# When nothing isn't

**URL:** <https://www.ruby-forum.com/t/when-nothing-isnt/89087>\
**Category:** Rails\
**Created:** [March 17, 2007, 3:45am UTC](https://www.ruby-forum.com/t/when-nothing-isnt/89087 "2007-03-17T03:45:46Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![jeffpritchard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/43a26b/32.png) [@jeffpritchard](https://www.ruby-forum.com/u/jeffpritchard)\
**Post date:** [March 17, 2007, 3:45am UTC](https://www.ruby-forum.com/t/when-nothing-isnt/89087/1 "2007-03-17T03:45:46Z")

</div>

One of the strangest things for me, coming from the “C” world to Ruby  
was how much attention “nil” gets. Sure, NULL is there in C, but it’s  
about as interesting as its value.

In Ruby, it seems as if nil is an extra value, stuffed in somewhere  
around zero, but not really there or anywhere else. It seems to be  
almost ubiquitous in Ruby. Quite strange for something that is so close  
to non-existent.

When writing rails, I find myself experimenting with various and sundry  
ways of checking for nil and not really satisfied with any of them. I  
would like to find out from the group here what are the most standard  
ways to check for nil in a couple of common situations.

One of the most common for me seems to be that of a session key that I  
want to do something with but I have to make sure it is not nil first.  
Let’s say we have a session key that is expected to have an array in it,  
and we want to check the size of the array, I wind up using a trinary:

the\_size = (session[:somethingorotherarray].nil?) ? 0 :  
session[:somethingorotherarray].size

Is there a more graceful way of doing that? I keep ending up with a  
bunch of those and also if statements with  
(!session[:somethingorotherarray].nil?). Looks really ugly to me.

Do others use “defined?” more often, and can you use that on a session  
key? Just seems like I’m working harder at avoiding nothing than I  
should. How are the rest of you dealing with nothing of this variety  
and other types of nothing that happen along frequently in a Rails app?

thanks,  
jp

---

<div class="post-metadata">

**Author:** ![jeffpritchard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/43a26b/32.png) [@jeffpritchard](https://www.ruby-forum.com/u/jeffpritchard)\
**Post date:** [March 17, 2007, 4:10am UTC](https://www.ruby-forum.com/t/when-nothing-isnt/89087/2 "2007-03-17T04:10:21Z")

</div>

On 3/16/07, Jeff P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> the\_size = (session[:somethingorotherarray].nil?) ? 0 :  
> session[:somethingorotherarray].size
> 
> Is there a more graceful way of doing that? I keep ending up with a  
> bunch of those and also if statements with  
> (!session[:somethingorotherarray].nil?). Looks really ugly to me.

So factor it out.

def my\_something  
@my\_something ||= (session[:somethingorotherarray] || [])  
end

def my\_something=(s)  
@my\_something = session[:somethingorotherarray] = s  
end

Now if it’s not defined in the session you’ll get a blank array back.

It’s a pretty basic principle…if you see duplication, factor it out.

Pat

p.s. it’s definitely not a good idea to stick a full array of objects  
in the session…at most you’d want a string/array containing the IDs  
of objects

---

<div class="post-metadata">

**Author:** ![jeffpritchard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/43a26b/32.png) [@jeffpritchard](https://www.ruby-forum.com/u/jeffpritchard)\
**Post date:** [March 17, 2007, 4:05am UTC](https://www.ruby-forum.com/t/when-nothing-isnt/89087/3 "2007-03-17T04:05:21Z")

</div>

I would dispute the notion that NULL or \0 are uninteresting in C. But  
that’s  
not the question.

The Ruby language defines the value of an unset hash member as nil.  
Therefore,

my\_hash[:bogus\_key] == nil =\> true

Further, Ruby defines the behavior of nil in a conditional as evaluating  
to  
false, e.g.:

if nil  
puts ‘it is nil’  
else  
puts ‘it has a value’  
end

The session key deal is typically done as:

login\_id = session[:user] || 0

Where the || conditional assignment provides some sensible default.

HTH

Wes G.-2 wrote:

> the\_size = (session[:somethingorotherarray].nil?) ? 0 :
> 
> thanks,  
> jp
> 
> –  
> Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).
> 
> >

–  
View this message in context:  
[http://www.nabble.com/When-nothing-isn't-tf3417890.html#a9525946](http://www.nabble.com/When-nothing-isn%27t-tf3417890.html#a9525946)  
Sent from the RubyOnRails Users mailing list archive at [Nabble.com](http://Nabble.com).

---

<div class="post-metadata">

**Author:** ![jeffpritchard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/43a26b/32.png) [@jeffpritchard](https://www.ruby-forum.com/u/jeffpritchard)\
**Post date:** [March 17, 2007, 6:51am UTC](https://www.ruby-forum.com/t/when-nothing-isnt/89087/4 "2007-03-17T06:51:14Z")

</div>

Thanks guys. The session[:unknown\_key] == nil == false thing will help  
clean up a lot of my code, but we’re still left with the ugliness of  
having to separately test something for nil before applying a method.

My example was .size on something that is expected to be an array.

Is this legal?  
(my\_maybe\_nil\_array || []).size

irb to the rescue:  
irb(main):002:0\> myhash = {}  
=\> {}  
irb(main):003:0\> myhash[:foo]  
=\> nil  
irb(main):004:0\> (myhash[:foo] || []).size  
=\> 0

So what do you guys think, is that an ugly construct, or is it a  
reasonable way to call a method on something that might be nil (assuming  
zero is a good default value for the whole mess)?

thanks,  
jp

---

<div class="post-metadata">

**Author:** ![jeffpritchard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/43a26b/32.png) [@jeffpritchard](https://www.ruby-forum.com/u/jeffpritchard)\
**Post date:** [March 17, 2007, 4:07am UTC](https://www.ruby-forum.com/t/when-nothing-isnt/89087/5 "2007-03-17T04:07:21Z")

</div>

Jeff P. wrote:

> ways of checking for nil and not really satisfied with any of them. I
> 
> Is there a more graceful way of doing that? I keep ending up with a  
> bunch of those and also if statements with  
> (!session[:somethingorotherarray].nil?). Looks really ugly to me.
> 
> Do others use “defined?” more often, and can you use that on a session  
> key? Just seems like I’m working harder at avoiding nothing than I  
> should. How are the rest of you dealing with nothing of this variety  
> and other types of nothing that happen along frequently in a Rails app?

A hash will return nil if you try to access it with a non-existent key:

irb(main):001:0\> hash = Hash.new  
=\> {}  
irb(main):002:0\> hash[:blah]  
=\> nil

This allows you get to rid of those .nil? methods on hashes if you don’t  
like them.

There is also “||=”. As in:

irb(main):003:0\> hash[:size] ||= 0  
=\> 0  
irb(main):004:0\> hash[:size] ||= 10  
=\> 0

which is the equivalent of

hash[:size] = hash[:size] || 0

so you could do something like:

irb(main):018:0\> hash = {}  
=\> {}  
irb(main):019:0\> size = (hash[:size] ||= 10)  
=\> 10  
irb(main):020:0\> size  
=\> 10  
irb(main):021:0\> hash[:size]  
=\> 10

–  
Michael W.

---

<div class="post-metadata">

**Author:** ![jeffpritchard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/43a26b/32.png) [@jeffpritchard](https://www.ruby-forum.com/u/jeffpritchard)\
**Post date:** [March 17, 2007, 7:16am UTC](https://www.ruby-forum.com/t/when-nothing-isnt/89087/6 "2007-03-17T07:16:32Z")

</div>

I’d be more tempted to write:

myhash[:foo].size rescue 0

Wes G.-2 wrote:

> Is this legal?  
> So what do you guys think, is that an ugly construct, or is it a

–  
View this message in context:  
[http://www.nabble.com/When-nothing-isn't-tf3417890.html#a9526876](http://www.nabble.com/When-nothing-isn%27t-tf3417890.html#a9526876)  
Sent from the RubyOnRails Users mailing list archive at [Nabble.com](http://Nabble.com).

---

<div class="post-metadata">

**Author:** ![jeffpritchard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/43a26b/32.png) [@jeffpritchard](https://www.ruby-forum.com/u/jeffpritchard)\
**Post date:** [March 17, 2007, 4:56pm UTC](https://www.ruby-forum.com/t/when-nothing-isnt/89087/7 "2007-03-17T16:56:09Z")

</div>

> the\_size = (session[:somethingorotherarray].nil?) ? 0 :  
> session[:somethingorotherarray].size
> 
> Is there a more graceful way of doing that?

Sure, just do:

(session[:foo] || []).size

> I keep ending up with a  
> bunch of those and also if statements with  
> (!session[:somethingorotherarray].nil?). Looks really ugly to me.

It’s not really that ugly to me! You can drop the inverse operator (!)  
if you use the ‘unless’ conditional instead if you want:

unless session[:foo].nil?

# run some code

else

# do something else

end