How do you initialize variables in Ruby?

Question

Are there any differences between the following ways of initialization of variables?

@var ||= []
@var = [] if @var.nil?
@var = @var || []

Please share your way initializing a variable and state the pros & cons.

Related question: http://stackoverflow.com/questions/963868/in-ruby-should-i-use-or-if-defined-for-memoization — Andrew Grimm, Sep 06 '10 at 23:40

score 5 · Accepted Answer · edited Sep 06 '10 at 23:42

5

@var ||= [] and @var = @var || [] are equal it will set var to [] if it's false or nil

@var = [] if @var.nil? is more specific - will re-set var to [] only if it's equal to nil

edited Sep 06 '10 at 23:42

Andrew Grimm

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answered Sep 06 '10 at 08:09

fantactuka

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So will rails automatically checks `@var` is null in the `||` case? or will `.nil?` a small small overhead? – PeterWong Sep 06 '10 at 08:46
Yes, all these examples checks for `nil`. 1st and 3rd also checks for `false` – fantactuka Sep 06 '10 at 14:11

score 5 · Answer 2 · edited May 23 '17 at 11:53

5

If you have warnings on (and you should!), @var ||= [] behaves differently to @var = @var || []:

irb(main):001:0> $VERBOSE = true
=> true
irb(main):002:0> @var ||= []
=> []
irb(main):003:0> @var2 = @var2 || []
(irb):3: warning: instance variable @var2 not initialized
=> []
irb(main):004:0>

If you wish to check whether @var is defined or not, and you're happy if it's nil or false, you can use

@var = [] unless defined?(@var)

This won't work with local variables though, as noted in In Ruby why won't foo = true unless defined?(foo) make the assignment?

edited May 23 '17 at 11:53

Community

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answered Sep 06 '10 at 23:39

Andrew Grimm

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Thanks! I didn't noticed the warning message before. BTW, how to show the warning messages in the console? Mine didn't show it. – PeterWong Sep 07 '10 at 01:42
If you're using version 1.9.2, `irb -w` or `irb -d` will turn on the warnings (I don't know whether the similar `irb -v` will work). For earlier versions, you have to set `$VERBOSE` yourself. (`irb -d` working was the result of [my bugfix](http://redmine.ruby-lang.org/issues/show/3634) that resulted from another person's SO question) – Andrew Grimm Sep 07 '10 at 02:42

How do you initialize variables in Ruby?

2 Answers2