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For 2023 - 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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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))
What are Tcl lists?

In Tcl, all variables (except 'arrays') are held as strings of text - and that includes lists. A list is a string which is treated as a collection of individual words, space separated, and you can then select individual words by their word number.

And this means that if you have a simple, space separated file of data you can read in the data line by line and use commands like lindex to refer, item by item, to the components of that line without needing to recourse to splits or joins. There's an example that shows that truly simple (almost naive) approach here which I wrote on the public Tcl programming course I presented earlier this week. There's a further example of analyzing the log file and building up a list of the hosts served here - again a naive example (as adding arrays to the mix is helpful in this case).

Actually, lists are rather more sophisticated than the example shown above will lead you to believe - they're interpreted within Tcl by the same syntax analyzer that analyses your code (why not - it keeps the footprint down!) so that you can include spaces and other special characters in your list elements - using {} and \ protection just as you would on your data. Tools such as the list and split and join commands let you manipulate lists easily, even if the list members include 'specials'.

Here is a summary of the main list commands:

Creating and modifying lists
As well as creating lists from first principles, you can construct and modify lists using:
list - provides automatic quoting
lappend - adds second and subsequent arguments to list named as first argument
linsert - inserts elements into a list at a given index position, returns a new list
lreplace - replaces elements in a list (first argument) with index numbers in a range (second and third arguments) with subsequent arguments
split - splits a string into list elements and returns the list. If called with two arguments, the second argument is the separator character

Extracting information from a list
Information can be returned from lists using:
lindex - first parameter, the list; second parameter, the index number of the element to be returned
llength - returns the length of a list
join - merges the elements of a list, separating each with the second parameter
lsearch - search a list for a value

Manipulating lists to create other lists
concat - takes multiple lists as parameters and returns a single joined list
lrange - returns part of the incoming list, from start index number to end index number
lsort - sort an incoming list and return a new sorted list

(This table is from our course notes - all delegates get a copy of our course notes to take away with them - approx 60 pages per day of training. The source code of all the examples is available on our web site too.)
(written 2009-10-22, updated 2009-10-24)

Associated topics are indexed as below, or enter for individual articles
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)
  [1282] Stringing together Tcl scripts - (2007-07-29)
  [1283] Generating traffic for network testing - (2007-07-29)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1402] Tcl - append v lappend v concat - (2007-10-23)
  [1405] Sorting in Tcl - lists and arrays - (2007-10-24)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [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)
  [3415] User defined sorting and other uses of callbacks in Tcl and Tk - (2011-09-02)
  [3582] Tcl collections - lists, dicts and array - (2012-01-16)
  [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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Windows 7 and Open Source Programming
Beyond the Pale
What are Tcl lists?
Tcl - catching an error before your program crashes
Tcl - passing arrays and strings in and back out of procs
Melksham Town - asleep or awake?
Beauty in pictures
Tcl - a true interpretive, command based language
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