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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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)) |
Adding a member to a Hash in Ruby
In Ruby, you must initialise your variables - in other words, you cannot use the content of a variable that doesn't exist and have the language assume it will be 0, as happened with Perl.
So if - for example - you're using a Hash to keep tabs of a number of counters, you can't just +=1 members and have them automatically created if the dont already exist - you get an exception:
irb(main):001:0> demo = {}
=> {}
irb(main):002:0> demo["Graham"] = 25
=> 25
irb(main):003:0> demo["Graham"] += 1
=> 26
irb(main):004:0> demo["Lisa"] += 1
NameError: undefined method `+' for nil
from (irb):4
The fetch method on a Hash allows you to fetch a value if one exists, but return a default if the element you're looking for is undefined and that's a really effective way of initialising new counters to 0:
irb(main):005:0> demo["Graham"] = demo.fetch("Graham",0) + 1
=> 27
irb(main):006:0> demo["Lisa"] = demo.fetch("Lisa",0) + 1
=> 1
irb(main):007:0>
In some circumstances, the fetch behaviour is what you'll want .... but in others, it's more likely to be better to process the circumstance via an exception with a rescue clause. (written 2006-12-16, updated 2006-12-21)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles R107 - Collections (Arrays and Hashes) in Ruby [2291] Collection objects (array and hash) in Ruby - (2009-07-16) [2606] Sorting arrays and hashes in Ruby - (2010-01-30) [2618] What are Ruby Symbols? - (2010-02-02) [2621] Ruby collections and strings - some new examples - (2010-02-03) [2976] Creating, extending, traversing and combining Ruby arrays - (2010-09-30) [3253] Is this number between? Does this list include? - Ruby - (2011-04-18) [3255] Process every member of an array, and sort an array - Ruby - (2011-04-21) [3257] All possible combinations from a list (Python) or array (Ruby) - (2011-04-23) [3435] Sorta sorting a hash, and what if an exception is NOT thrown - Ruby - (2011-09-12) [3757] Ruby - a teaching example showing many of the language features in short but useful program - (2012-06-09) [4368] Shuffling a list - Ruby and Python - (2014-12-28) [4499] Significant work - beyond helloworld in Ruby - (2015-05-27) [4502] Reading and parsing a JSON object in Ruby - (2015-06-01)
Some other Articles
Ruby's case - no breakTraining on Cascading Style SheetsPositioning with Cascading Style SheetsEnthusiastic, but ....Adding a member to a Hash in RubyRuby - Totally TopicalMelksham QuizYou should think you're first in a hotel roomRuby v Perl - interpollating variablesputs - opposite of chomp in Ruby
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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
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releases, price changes, new techniques. Please check back via
our main site for current courses,
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