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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
  [4502] Reading and parsing a JSON object in Ruby - (2015-06-01)
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  [3757] Ruby - a teaching example showing many of the language features in short but useful program - (2012-06-09)
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  [3255] Process every member of an array, and sort an array - Ruby - (2011-04-21)
  [3253] Is this number between? Does this list include? - Ruby - (2011-04-18)
  [2976] Creating, extending, traversing and combining Ruby arrays - (2010-09-30)
  [2621] Ruby collections and strings - some new examples - (2010-02-03)
  [2618] What are Ruby Symbols? - (2010-02-02)
  [2606] Sorting arrays and hashes in Ruby - (2010-01-30)
  [2291] Collection objects (array and hash) in Ruby - (2009-07-16)


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