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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 http://melksh.am/nnnn for individual articles
T206 - Tcl/Tk - Lists
  [4455] Working out distance between places, using OS grid references and a program in Tcl - (2015-03-11)
  [4454] Everything is a string - even a list - (2015-03-11)
  [4209] Lists in Tcl - fundamentals in a commented source code example - (2013-11-16)
  [3618] lists and struct::list in Tcl - Introduction to struct::list and examples - (2012-02-18)
  [3583] Expanding a list of parameters in Tcl - {*} and eval - (2012-01-17)
  [3582] Tcl collections - lists, dicts and array - (2012-01-16)
  [3415] User defined sorting and other uses of callbacks in Tcl and Tk - (2011-09-02)
  [3394] The difference between lists and strings - Tcl - (2011-08-16)
  [3285] Extracting data from a string / line from file - Tcl - (2011-05-10)
  [2472] split and join in tcl and expect - (2009-10-23)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [1405] Sorting in Tcl - lists and arrays - (2007-10-24)
  [1402] Tcl - append v lappend v concat - (2007-10-23)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1283] Generating traffic for network testing - (2007-07-29)
  [1282] Stringing together Tcl scripts - (2007-07-29)
  [781] Tcl - lappend v concat - (2006-06-27)
  [463] Splitting the difference - (2005-10-13)
  [144] Tcl sandwich - lists in Tcl - (2004-12-08)

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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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