From: A Love of Surf <wetlife@...>
Date: 2007-10-11T15:05:06+09:00
Subject: Re: |how do you know when to put a variable between pipes?|

On Oct 10, 7:46 pm, 7stud -- <dol...@excite.com> wrote:
> A Love of Surf wrote:
>
> > Hi i am at about page fifty in the pickaxe book, and i have a
> > question.  I see a lot of examples of a block that starts with
> > |SomeVar|
> > and I dont really understand how you know when you can do
> > this
>
> Whenever a method specified to the left of the block has a yield
> statement in its definition, the method is going to pass a value to the
> block.  For example:
>
> def my_method
>   yield 10
> end
>
> my_method {|num| puts num * 2}       #20
>
> How do you know if one of ruby's methods has a yield statement in its
> definition?  You could look at the source code, but for now just
> remember that when the book says that a method is an iterator, then the
> method is going to send one or more values to a block.  Or, if the book
> introduces a method and also uses a block after the method call, then
> you know the definition of the method has a yield statement in it, which
> just means it is going to send one or more values to the block.  What
> you should be trying to figure out is:
>
> 1) what values the method call sends to the block
>
> 2) what the return value of method call is
>
> The most common iterator is 'each'.  When called on an array, each sends
> each element of the array to the block one at a time.
>
> You should make frequent use of the reference section in the back of the
> book to look up methods and read about what values they send to a block,
> and what the return value of the method call is.
>
> > or what the value will be that is passed to the variable.
>
> It depends on how the method you call is defined:
>
> def my_method
>   yield 20
>   yield 30
> end
>
> my_method {|num| puts num * 2}    #40  60
>
> > What
> > is the name of  this construct or the var that goes in pipes?
>
> The construct is exactly like a method definition, and it is called a
> "block".  The thing in the pipes is the same thing as a parameter
> variable in a method definition.
>
> You are calling a method:
>
> my_method
>
> with a block specified on the right:
>
> {|num| puts num * 2}
>
> The block is just like a method, but it doesn't have a name.
>
> Backing up a little, you can define methods that call each other like
> this:
>
> def my_method
>   calculate(10)
> end
>
> def calculate(num)
>   puts num * 2
> end
>
> my_method     #20
>
> The same thing is happening with a method call that is followed by a
> block: you are calling a method that sends values to another method(the
> block).
>
> --
> Posted viahttp://www.ruby-forum.com/.

Thank you so much for your help, I was having a tough time with that
but you cleared it up for me.