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Tcl/Tk module T203
Conditionals and Loops
Exercises, examples and other material relating to training module T203. This topic is presented on public courses Learning to program in Tcl, Tcl Programming, Tcl Programming

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 subjectupdated
4455Working out distance between places, using OS grid references and a program in Tcl
In Great Britain, locations have Ordnance Survey grid reference positions which place them to within 100 metres. Our training centre / hotel is at ST907633 - that's   100 km square ST   90.7 km into that square from the west side   63.3 km into that squate from the south ...
4323Learning to program - Loop statements such as while
If your program always ran each statement just once (indeed skipping over statements which were in blocks in false conditions) it would run very quickly and would have little use. You couldn't (for example) run a program which went through a whole series of results from a database query and displayed ...
4322Learning to Program - the conditional statement (if)
Every language has some sort of conditional statement. That's a way of looking at some sort of setting or status in the program and performing some sort of action based on that setting or status. such statements take the form ...   if {some sort of condition is true} then {run a group of ...
3397Does 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 ...
3570Trapping 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 ...
3571Comparing 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 ...
3189Tcl - 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) ...
2681Tcl - 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 ...
1401Tcl - 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 ...
2471A 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, ...
2261Tcl - 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 ...
1696Saying 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. ! ...
1477Decisions - 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 ...
210Joining 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 ...
Examples from our training material
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
Tcl course group
A private Tcl Course in Dublin
Background information
Some modules are available for download as a sample of our material or under an Open Training Notes License for free download from [here].
Topics covered in this module
The if command.
deferred substitution.
The else and elseif parameters.
The while command.
Getting out of loops.
The for command.
The foreach command.
The switch command.
Conditional operators.
Combining conditions.
Locating errors in your Tcl program.
Complete learning
If you are looking for a complete course and not just a information on a single subject, visit our Listing and schedule page.

Well House Consultants specialise in training courses in Tcl/Tk, Expect,Ruby, Lua, Python, Perl, PHP, and MySQL. We run Private Courses throughout the UK (and beyond for longer courses), and Public Courses at our training centre in Melksham, Wiltshire, England. It's surprisingly cost effective to come on our public courses - even if you live in a different country or continent to us.

We have a technical library of over 700 books on the subjects on which we teach. These books are available for reference at our training centre.

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