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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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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))
Setting a safety net or fallback value in Perl

Let's say that you want a Perl variable to contain a result which varies depending in the type and values of data fed in. Easy enough - but you need to think about what value you want it to hold if the inputs don't match any of the acceptable values. And there are then two approaches:

1. You can set a Perl variable to a default value early in the code, then override it if you find a matching type and value entered by the user.

2. You can set the variable if you find matching data, and then conclude your code with a "safety net" assignment to set a fallback value if nothing matched at all:
  $name ||= "[Not identified. Not usual colours]";

If you're using the second method, though, you need to take care if an empty string or zero (a false value) is a valid matched value, and you should really rewrite your code:
  defined ($name) or $name = "[Not identified. Not usual colours]";

From Perl 5.10 onwards, a new operator - // - is provided to provide you with a shorthand for "defined or", and you can now write:
  $name //= "[Not identified. Not usual colours]";

We have provided a complete example - [source code here].

(written 2010-06-19, updated 2010-06-20)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P256 - Perl 6 Look Ahead
  [89] When will Perl 6 be available - (2004-10-15)
  [113] A Parallel for Perl 6 - (2004-11-09)
  [550] 2006 - Making business a pleasure - (2006-01-01)
  [582] DWIM and AWWO - (2006-01-30)
  [995] Ruby's case - no break - (2006-12-17)
  [1215] An update on Perl - where is it going? - (2007-06-03)
  [1417] What software version do we teach? - (2007-10-31)
  [1721] Perl 6 - When will we have a production release? - (2008-07-26)
  [2559] Moving the product forward - ours, and MySQL, Perl, PHP and Python too - (2010-01-01)
  [2815] switch and case, or given and when in Perl - (2010-06-17)
  [2816] Intelligent Matching in Perl - (2010-06-18)
  [2967] Multiway branches in Perl - the given and when syntax - (2010-09-22)
  [3077] Perl 6 - significantly nearer, and Rakudo looks very good - (2010-12-02)

P206 - Perl - More Loops and Conditionals
  [138] Perl - redo and last without a loop - (2004-12-02)
  [299] What - no switch or case statement? - (2005-05-03)
  [657] The ternary operator in Python - (2006-03-25)
  [962] Breaking a loop - Ruby and other languages - (2006-12-03)
  [1191] Smart English Output - via PHP and Perl ? : operator - (2007-05-18)
  [1220] for loop - how it works (Perl, PHP, Java, C, etc) - (2007-06-06)
  [1582] Ruby, C, Java and more - getting out of loops - (2008-03-19)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [1825] Question Mark - Colon operator (Perl and PHP) - (2008-10-08)
  [2711] For loop - checked once, or evety time? Ruby v Perl comparison and contrast - (2010-04-07)
  [2824] A pint of Black Rat, and a lazy barman - (2010-06-25)
  [2832] Are you learning Perl? Some more examples for you! - (2010-06-27)
  [2892] Alternative loops and conditionals in Ruby and Perl - (2010-07-28)
  [2972] Some more advanced Perl examples from a recent course - (2010-09-27)
  [3200] How a for loop works Java, Perl and other languages - (2011-03-12)
  [3398] Perl - making best use of the flexibility, but also using good coding standards - (2011-08-19)
  [3619] Ruby v Perl - a comparison example - (2012-02-21)
  [3914] While, for, foreach or something else to loop. - (2012-11-06)


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