From: "David A. Black" <dblack@...>
Date: 2010-08-19T06:17:02+09:00
Subject: Re: Check existence of object and it's property at the same time

Hi --

On Thu, 19 Aug 2010, Cory Patterson wrote:

> I run into this from time to time and I was wondering if there is a
> cleaner way to do check that an object is not nil and also check a
> property of that object.
>
> I want to do this:
>
> if !@member.nil? and @member.is_active?
>  ... do something ...
> else
>  ... do something else ...
> end
>
> But this obviously raises a no method error if @member is nil.

It doesn't, actually, because if @member is nil, the "and" is
short-circuited so the right-hand expression doesn't get evaluated:

>> if !@member.nil? and @member.is_active?; end
=> nil
>> @member
=> nil

A somewhat more common way of doing this is:

   if @member && @member.is_active? ... end

There's a difference, though, which is that "if @member" is false if
@member is false, not just if @member is nil. I'm a big non-fan of
tri-state booleans involving true/false/nil, so I try to avoid
situations where I have to distinguish between nil and false for
condition-testing purposes.


David

-- 
David A. Black, Senior Developer, Cyrus Innovation Inc.

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