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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
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)
  [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)
  [2468] What are Tcl lists? - (2009-10-22)
  [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)

T208 - Tcl/Tk - Arrays and dicts
  [3638] Sorting dicts and arrays in Tcl - (2012-03-04)
  [3614] Tcl - dicts - a tutorial and examples - (2012-02-14)
  [3192] Tcl - Some example of HOW TO in handling data files and formats - (2011-03-04)
  [2466] Tcl - passing arrays and strings in and back out of procs - (2009-10-22)
  [1614] When an array is not an array - (2008-04-17)
  [1427] Arrays in Tcl - a demonstration - (2007-11-10)
  [779] The fragility of pancakes - and better structures - (2006-06-26)
  [122] Passing arrays to procs in Tcl - (2004-11-18)


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