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DesertFox

2006-Oct-02 22:23 UTC

Newbie Code question

Hello,

I reading the Agile Development Ruby book and came accross a line of 
code that I dont quite understand...

errors.add(:price, "should be positive") unless price.nil? || price
>
0.0

The line of code works but I dont quite understand the logic behind it. 
It prints the error message "price should be positive" when the price 
entered is either 0 or less than 0.

This is where i dont understand the logic - the price.nil operator 
returns a true if price is zero and the price > 0 returns a true if the 
price is more than zero (logically this would have to be price < 0)?

If someone could explain the logic here then that would be great.

Thanks in advance!

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Daniel N

2006-Oct-02 23:22 UTC

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Re: Newbie Code question

On 10/3/06, DesertFox
<rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:>
>
> errors.add(:price, "should be positive") unless price.nil? ||
price >
> 0.0
>
> The line of code works but I dont quite understand the logic behind it.
> It prints the error message "price should be positive" when the
price
> entered is either 0 or less than 0.
>
> This is where i dont understand the logic - the price.nil operator
> returns a true if price is zero and the price > 0 returns a true if the
> price is more than zero (logically this would have to be price < 0)?

price.nil? should not return true if the value of price is 0.  nil is an
object that is the anti-matter of objects.  It is something that isn''t
really there.  It''s not 0, it''s not a blank string, they would
be
something.   No, this is nothing.  ( does anyone have a racing snail? )  The
nil? method, available on all objects, will only return true if the return
value is nil ( ie nothing )

So the logic of the code says

add an error to the price field unless price is nil (nothing) or if price is
greater than 0.

This means that price can be nil (or blank or however you want to think
about it but blank is another method in rails) or it can be positive.  Any
other value and the error gets added to the price attribute.

Hope that helps a bit.

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Michael Campbell

2006-Oct-02 23:29 UTC

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Re: Newbie Code question

On 10/2/06, DesertFox
<rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:>
> Hello,
>
> I reading the Agile Development Ruby book and came accross a line of
> code that I dont quite understand...
>
> errors.add(:price, "should be positive") unless price.nil? ||
price >
> 0.0
> This is where i dont understand the logic - the price.nil operator
> returns a true if price is zero
No.  0 is not nil.

irb(main):001:0> a = nil
=> nil
irb(main):002:0> a.nil?
=> true
irb(main):003:0> a = 0
=> 0
irb(main):004:0> a.nil?
=> false



Ugh, how I hate "unless"...

Unless is like "if not".  So...

errors.add()  <if not> (price.nil? || price > 0.0)

So, when your price is <= 0.0...

errors.add()  <if not> (false || false) # since price.nil? is false,
and price > 0.0 is false

errors.add()  <if not> (false) # since (false || false) is false

so errors are added.

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DesertFox

2006-Oct-03 07:53 UTC

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Re: Newbie Code question

Michael Campbell wrote:> Ugh, how I hate "unless"...
> 
> Unless is like "if not".  So...
> 
> errors.add()  <if not> (price.nil? || price > 0.0)
> 
> so errors are added.
Thanks for the replies guys.  So, what would be a more sensible way to
write that line?

Thanks in advance.

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Chris Gernon

2006-Oct-03 19:33 UTC

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Re: Newbie Code question

DesertFox wrote:> Thanks for the replies guys.  So, what would be a more sensible way to
> write that line?
> 
> Thanks in advance.
I would say something like:

errors.add(:price, "should be positive") if !price.nil? &&
price <= 0.0

Or, to be a bit more verbose:

if !price.nil?
  if price <= 0.0
    errors.add(:price, "should be positive")
  end
end

I guess that''s not a huge improvement. Basically, one way or another, 
you have to check whether price is nil first, and then, if it isn''t, 
check whether it''s less than or equal to 0, and that''s what
makes the
line a bit complicated. Hope this helps!

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Philip Hallstrom

2006-Oct-03 20:00 UTC

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Re: Newbie Code question

>> Thanks for the replies guys.  So, what would be a more sensible way to
>> write that line?
>>
>> Thanks in advance.
>
> I would say something like:
>
> errors.add(:price, "should be positive") if !price.nil?
&& price <= 0.0
>
> Or, to be a bit more verbose:
>
> if !price.nil?
>  if price <= 0.0
>    errors.add(:price, "should be positive")
>  end
> end
>
> I guess that''s not a huge improvement. Basically, one way or
another,
> you have to check whether price is nil first, and then, if it
isn''t,
> check whether it''s less than or equal to 0, and that''s
what makes the
> line a bit complicated. Hope this helps!
Why not...

errors.add(:price, "should be positive") if price.to_f < 0

This should work since...
>> nil.to_f=> 0.0


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DesertFox

2006-Oct-04 04:14 UTC

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Re: Newbie Code question

Thanks guys.  That helped.  Now i can continue with the book.


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njmacinnes-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org

2006-Oct-04 06:10 UTC

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Re: Newbie Code question

Or,

errors.add(:price, "should be positive") if !(price > 0)

-Nathan



On 03/10/06, Philip Hallstrom
<rails-SUcgGwS4C16SUMMaM/qcSw@public.gmane.org>
wrote:>
> >> Thanks for the replies guys.  So, what would be a more sensible
way to
> >> write that line?
> >>
> >> Thanks in advance.
> >
> > I would say something like:
> >
> > errors.add(:price, "should be positive") if !price.nil?
&& price <= 0.0
> >
> > Or, to be a bit more verbose:
> >
> > if !price.nil?
> >  if price <= 0.0
> >    errors.add(:price, "should be positive")
> >  end
> > end
> >
> > I guess that''s not a huge improvement. Basically, one way or
another,
> > you have to check whether price is nil first, and then, if it
isn''t,
> > check whether it''s less than or equal to 0, and
that''s what makes the
> > line a bit complicated. Hope this helps!
>
> Why not...
>
> errors.add(:price, "should be positive") if price.to_f < 0
>
> This should work since...
>
> >> nil.to_f
> => 0.0
>
>
> >
>
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njmacinnes-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org

2006-Oct-04 06:14 UTC

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Re: Newbie Code question

Oops, sorry. Just realised nil is supposed to be a valid answer.

errors.add(:price, "should be positive") if !(price > 0 ||
price.nil?)

-Nathan

On 04/10/06, njmacinnes-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
<njmacinnes-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:> Or,
>
> errors.add(:price, "should be positive") if !(price > 0)
>
> -Nathan
>
>
>
> On 03/10/06, Philip Hallstrom
<rails-SUcgGwS4C16SUMMaM/qcSw@public.gmane.org> wrote:
> >
> > >> Thanks for the replies guys.  So, what would be a more
sensible way to
> > >> write that line?
> > >>
> > >> Thanks in advance.
> > >
> > > I would say something like:
> > >
> > > errors.add(:price, "should be positive") if !price.nil?
&& price <= 0.0
> > >
> > > Or, to be a bit more verbose:
> > >
> > > if !price.nil?
> > >  if price <= 0.0
> > >    errors.add(:price, "should be positive")
> > >  end
> > > end
> > >
> > > I guess that''s not a huge improvement. Basically, one
way or another,
> > > you have to check whether price is nil first, and then, if it
isn''t,
> > > check whether it''s less than or equal to 0, and
that''s what makes the
> > > line a bit complicated. Hope this helps!
> >
> > Why not...
> >
> > errors.add(:price, "should be positive") if price.to_f <
0
> >
> > This should work since...
> >
> > >> nil.to_f
> > => 0.0
> >
> >
> > > >
> >
>
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Alan C Francis

2006-Oct-04 09:09 UTC

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Re: Newbie Code question

Chris Gernon wrote:> DesertFox wrote:
>> Thanks for the replies guys.  So, what would be a more sensible way to
>> write that line?
>> 
>> Thanks in advance.
> 
> I would say something like:
> 
> errors.add(:price, "should be positive") if !price.nil?
&& price <= 0.0
> 
> Or, to be a bit more verbose:
> 
> if !price.nil?
>   if price <= 0.0
>     errors.add(:price, "should be positive")
>   end
> end
> 
> I guess that''s not a huge improvement. Basically, one way or
another,
> you have to check whether price is nil first, and then, if it
isn''t,
> check whether it''s less than or equal to 0, and that''s
what makes the
> line a bit complicated. Hope this helps!
*cough*Java*cough*

What makes the line above a bit complicated is not using the beauty and 
power of ''unless'' which exists entirely to stop you having to
contort
boolean logic like this :-)

"Add the error unless the price is nil or positive"

versus

"If the price is not nil or if the price is less than or equal to zero, 
add an error."

I know which reads more easily for me :-)

A.


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Al Cholic

2006-Oct-06 05:04 UTC

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Re: Newbie Code question

So, what does this .nil statement do exactly?  I think thats whats 
causing the confusion for me.


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Daniel N

2006-Oct-06 05:22 UTC

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Re: Newbie Code question

On 10/6/06, Al Cholic
<rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:>
>
> So, what does this .nil statement do exactly?  I think thats whats
> causing the confusion for me.

It''s actuall a question posed to the object
something.nil?

The method includes the ?

It says to the object "Hey, are you nil?"


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