A little Ruby help

[Mark H]

I see the construct below in many of the RubyMotion samples & open source code.  Can anyone explain what is going on here.  Does this assign a block to an instance variable?  If that's the case, then every time the instance variable is referenced does the block execute?

  def fonts

    @fonts ||= begin

      UIFont.familyNames.sort.map do |family|

        fonts = UIFont.fontNamesForFamilyName(family).sort

        {:family => family, :fonts => fonts}

      end

    end

  end

[Alan deLevie]

The "||=" (instead of just "=") means it's memoizing--a fancy word for "if the variable is false, assign it this value. Otherwise, don't do anything." 

[Mark H]

Thanks!  I was confused by the "||=", thanks for explaining it.  I have the Pickaxe book, but cannot find an explanation for "||=" anywhere in the book.

[Bruce Hobbs]

On Jul 29, 2012, at 9:04 AM, Mark H <meh...@gmail.com> wrote:

I have the Pickaxe book, but cannot find an explanation for "||=" anywhere in the book.

The index entry on page 812 points to section 9.4, "Conditional Execution," on page 126. The discussion begins at the bottom of that page.

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[Ben Taylor]

To understand better how this construct works, try writing in irb

>> nil || "steve"

>> "steve" || "john"

>> "steve" || nil

>> nil && "steve"

>> "steve" && nil

>> "steve" && "john"

The construct ||= is shorthand for

>> @blah = @blah || "something"

It's common practice in many programming languages to use boolean operators like this as shorthand for using an if statement.

>> @blah ||= "something"

Can also be written as

>> @blah = "something" unless @blah

In more verbose languages it would need to be a full if statement.

[Bruce Hobbs]

On Jul 29, 2012, at 6:02 PM, Ben Taylor <m...@taybenlor.com> wrote:

The construct ||= is shorthand for

>> @blah = @blah || "something"

Not that it's a big difference, but in the "Magic Defined" section of the article "Even More Eloquent Ruby" <http://subinterest.com/rubies-in-the-rough/5-even-more-eloquent-ruby> from his unfortunately short-lived newsletter Rubies in the Rough, James Edward Gray III turned on Ruby warnings to demonstrate that it's actually closer to

@blah = (defined?(@blah) && @blah) || "something"