# Explaination of a bit of code

**URL:** <https://www.ruby-forum.com/t/explaination-of-a-bit-of-code/149122>\
**Category:** Ruby\
**Created:** [September 20, 2008, 11:40pm UTC](https://www.ruby-forum.com/t/explaination-of-a-bit-of-code/149122 "2008-09-20T23:40:24Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![paradise](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/8e7dd6/32.png) [@paradise](https://www.ruby-forum.com/u/paradise)\
**Post date:** [September 20, 2008, 11:40pm UTC](https://www.ruby-forum.com/t/explaination-of-a-bit-of-code/149122/1 "2008-09-20T23:40:24Z")

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Hey, another question. This is kinda putting several concepts together  
that i understand but the code is throwing me for some reason:

class Periods  
def each  
yield “Classical”  
yield “Jazz”  
yield “Rock”  
end  
end

periods = Periods.new  
for genre in periods  
print genre, " "  
end

produces:  
Classical Jazz Rock

So a simple for…in loop which is the same as using the .each for an  
array/Hash, etc.

I guess I am having trouble thinking how this would work out. Each is  
suppose to go through a set of items. however, where are the items? I  
see three strings but they are used in yield statements. How is this  
method actually working?

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<div class="post-metadata">

**Author:** ![paradise](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/8e7dd6/32.png) [@paradise](https://www.ruby-forum.com/u/paradise)\
**Post date:** [September 20, 2008, 11:57pm UTC](https://www.ruby-forum.com/t/explaination-of-a-bit-of-code/149122/2 "2008-09-20T23:57:35Z")

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Dave L. wrote:

> Each is suppose to go through a set of items.

each is supposed to whatever it has been defined to do. It’s not  
special.  
It’s not different from any other method. You can define it to do  
whatever you  
want. And in this case it is defined to yield three strings so that’s  
what it  
does.

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**Author:** ![paradise](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/8e7dd6/32.png) [@paradise](https://www.ruby-forum.com/u/paradise)\
**Post date:** [September 20, 2008, 11:58pm UTC](https://www.ruby-forum.com/t/explaination-of-a-bit-of-code/149122/3 "2008-09-20T23:58:01Z")

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And while we are at little samples of code…

if false  
a = 1  
end  
3.times {|i| a = i }

a --\> 2

I was looking at an example and was a bit confused why variable a got 2.  
I undrestand the basic idea of this example (as long as a is defined  
somewhere, evne if it’s value is never reached, it exists), but why 2?  
Is the times method going from 0 to X-1 where X is the number in  
X.times?

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<div class="post-metadata">

**Author:** ![paradise](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/8e7dd6/32.png) [@paradise](https://www.ruby-forum.com/u/paradise)\
**Post date:** [September 21, 2008, 12:06am UTC](https://www.ruby-forum.com/t/explaination-of-a-bit-of-code/149122/4 "2008-09-21T00:06:57Z")

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Dave L. wrote:

> Is the times method going from 0 to X-1 where X is the number in  
> X.times?

Yes.  
Try this: 3.times {|i| p i}  
Prints:  
0  
1  
2

HTH,  
Sebastian

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**Author:** ![paradise](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/8e7dd6/32.png) [@paradise](https://www.ruby-forum.com/u/paradise)\
**Post date:** [September 20, 2008, 11:59pm UTC](https://www.ruby-forum.com/t/explaination-of-a-bit-of-code/149122/5 "2008-09-20T23:59:52Z")

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Dave L. wrote:

> I guess I am having trouble thinking how this would work out. Each is  
> suppose to go through a set of items. however, where are the items? I  
> see three strings but they are used in yield statements. How is this  
> method actually working?

The method each is working just as you programme it. For Arrays (and  
Enumerables, in general) it calls the passed block with each element of  
the object (self) in turn, so in some useless pseudocode, Array#each  
works like this:  
for each element of self in e  
yield(e)  
end  
Now you can see that your own each works just like  
[“Classical”,“Jazz”,“Rock”].each{|e| yield(e)}  
So, your Periods::new works like the array [“Classical”,“Jazz”,“Rock”]  
itself. Only your object is a dummy array - it does not read the  
elements from any kind of set or anything, it is just hardcoded to call  
yield with these three strings. But from outside you cannot tell your  
Period::new from [“Classical”,“Jazz”,“Rock”], they just behave in the  
same way when we call the method each on them. And that’s why the for  
loop sees your object as if it was the array of the three strings.

TPR.

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<div class="post-metadata">

**Author:** ![paradise](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/8e7dd6/32.png) [@paradise](https://www.ruby-forum.com/u/paradise)\
**Post date:** [September 21, 2008, 12:37am UTC](https://www.ruby-forum.com/t/explaination-of-a-bit-of-code/149122/6 "2008-09-21T00:37:31Z")

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Thanks all.

I should note that these are not my programs but snippets from a book  
(Programming Guide to Ruby) that I am trying to make sure I understand.

Here’s another snippet I am wondering on:

catch (:done) do  
while line = gets  
throw :done unless fields = line.split(/\t/)  
songlist.add(Song.new(\*fields))  
end  
songlist.play  
end

So, according to the book, it’s looking for badly formatted strings. If  
there is a badly formatted string, the throw clause is invoked. Now,  
the question is, where does the throw clause go. I see that catch  
(:done) is right there, but how does the throw clause “find” the catch  
clause.

In other words, I am trying to understand howt his works compared to…

while line = gets  
throw :done unless fields = line.split (/\t/)  
songlist.add(Song.new(\*fields))  
end  
end

catch :done  
#handle issues  
end

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**Author:** ![paradise](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/8e7dd6/32.png) [@paradise](https://www.ruby-forum.com/u/paradise)\
**Post date:** [September 21, 2008, 10:01pm UTC](https://www.ruby-forum.com/t/explaination-of-a-bit-of-code/149122/7 "2008-09-21T22:01:48Z")

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Dave L. wrote:

> Here’s another snippet I am wondering on:
> 
> catch (:done) do  
> while line = gets  
> throw :done unless fields = line.split(/\t/)  
> songlist.add(Song.new(\*fields))  
> end  
> songlist.play  
> end
> 
> So, according to the book, it’s looking for badly formatted strings. If  
> there is a badly formatted string, the throw clause is invoked. Now,  
> the question is, where does the throw clause go.

The throw has to be _inside_ a corresponding catch block, and it drops  
you to the end of the catch block.

You can also give a value alongside the throw; if so, this is the value  
“returned” by the catch block itself.

ans = catch(:hello) do  
throw :hello, 2  
puts “Never reached”  
3  
end  
p ans # 2

> I see that catch  
> (:done) is right there, but how does the throw clause “find” the catch  
> clause.

It’s pretty much the same as an exception. The interpreter winds back  
the stack looking for a matching ‘catch’ block.

> In other words, I am trying to understand howt his works compared to…
> 
> while line = gets  
> throw :done unless fields = line.split (/\t/)  
> songlist.add(Song.new(\*fields))  
> end  
> end
> 
> catch :done  
> #handle issues  
> end

That won’t work, because when the throw executes, there is no enclosing  
matching catch block.