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Expanding a list of parameters in Tcl - {*} and eval

In Tcl commands, parameters are separated by spaces, just like Tcl lists are. So it would sometimes be very useful to be able to write
  set action {piemiddle apple}
  set $action

to take a list of parameters from a Tcl list - in this case setting the "piemiddle" variable to the value "apple".

But it doesn't work. The problem is that the variable $action is only expanded into the string "piemiddle apple" after the command line has been split into its individual parameters, so that the Tcl program attempts to refer to a single variable called "piemiddle apple" with a space in the middle of it, giving the message:
  can't read "piemiddle apple": no such variable
which is a huge pity.

However, as of Tcl version 8.5 the expand operator was introduced, which allows you to specify that a variable is to be expanded before the command line is split into individual parameters. So if you write
  set action {piemiddle apple}
  set {*}$action

you will actually end up setting a variable called "piemiddle" to the value "apple".

The sequence {*} is, then, know as the expand operator, and forms a part of a parameter that's to be expanded early. In other words, you may not add a space between the {*} and the name of the variable to be expanded. Full source code example [here].

Note - in earlier versions of Tcl, the eval command was the recommended alternative and it remains available today. eval is often very messy to implement, as it applies to the whole command rather than just an individual scalar list, and so you end up having to add all sorts of extra protections. Here's a simple eval example - equivalent of the examples above:
  set pudding {rolypolymiddle jam}
  eval set $pudding
  puts "In rolypoly we have $rolypolymiddle"

(written 2012-01-17, updated 2012-01-28)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
T214 - Tcl/Tk - Other Facilities in Tcl
  [239] What and why for the epoch - (2005-03-08)
  [364] pu daily and p hourly - (2005-06-30)
  [366] Error handling in Tcl through catch - (2005-07-02)
  [461] Shortened interactive commands - (2005-10-11)
  [748] Getting rid of variables after you have finished with them - (2006-06-06)
  [782] Converting between Hex and Decimal in Tcl - (2006-06-28)
  [1277] AgtInvoke - a command to drive Agilent Tcl software extensions - (2007-07-26)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1338] Handling Binary data in Tcl (with a note on C) - (2007-09-09)
  [2467] Tcl - catching an error before your program crashes - (2009-10-22)
  [3287] Exceptions - Tcl style - (2011-05-12)
  [3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
  [4207] Exception handling in Tcl - (2013-11-14)
  [4523] Catching failed commands and not crashing the program in Tcl - (2015-10-10)
  [4525] What does Tcl do if you try to run a command that is not defined? - (2015-10-10)
  [4762] Coverage map in Tcl - how many times has each proc been called? - (2017-09-28)

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)
  [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)
  [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)
  [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)
  [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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