Joining lists in Tcl. Indirect variables in Tcl.
Archive - Originally posted on "The Horse's Mouth" - 2005-02-12 10:08:25 - Graham EllisWith 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 $containsThe 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.