From: Stefano Crocco <stefano.crocco@...>
Date: 2009-09-25T18:10:02+09:00
Subject: Re: Roulette & rand

On Friday 25 September 2009, Semih Ozkoseoglu wrote:
> |Hi again Stefano,
> |
> |I was looking at the code and as I'm quite new to both programming and
> |Ruby, I can't seem to understand two things. First is the |i| in
> |
> |> [2, 4, 6, 8, 10, 11, 13, 15, 17, 20, 22, 24, 26, 28, 29, 31,33,35].each
> |> do |i|

I don't know whether you've already read about blocks in ruby. Given your 
question, I don't think so but, in case you did, you can skip the first part 
of my answer.

One of the most peculiar characteristics of ruby are blocks. As the name says, 
they are blocks of code which can be passed to methods and will be called by 
those methods at a later time. They can be introduced either with the syntax

do
...
end

or with
{...}

The ... are actually the body of the block, that is the code which will be 
executed later. For example, the code

5.times do 
  puts "Hello"
end

passes a block whose body is

puts "Hello"

to the "times" method of the object 5. The documentation for the times method 
says that n.times calls the block n times, that is it executes the contents of 
the block n times. In the example above, n is 5, so times will call execute 
the body of the block 5 times. The output is then:

Hello
Hello
Hello
Hello
Hello

Now, I can answer your question: what does 
[2,4,6].each do |i|
...
end
mean? As you can see, there are a do and an end keyword, so this must be a 
block. The difference with the example above is the |i|. At the very beginning 
of the block, immediately after the do keyword or the opening brace, a number 
of comma-separated variables (just one, in this case) enclosed in a pair of 
pipe characters (|) are the arguments passed to the block. Passing arguments 
to a block works in a similar way as passing arguments to a method. In the 
code above, for example, the each method of class Array is called. According 
to the documentation, the each method calls the block for every element of the 
array, passing the element itself to the block. This means that, inside the 
body of the block, the variable i will contain one of the elements of the 
array in turn. For example:

[2,4,6].each do |i|
  puts i
end

outputs:
2
4
6

"each" calls the block three times: the first the argument is 2, so the 
variable i is set to 2; the second time i is set to 4 and the third time i is 
set to 6.

This is only a very short introduction to blocks. For more, you can look at 
http://www.ruby-doc.org/docs/ProgrammingRuby/html/tut_containers.html (the 
section called "Block and Iterators")


> |and second is the ':' in why not just 'black' for example.

In ruby, a sequence of letters, number, underscores (and some other 
characters) preceeded by a column (for example, :black) is a Symbol. Symbols 
are similar to strings. The main differences is that while every time I write 
a string literal, for example "hello", I create a different string, with 
symbols this doesn't happen: if I write the symbol :hello one hundred times, I 
still get the same object every time. If I had written "hello" one hundred 
times, I'd have got one hundred strings all containing the word "hello". 
Expecially when beginning programming in ruby, it may be difficult to 
understand when it's better to use a symbol rather than a string (you can find 
a lot of material on this topic searching google for something like "strings 
and symbols"). Symbols are often used as hash keys when you know in advance 
which keys are allowed. For example, in your program you already know that the 
only possible outcomes of a roll are black, red or zero, so you can use a 
symbols as keys. A situation in which symbols wouldn't be a good choice is if 
you're storing information about people in the hash, using each person's name 
as keys. Obviously, in this case you don't know in advance which would be the 
keys of the array, so you should use strings instead.

If you know C or C++, symbols are somewhat similar to enums, except that they 
don't have a numerical value associated with them.

> |
> |>   colors[i] = :black
> |
> |If you can find the time to tell me what do these do to make the code
> |work it would be much apriciated.

I hope this helps

> |Thanks again
> |Semih

Stefano