Exercises, examples and other material relating to training module T203. This topic is presented on public courses Learning to program in Tcl, Tcl Basics

Every programming language needs conditional commands such as "if" and the ability to repeat sections of code using commands such as "while". We cover these commands in this module, together with "for", "foreach", "switch" and conditional operators.

Articles and tips on this subject		updated
3570	Trapping errors in Tcl - the safety net that catch provides What could possibly go wrong when you open a file in a program? Lots of things, actually. * You could try to open a file in directory you cannot access * You could try to read from a file that does not exist * You could try to read from something that's not a plain file - e.g. a directory * You could ...	2012-01-07
3571	Comparing loop commands in Tcl Tcl offers you three different loop commands - here they are generating the same output: while A condition is tested, and if true the commands in a loop are performed. The condition is then retested (provided that you have passed the test into while as a deferred block)   set month 1   while ...	2012-01-06
3397	Does a for loop evaluate its end condition once, or on every iteration? All the languages that we teach have a for loop or the equivalent, which is a clean way of repeating a block of code with a rising or falling index number. It's used in many circumstances - for example in iterating through the months of the year (for m goes from 1 to 12) of in stepping through all ...	2011-08-18
3189	Tcl - the danger of square brackets in a while command In Tcl, every statement is a command - with the parameters separated by spaces. Elements which are written in square brackets are commands which are evaluated before the rest of the statement, and elements which are written in curly braces are deferred - held back - and are only evaluated (if at all) ...	2011-03-04
2681	Tcl - a great engineering language It might be a little old and out of fashion - but Tcl is still very much in use, we use it and we can offer you a learning to program in Tcl training course if you're new to programming, or a Tcl Programming course if you've programmed in another language before. We also offer a two day Tk course as ...	2011-03-01
1401	Tcl - using [] or {} for conditions in an if (and while) In Tcl, the use of square brackets tells the interpreter to "do this first" whereas curly braces are a request to defer execution of a block - perhaps suppressing it completely as in the action on an if statement. So the if statement's action will NEVER be written in square brackets - [ ] - as that ...	2011-03-01
2471	A short form of if ... then ... else There are so many times you want to say "if ... then ... else" to do no more than choose between the word "is" and "are", to say "child" or "children", or to say "may" or "may not" in your output. Using an if statement for that's a lot of code for a little job. In most of the languages we teach (Perl, ...	2009-10-23
2261	Tcl - nice and nasty Tcl is a lovely engineers language ... but goodness you can do some nasty things with it, and write some unmaintainable code. set count 1 foreach type {if while} {   $type {$count < 5} {     puts "yay $type $count"     incr count   } } Which ...	2009-06-30
1696	Saying NOT in Perl, PHP, Python, Lua ... "Isn't there one standard way to say NOT?" asked one of my delegates on today's course - and it's an excellent question. But the answer to a question about a negative subject is itself in the negative - no, there isn't just a single way! In fact .. I can think of no fewer that 12 ways! • 1. ! ...	2008-07-05
1477	Decisions - small ones, or big ones? When you're traveling, you'll sometimes come to a point at which you make a decision - to go one way or to go another. Sometimes the decision is a small one - for example, if I'm driving through Marlborough there's a choice of the town centre or the road around the back; whichever I take, they come ...	2007-12-19
210	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 ...	2006-06-05

diy	The for command
diy2	The foreach command
dthrow	The switch command
flt	for while and foreach loops compared
food	Alternative conditions - while loop
ftoc	F to C conversion - exercise
ftoc2	Is water a solid, a liquid or a gas?
globber	Switch - exact, glob and regex
group	The if command in Tcl
group4	deferred execution blocks with { and }
group5	if, elseif and else in Tcl
lazy	The ? : conditional operator
lunches	Lunches for each day of the week
minimax	Answer to practical - minimum and maximum
morelunches	Lunches - different places to go
older	Conditional Operator ? and :
validlunches	Lunches - validating user input within range
wd	The while command in Tcl
wipats	More flexible alternative to a switch
wtot	breaking out of a Tcl loop

The Tcl if command
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