From: Dave Aronson <rubytalk2dave@...>
Date: 2011-09-29T03:18:19+09:00
Subject: Re: Ruby Syntax @keywords ||= [ ]

On Wed, Sep 28, 2011 at 13:07, Bhavesh Sharma <sharmabhavesh@gmail.com> wrote:

> Sorry if this comes across as a dumb question, but what does the
> following syntax mean in ruby.
>
> @keywords ||= []
>
> I understand that its setting the instance variable as an array but what
> is the logical operator '||' doing in there.

"x ||= y" is a common idiom for "if x is nil, set it to y".

An operator followed by the = sign is like saying "use the thing on
the left, as the left side of the operator".  In other words, the
above is equal to:

  @keywords = @keywords || []

Similarly, "x += 1" is equal to "x = x + 1", and so on for all other operators.

The || operator specifically won't bother evaluating its right-side
operator, if the left is true.  In Ruby, two things are not true:
false, and nil.  So, if the thing on the left is nil (or boolean
false), it gets the value on the right, else it gets left alone.

-Dave

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