Joining lists in Tcl. Indirect variables in Tcl.

With all programming languages, there seem to be a few things that you end up saying "I'm SURE there's a way to do that" .... yet you can't find out how. Here are two quick solutions in Tcl - a specialist subject, I know, but one of the busiest on our forum.

# Two quick solutions in Tcl ...



# How to join a list onto the end of another list



set first {a b c}

set second {d e f}

set together [concat $first $second]

puts $together



# How to reference a variable named within another variable



set person "John Jones"

set elholder person



# Following line is how NOT to do it - Tcl expands $s just ONCE

set contains $$elholder

puts $contains

# Following line is how you SHOULD do it

set contains [set $elholder]

puts $contains

The concat command lets you concatanate lists. There's a temptation to try to join lists up using the append or lappend commands, but this always turns out to be a bit of a fiddle as you manipulate the list structure yourself, perhaps having to add in extra spaces to the syntax.

Tcl only goes through each command once as it interprets it (it's NOT recursive). That's efficient and what you want most of the time, but just occasionally - for example when you have one variable within another - it's a nuisance. Tcl's set command return the contents of a variable, though, so writing code as in the example above provide a useful is slightly inelegant solution.

We have links to more Tcl List examples and more Tcl interpretter examples available.

(written 2005-02-12, updated 2006-06-05)

Associated topics are indexed under

T202 - Tcl/Tk - Tcl Fundamentals
  [3917] BODMAS - the order a computer evaluates arithmetic expressions - (2012-11-09)
  [2442] Variable storage - Perl, Tcl and Python compared - (2009-10-08)
  [1469] Curley brackets v double quotes - Tcl, Tk, Expect - (2007-12-12)
  [1426] Buffering up in Tcl - the empty coke can comparison - (2007-11-10)
  [1282] Stringing together Tcl scripts - (2007-07-29)
  [1136] Buffering output - why it is done and issues raised in Tcl, Perl, Python and PHP - (2007-04-06)
  [782] Converting between Hex and Decimal in Tcl - (2006-06-28)
  [362] The ireallyreallywanna operator - (2005-06-28)
  [349] Comments in Tcl - (2005-06-16)
  [328] Making programs easy for any user to start - (2005-05-29)
  [3] Looking for a donkey - (2004-08-05)

T203 - Tcl/Tk - Conditionals and Loops
  [3571] Comparing loop commands in Tcl - (2012-01-06)
  [3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [3189] Tcl - the danger of square brackets in a while command - (2011-03-02)
  [2681] Tcl - a great engineering language - (2010-03-17)
  [2471] A short form of if ... then ... else - (2009-10-23)
  [2261] Tcl - nice and nasty - (2009-06-29)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [1401] Tcl - using [] or {} for conditions in an if (and while) - (2007-10-23)

534d

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