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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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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))
Error handling in Tcl through catch

Tcl is a truly interpretive language - and that means that you can get a syntax error if a user makes an illegal entry ... even in the middle of a program running. For example


puts -nonewline "please enter a number: "
flush stdout
gets stdin value
set doubled [expr $value * 2]
puts "Result is $doubled"
puts "application continues ..."


Will run correctly if you enter a number to the prompt as requested:


earth-wind-and-fire:~/jul05 grahamellis$ wish ifex
please enter a number: 17
Result is 34
application continues ...
earth-wind-and-fire:~/jul05 grahamellis$


but will fail if you enter something that isn't a number:


earth-wind-and-fire:~/jul05 grahamellis$ wish ifex
please enter a number: lots
Error in startup script: syntax error in expression "lots * 2": variable references require preceding $
  while executing
"expr $value * 2"
  invoked from within
"set doubled [expr $value * 2]"
  (file "ifex" line 6)
earth-wind-and-fire:~/jul05 grahamellis$


If you want to make your Tcl code rugged against such errors, embed commands that may cause a problem within catch commands. Tcl's catch command returns a true of false result depending on whether or not the embedded command succeeds, and if it fails it also returns the error message describing the problem in a variable of your choice. Here's the program above, enhanced to catch any errors:


puts -nonewline "please enter a number: "
flush stdout
gets stdin value
if {[catch {set doubled [expr $value * 2]} errmsg]} {
  puts "Failed - $errmsg"
} else {
  puts "Succeeded and result is $doubled"
}
puts "application continues ..."


And let's try running that. Firstly, with correct data:


earth-wind-and-fire:~/jul05 grahamellis$ wish ifcat
please enter a number: 17
Succeeded and result is 34
application continues ...
earth-wind-and-fire:~/jul05 grahamellis$


And now with data that would previously have caused a crash:


earth-wind-and-fire:~/jul05 grahamellis$ wish ifcat
please enter a number: many
Failed - syntax error in expression "many * 2": variable references require preceding $
application continues ...
earth-wind-and-fire:~/jul05 grahamellis$


Complete source code example - [here].

This approach described here could be described as an expection handling system as it tries to perform an operation and traps if a failure occurs, rather than trying to predict the error ahead of time. An alternative (error handling) system could have been used, in which the code check the input text string to make sure that it's numeric before the expr command is run. We have a comparison of exceptions v errors in our solution centre.
(written 2005-07-02, updated 2012-03-04)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
T214 - Tcl/Tk - Other Facilities in Tcl
  [239] What and why for the epoch - (2005-03-08)
  [364] pu daily and p hourly - (2005-06-30)
  [461] Shortened interactive commands - (2005-10-11)
  [748] Getting rid of variables after you have finished with them - (2006-06-06)
  [782] Converting between Hex and Decimal in Tcl - (2006-06-28)
  [1277] AgtInvoke - a command to drive Agilent Tcl software extensions - (2007-07-26)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1338] Handling Binary data in Tcl (with a note on C) - (2007-09-09)
  [2467] Tcl - catching an error before your program crashes - (2009-10-22)
  [3287] Exceptions - Tcl style - (2011-05-12)
  [3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
  [3583] Expanding a list of parameters in Tcl - {*} and eval - (2012-01-17)
  [4207] Exception handling in Tcl - (2013-11-14)
  [4523] Catching failed commands and not crashing the program in Tcl - (2015-10-10)
  [4525] What does Tcl do if you try to run a command that is not defined? - (2015-10-10)
  [4762] Coverage map in Tcl - how many times has each proc been called? - (2017-09-28)


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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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