Joining lists in Tcl. Indirect variables in Tcl.

For 2023 (and 2024 ...) - 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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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))

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 as below, or enter http://melksh.am/nnnn for individual articles

T203 - Tcl/Tk - Conditionals and Loops
  [1401] Tcl - using [] or {} for conditions in an if (and while) - (2007-10-23)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [2261] Tcl - nice and nasty - (2009-06-29)
  [2471] A short form of if ... then ... else - (2009-10-23)
  [2681] Tcl - a great engineering language - (2010-03-17)
  [3189] Tcl - the danger of square brackets in a while command - (2011-03-02)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
  [3571] Comparing loop commands in Tcl - (2012-01-06)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4455] Working out distance between places, using OS grid references and a program in Tcl - (2015-03-11)

T202 - Tcl/Tk - Tcl Fundamentals
  [3] Looking for a donkey - (2004-08-05)
  [328] Making programs easy for any user to start - (2005-05-29)
  [349] Comments in Tcl - (2005-06-16)
  [362] The ireallyreallywanna operator - (2005-06-28)
  [782] Converting between Hex and Decimal in Tcl - (2006-06-28)
  [1136] Buffering output - why it is done and issues raised in Tcl, Perl, Python and PHP - (2007-04-06)
  [1282] Stringing together Tcl scripts - (2007-07-29)
  [1426] Buffering up in Tcl - the empty coke can comparison - (2007-11-10)
  [1469] Curley brackets v double quotes - Tcl, Tk, Expect - (2007-12-12)
  [2442] Variable storage - Perl, Tcl and Python compared - (2009-10-08)
  [3917] BODMAS - the order a computer evaluates arithmetic expressions - (2012-11-09)
  [4324] Learning to program - variables and constants - (2014-11-22)
  [4453] Tcl variable names - no real limits! - (2015-03-10)

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