in your program - (2010-10-21)
  [3072] Finding elements common to many lists / arrays - (2010-11-26)
  [3105] Adventure with references to lists and lists of references - (2010-12-26)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [3399] From fish, loaves and apples to money, plastic cards and BACS (Perl references explained) - (2011-08-20)
  [3444] Take the dog on a lead - do not carry her. Perl references. - (2011-09-17)
  [3577] How to do multidimensional arrays (or rather lists and hashes) in Perl - (2012-01-14)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)


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Perl - a quick reminder and revision. Test yourself!
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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