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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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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))
Binary data handling with unpack in Perl

During today's Perl course, I was asked to provide an example of the unpack function for extracting multiple values from a piece of data - typically binary data extracted from a file into a scalar variable.


Dorothy-2:de$ perl imgsize sd*.gif
sd1.gif is GIF, 89a, 450 by 250
sd2.gif is GIF, 89a, 250 by 300
sd3.gif is GIF, 89a, 300 by 250
sd4.gif is GIF, 89a, 333 by 333
Dorothy-2:de$



And what better images to illustrate the article that ones of the delegates on the course?


Here's the program:

for $file(@ARGV) {
  open (FH,$file);
  read (FH,$header,10);
  ($type,$st,$wid,$hei) = unpack("a3a3vv",$header);
  print ("$file is $type, $st, $wid by $hei\n");
}


From memory (!) I recalled that a .gif file starts with 3 bytes to confirm that it's a GIF file, 3 bytes to tell us which subformat has been used, and 2 2-byte 16 bit unsigned integers, bigendian, which tell the width and height of the image in pixels.

You'll notice that the course had a lot of delegate on it. Where there's a high number of delegates, all at a similar level, all from the same company, and all doing similar applications, I will let the customer place up to 20 delegates on a course. Where the delegates background differs, I request a reduced top number. And on a public course, where people from differnt companies book, we restrict delegates to just 8 so that there's a good opportunity for the tutor to talk to each and every delegate about their own individual use of the subject being taught.


P.S. There's another example of unpack on a .gif file - [here] - that I wrote a wile back; the techniques used are slightly different, and the program is one that came from a series named after British Rivers - in this case the Ystwyth from Aberystwyth fame.

(written 2010-06-10)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P212 - Perl - More on Character Strings
  [453] Commenting Perl regular expressions - (2005-09-30)
  [583] Remember to process blank lines - (2006-01-31)
  [586] Perl Regular Expressions - finding the position and length of the match - (2006-02-02)
  [597] Storing a regular expression in a perl variable - (2006-02-09)
  [608] Don't expose your regular expressions - (2006-02-15)
  [737] Coloured text in a terminal from Perl - (2006-05-29)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [1222] Perl, the substitute operator s - (2007-06-08)
  [1230] Commenting a Perl Regular Expression - (2007-06-12)
  [1251] Substitute operator / modifiers in Perl - (2007-06-28)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1510] Handling Binary data (.gif file example) in Perl - (2008-01-17)
  [1727] Equality and looks like tests - Perl - (2008-07-29)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1947] Perl substitute - the e modifier - (2008-12-16)
  [2230] Running a piece of code is like drinking a pint of beer - (2009-06-11)
  [2379] Making variables persistant, pretending a database is a variable and other Perl tricks - (2009-08-27)
  [2657] Want to do a big batch edit? Nothing beats Perl! - (2010-03-01)
  [2834] Teaching examples in Perl - third and final part - (2010-06-27)
  [2874] Unpacking a Perl string into a list - (2010-07-16)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2993] Arrays v Lists - what is the difference, why use one or the other - (2010-10-10)
  [3059] Object Orientation in an hour and other Perl Lectures - (2010-11-18)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3322] How much has Perl (and other languages) changed? - (2011-06-10)
  [3332] DNA to Amino Acid - a sample Perl script - (2011-06-24)
  [3411] Single and double quotes strings in Perl - what is the difference? - (2011-08-30)
  [3546] The difference between dot (a.k.a. full stop, period) and comma in Perl - (2011-12-09)
  [3630] Serialsing and unserialising data for storage and transfer in Perl - (2012-02-28)
  [3650] Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12)
  [3707] Converting codons via Amino Acids to Proteins in Perl - (2012-04-25)
  [3927] First match or all matches? Perl Regular Expressions - (2012-11-19)
  [4452] Binary data handling - Python and Perl - (2015-03-09)

G401 - Well House Consultants - On Site courses
  [139] Just provide a room and the students - (2004-12-03)
  [191] Setting up a training room for a course - (2005-01-27)
  [213] Off to Munich - (2005-02-13)
  [241] Dress for training - (2005-03-10)
  [382] Central London Courses - Perl, PHP, Python, Tcl, MySQL - (2005-07-18)
  [438] Fair system for travel and accommodation expenses - (2005-09-07)
  [439] You cant - (2005-09-08)
  [1027] Cue the music, I'm happy. - (2007-01-09)
  [1465] Perl, PHP, Python, Tcl, Linux, MySQL, Ruby courses ... - (2007-12-09)
  [1600] Cambidge - Tcl, Expect and Perl courses - (2008-04-04)
  [1781] Reception - (2008-09-01)
  [2041] httpd, Tomcat and PHP course enhancements - (2009-02-14)
  [2309] Lua training class in Spanish - (2009-07-29)
  [2311] Floor to ceiling - (2009-07-30)
  [2810] A course review - for the tutor to complete - (2010-06-14)
  [2928] Public (scheduled) or private course? Book direct, or through a third party? - (2010-08-19)
  [2956] On site course - travel and accommodation expenses - (2010-09-14)
  [3131] Lua, Tcl, Python; Worldwide training classes - (2011-01-14)
  [3137] Training Classes - should the training company provide a system for each delegate to use? - (2011-01-18)
  [3691] Back in Cambridge to give a Lua course - (2012-04-09)
  [3715] Changing face - Filton - (2012-05-01)
  [3898] The course must go on - improvements to tutor travel plans, with immediate effect - (2012-10-17)
  [3950] 10 reasons why I travel the night before a course starts - (2012-12-10)
  [4132] Using your own laptop on our courses - now even easier! - (2013-07-05)
  [4230] Well House Consultants - course prices for 2014 - (2013-12-26)
  [4602] Training course and hotel room prices - 2016 - (2015-12-31)
  [4739] A year of changes for Lisa and Graham Ellis, and Well House - (2017-05-27)
  [4769] Python, Tcl, Lua and other Open Source programming courses for 2018 - (2017-12-31)


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After the Perl course in Nurnberg
Some other Articles
How are you getting on?
Regular Expression Myths
Travelling across Europe
After the Perl course in Nurnberg
Binary data handling with unpack in Perl
The Merchants And
Nuremberg - some pictures
Perl - skip the classics and use regular expressions
Sur la T.G.V.
Sunday - waiting for the ferry.
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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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