More on the fundamentals


#1

Please have a look at the comments below to see that I’m understanding
things correctly;

#!/usr/bin/ruby
#Tue Jan 3 14:04:28 GMT 2006
class Greeter
# a new class cllaed Greeter
def initialize(name)
# creating a new method called name
@name = name
# we define a new @instance variable called name
end
def say(phrase)
# creating a new method called phrase…
puts “#{phrase}, #{@name}”
# … which uses doube quotes to substitute
end
end

say hello

g1 = Greeter.new(“jayeola”)
g2 = Greeter.new(“buddy”)
g3 = Greeter.new(“vimmer”)
g4 = Greeter.new(“slammer”)

we have just created four new (object) instances belonging to the

class

Greeter. They can now use the instance variables that have been

created

above

g1.say(“Hello”)
g2.say(“Wotcha”)
g3.say(“Ire!”)
g4.say(“Elake”)


#2

Hi –

On Tue, 3 Jan 2006, John M. wrote:

Please have a look at the comments below to see that I’m
understanding things correctly;

#!/usr/bin/ruby
#Tue Jan 3 14:04:28 GMT 2006
class Greeter
# a new class cllaed Greeter
def initialize(name)
# creating a new method called name

There’s no method called name.

           @name = name
           # we define a new @instance variable called name

Move the @ from @instance to name and you’re set :slight_smile:

   end
   def say(phrase)
           # creating a new method called phrase...

There’s no method called phrase.

           puts "#{phrase}, #{@name}"
           # ... which uses doube quotes to substitute
   end

end

It seems that you’re confusing instance variable assignment with
method creation. They’re unrelated. You can, of course, write a
method that returns the current value of an instance variable:

def name
@name
end

and you can even get Ruby to do it semi-automatically for you:

attr_reader :name # creates the above “name” method

But @name = name is just a variable assignment.

say hello

g1 = Greeter.new(“jayeola”)
g2 = Greeter.new(“buddy”)
g3 = Greeter.new(“vimmer”)
g4 = Greeter.new(“slammer”)

we have just created four new (object) instances belonging to the class

Greeter. They can now use the instance variables that have been created

I would say “instances of the class Greeter”.

above

g1.say(“Hello”)
g2.say(“Wotcha”)
g3.say(“Ire!”)
g4.say(“Elake”)

Have another look at the attr_* family of methods, and keep in mind
that they’re essentially shortcuts for writing your own get-and/or-set
methods (using instance variables), and I think that part of it will
fall into place.

David


David A. Black
removed_email_address@domain.invalid

“Ruby for Rails”, from Manning Publications, coming April 2006!


#3

On Jan 3, 2006, at 14:51, John M. wrote:

Please have a look at the comments below to see that I’m
understanding things correctly;

You’ve made a couple of fundamental mistakes regarding how methods
are defined. The basic schema is:

def method_name(parameter_name)

Where you can have a whole list of parameters, or none at all. See
the revised (and very verbose comments. Hope this helps.

matt smillie.

Also, I’ve changed the indented to two spaces, since that’s the Ruby
norm.

class Greeter

open class Greeter (it’s not necessarily new)

def initialize(name)
# defining the initialise method, which is called when a new
# object is created. The method takes one parameter, ‘name’.
# Example (such as: foo = Greeter.new(“Mark”).
@name = name
# define an instance variable called ‘@name’ and assign it
# the value of the parameter ‘name’
end

def say(phrase)
# defining a new method called ‘say’ with a parameter
# called ‘phrase’.
puts “#{phrase}, #{@name}”
# The double quotes define a string, and the #{} sections are
# used for interpolation (substitution).
end
end


#4

John M. removed_email_address@domain.invalid writes:

In this line of code can anyone tell me why “food” is a method? I
thought that they are preceded by a dot.

It is not a method, it’s a variable, very possibly a block-local one.

__  an array           __ the method   __ parameters for code block

/ / /
[cornflakes, yam, rice].each { |food| eat food}
/
code block __/ __block args

^^^ is parsed as: [corflakes, yam, rice].each {|food| eat(food)}

YS.


#5

Quoting John M. removed_email_address@domain.invalid:

In this line of code can anyone tell me why “food” is a method? I
thought that they are preceded by a dot.

__  an array           __ the method   __ parameters for code

block
/ / /
[cornflakes, yam, rice].each { |food| eat food}
/
code block __/ __block args

Within the context of the block, “food” is a variable. “eat”,
however, will be interpreted as a private method call (i.e. as a
call to the “eat” method on self), provided an “eat” variable does
not already exist in that scope.

Generally, given a bare identifier, the rule is:

  1. If a previous assignment to the name exists in the current scope
    (whether or not it was actually performed), the name is a variable
    (or a parse error if you try to give it parameters)

  2. otherwise, it is a call to a private method on self

Examples:

  1. foo # method

  2. foo = 1
    foo # variable

  3. if false
    foo = 1 # never executed
    end
    foo # still a variable (initialized to nil)

  4. [1, 2, 3].each { |foo| # introduced as block param
    foo # variable here
    }
    foo # method call here

  5. foo = 1
    foo # variable
    [1, 2, 3].each { |foo| # same variable
    foo # yes, same variable
    }
    foo # still same variable

-mental


#6

In this line of code can anyone tell me why “food” is a method? I
thought that they are preceded by a dot.

__  an array           __ the method   __ parameters for code block

/ / /
[cornflakes, yam, rice].each { |food| eat food}
/
code block __/ __block args

On Tue, 3 Jan 2006 23:51:15 +0900