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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
User defined sorting and other uses of callbacks in Tcl and Tk

When I output a table of results, I usually want it to be sorted in some way.

In Tcl, I can use lsort to sort a list - there's an example of it running in a default way [here]. However, there's often a need to order records according to a non-default algorithm, and there are switches such as -integer for cases like this.

  % set values "4 7 12 7 12 5 4"
  4 7 12 7 12 5 4
  % puts [lsort $values]
  12 12 4 4 5 7 7
  % puts [lsort -integer $values]
  4 4 5 7 7 12 12
  %


Good - but what about more complex cases? I can use the -command option to pass in to lsort the name of a command that takes two parameters and returns negative / zero / positive depending on whether the first parameter passed in is to come first in the sorted result set, if the two values are of the same value as far as sorting is concerned, or if the second parameter passed is to be first on the output.

The code for this can be quite short - there's an example from yesterday's Tcl course that I've uploaded [here]. But it can also be quite baffling for the newcomer, as it's often his / her first experience of what's known as a callback - that's where a call is made to a Tcl-provided command (lsort in this case) which then calls - possibly multiple times, and possibly later on the extra piece of user provided code.

Purely as a training demonstration of the guts of how this works, I re-implemented elements of lsort in my own proc called mysort and I have placed that code on our website [here].

Callbacks turn out to be very important in Tk - the Graphic User Interface that's often used with Tcl. When you set up a button, you'll usuaully tell Tcl/Tk what the button is to do when pressed via a callback. There's a shortish example [here] ...
  button .tog -text Marmite -command flip
... in which a used defined proc called flip is to be run whenever the button called .tog is pressed. The definition of the code to be run is done early in the setup; it may be run (if the button is ever pressed!) much later on, as described above. That's often referred to as a deferred callback.


Illustraton - the Tcl/Tk demonstration program in use.
(written 2011-09-02)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
T217 - Tcl/Tk - First Widgets, Geometry and Events
  [309] Writing a Tcl/Tk GUI is as easy as baking a cake - (2005-05-12)
  [3575] Multiple buttons calling the same proc in wish (tcl/tk) - (2012-01-12)
  [4458] A new Tcl/tk example - a window to show system status - (2015-03-11)

T208 - Tcl/Tk - Arrays and dicts
  [122] Passing arrays to procs in Tcl - (2004-11-18)
  [779] The fragility of pancakes - and better structures - (2006-06-26)
  [1282] Stringing together Tcl scripts - (2007-07-29)
  [1283] Generating traffic for network testing - (2007-07-29)
  [1405] Sorting in Tcl - lists and arrays - (2007-10-24)
  [1427] Arrays in Tcl - a demonstration - (2007-11-10)
  [1614] When an array is not an array - (2008-04-17)
  [2466] Tcl - passing arrays and strings in and back out of procs - (2009-10-22)
  [3192] Tcl - Some example of HOW TO in handling data files and formats - (2011-03-04)
  [3582] Tcl collections - lists, dicts and array - (2012-01-16)
  [3614] Tcl - dicts - a tutorial and examples - (2012-02-14)
  [3638] Sorting dicts and arrays in Tcl - (2012-03-04)

T206 - Tcl/Tk - Lists
  [144] Tcl sandwich - lists in Tcl - (2004-12-08)
  [463] Splitting the difference - (2005-10-13)
  [781] Tcl - lappend v concat - (2006-06-27)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1402] Tcl - append v lappend v concat - (2007-10-23)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [2468] What are Tcl lists? - (2009-10-22)
  [2472] split and join in tcl and expect - (2009-10-23)
  [3285] Extracting data from a string / line from file - Tcl - (2011-05-10)
  [3394] The difference between lists and strings - Tcl - (2011-08-16)
  [3583] Expanding a list of parameters in Tcl - {*} and eval - (2012-01-17)
  [3618] lists and struct::list in Tcl - Introduction to struct::list and examples - (2012-02-18)
  [4209] Lists in Tcl - fundamentals in a commented source code example - (2013-11-16)
  [4454] Everything is a string - even a list - (2015-03-11)
  [4455] Working out distance between places, using OS grid references and a program in Tcl - (2015-03-11)


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Passing back multiple results in Tcl - upvar and uplevel
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Storing Tcl source code encoded, and running via your own C program
Some other Articles
Data that we use during our training courses, and other training resources
Tcl packages, pkg_mkIndex, pkgIndex.tcl -what are they and why use them.
What is a namespace and why do we need them?
Storing Tcl source code encoded, and running via your own C program
User defined sorting and other uses of callbacks in Tcl and Tk
Passing back multiple results in Tcl - upvar and uplevel
If its Sunday, must it be Weymouth?
Handling binary data in Perl is easy!
Single and double quotes strings in Perl - what is the difference?
A review of the Summer Sunday extra trains on the TransWilts line
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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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