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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.
Tcl collections - lists, dicts and array

In Tcl, almost all variables are what is described as "pure" - which means that they hold data as strings of text which can be passed, substituted with a $ prefix, etc. Special interpretation on pure variables allows them to be treated as:
• integers
• floats
• lists
• dicts
• file handles

Arrays, on the other hand, are NOT pure variables - they are special cases and this means they cannot be substituted with a $, nor passed into procs by value, nor passed back via return (you have to use upvar to get them in and out of a proc)

List example:

  set demo {Anne Bill Charlotte Dennis Enid Fred Gwynn Henry Isobel}
  set fourth [lindex $demo 3]
  puts $fourth


Dict example:

  set demo {Anne Jones Bill Brown Charlotte Steeple Dennis {The Menace} Enid Blyton
    Fred Dibnah Gwynn Grimm Henry Eighth Isobel Quietly}
  set surname [dict get $demo Enid]
  puts $surname


With both lists and dicts, you can easily use single word values (and keys in the case of dicts) but need to apply protection if you want embedded special characters - and for this purpose the space is a special character.

lists are handled with a whole series of commands such as lsort, lindex and lappend, whereas with dictionaries all the actions are handled with the single dict command via a series of sub-commands.

The example code above may be seen in a complete but short sample program [here].

arrays are handles through the array command, and its sub-commands, and also with the use of round brackets to address individual members, which are pure strings. There's an example from our training notes [here] and another [here].

dicts were introduced into Tcl at version 8.5 ... and that's now been around long enough for it to be pretty unusual for us to come across 8.4 or earlier.

Advantages of dicts
- can use just like other pure variables
- ability to have dicts of dicts, lists of dicts, dicts of lists, etc

Advantages of arrays
- likely to be more efficient for larger volumes of data
- no need to worry if you've got Tcl 8.5 or not

See our Tcl courses if you want to come along and learn Tcl with us!
(written 2012-01-16, updated 2012-01-28)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
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)
  [3415] User defined sorting and other uses of callbacks in Tcl and Tk - (2011-09-02)
  [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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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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