I thought that defining a to_s method for a custom class meant that calling the puts method on that class would return an output as specified by to_s. In this program, however, I only get the result I crave if I write puts bingo_board.to_s. What is going on?
class BingoBoard < Array
##letters = %w[B I N G O]
def initialize
# populates an 5x5 array with numbers 1-100
# to make this accessible across your methods within this class, I made
# this an instance variable. # = instance variable
#bingo_board = Array.new(5) {Array.new(5)}
#bingo_board.each_with_index do |column, i|
rangemin = 15 * i + 1
#bingo_board[i] = (rangemin..(rangemin+14)).to_a.sample(5)
end
#bingo_board[2][2] = "X" # the 'free space' in the middle
#game_over = false
end
def game_over?
#game_over
end
def generate_call
....
end
def compare_call(call)
#bingo_board[##letters.index(call[0])].include? call[1]
end
def react_to_call(call)
...
end
def check_board
...
end
def show_column(num)
...
end
def to_s
result = ""
0.upto(4) do |val|
result += " " + ##letters[val] + " "
end
result += "\n\n"
0.upto(4) do |row|
0.upto(4) do |col|
val = #bingo_board[col][row]
result += " " if val.to_i < 10
result += val.to_s + " "
end
result += "\n"
end
result
end
end
my_board = BingoBoard.new
counter = 0
until my_board.game_over?
puts my_board.to_s # renders the board in accordance with my to_s method
call = my_board.generate_call
counter += 1
puts "\nThe call \# #{counter} is #{call[0]} #{call[1]}"
my_board.react_to_call(call)
gets.chomp
end
puts my_board # renders bubkes (i.e., nothing)
puts "\n\n"
puts "Game over"
Its because you'r extending from Array. That's why you're getting the wierd behavior. I don't see where you need the extending from so just remove that and things will work as you expect.
Here's a more detaled answer if you'd like to know why this is happening. Basically puts makes an exception for arrays so when an array is passed puts is called on each member. Ruby Array#puts not using overridden implementation?
As #jörgwmittag said, this is a special case. The IO#puts method treats arrays - which means anything that responds to to_ary - differently. It calls to_ary first and then iterates over each element of the resulting array, and only calls to_s on them. It never calls to_s on the array itself.
If you delegate to a member array instead of subclassing from Array, you have finer-grained control over what gets "inherited" (delegated). Then you can exclude to_ary from the delegation, which will prevent puts from seeing your object as an Array and triggering this behavior.
Other general solutions:
Use string interpolation or explicit to_s calls so that what puts receives is already a string:
puts "#{bingo_board}"
puts bingo_board.to_s
Use print or printf instead of puts:
print bingo_board,"\n"
printf "%s\n",bingo_board
If the object is an Array or can be converted to one (i.e. it implements to_ary), then puts will not call to_s on the object, but rather iterate over the object and print each object within by calling to_s on it.
See:
puts [1, 2]
# 1
# 2
[1, 2].to_s
# => '[1, 2]'
This is actually documented, although somewhat implicitly:
If called with an array argument, writes each element on a new line.
Looks like it runs Array#inspect method instead of your custom to_s. Adding alias_method :inspect, :to_s just after ending of the to_s definition will help you.
But it'll work only with p, because puts runs each(&:inspect).
def any?
if block_given?
method_missing(:any?) { |*block_args| yield(*block_args) }
else
!empty?
end
end
In this code from ActiveRecord, what is the purpose of a yield statement that exists within a block?
Basically if the current method has been given a code-block (by the caller, when it was invoked), the yield executes the code block passing in the specified parameters.
[1,2,3,4,5].each { |x| puts x }
Now { |x| puts x} is the code-block (x is a parameter) passed to the each method of Array. The Array#each implementation would iterate over itself and call your block multiple times with x = each_element
pseudocode
def each
#iterate over yourself
yield( current_element )
end
Hence it results
1
2
3
4
5
The *block_args is a Ruby way to accept an unknown number of parameters as an array. The caller can pass in blocks with different number of arguments.
Finally let's see what yield within a block does.
class MyClass
def print_table(array, &block)
array.each{|x| yield x}
end
end
MyClass.new.print_table( [1,2,3,4,5] ) { |array_element|
10.times{|i| puts "#{i} x #{array_element} = #{i*array_element}" }
puts "-----END OF TABLE----"
}
Here Array#each yields each element to the block given to MyClass#print_table...
It does not mean anything special. It's just a yield like any other yield.
def test_method
["a", "b", "c"].map {|i| yield(i) }
end
p test_method {|i| i.upcase }
# => ["A", "B", "C"]
In the code snippet from active record, the purpose is to yield every time the block of any? is called.
This helped me understand: yield is a way to insert blocks into a method you already wrote, which means "execute something here". For instance,
Without parameter
def original_method
puts "a"
puts "b"
end
What if you want to execute some unknown block between those two p?
def updated_method
puts "a"
yield
puts "b"
end
# these two are the same:
updated_method { puts "execute something here" }
updated_method do puts "execute something here" end
The result would be
a
execute something here
b
You can have as many yields in a method as you like.
With parameter
def updated_method
puts "a"
yield
puts "b"
end
What if you want to execute some unknown block on the string "Execute something on me!"?
def updated_method_with_argument
puts "a"
yield("Execute something on me!")
puts "b"
end
updated_method_with_argument do |argument|
puts "This argument gets put out: " << argument
end
The result would be
a
This argument gets put out: Execute something on me!
b
Assuming I have the following proc:
a = Proc.new do
puts "start"
yield
puts "end"
end
Also assuming I pass a to another method which subsequently calls instance_eval on another class with that block, how can I now pass a block onto the end of that method which gets yielded in a.
For example:
def do_something(a,&b)
AnotherClass.instance_eval(&a) # how can I pass b to a here?
end
a = Proc.new do
puts "start"
yield
puts "end"
end
do_something(a) do
puts "this block is b!"
end
Output should of course should be:
start
this block is b!
end
How can I pass the secondary block to a in the instance_eval?
I need something like this for the basis of a Ruby templating system I'm working on.
You can't use yield in a. Rather, you have to pass a Proc object. This would be the new code:
def do_something(a,&b)
AnotherClass.instance_exec(b, &a)
end
a = Proc.new do |b|
puts "start"
b.call
puts "end"
end
do_something(a) do
puts "this block is b!"
end
yield is only for methods. In this new code, I used instance_exec (new in Ruby 1.9) which allows you to pass parameters to the block. Because of that, we can pass the Proc object b as a parameter to a, which can call it with Proc#call().
a=Proc.new do |b|
puts "start"
b.call
puts "end"
end
def do_something(a,&b)
AnotherClass.instance_eval { a.call(b) }
end
def any?
if block_given?
method_missing(:any?) { |*block_args| yield(*block_args) }
else
!empty?
end
end
In this code from ActiveRecord, what is the purpose of a yield statement that exists within a block?
Basically if the current method has been given a code-block (by the caller, when it was invoked), the yield executes the code block passing in the specified parameters.
[1,2,3,4,5].each { |x| puts x }
Now { |x| puts x} is the code-block (x is a parameter) passed to the each method of Array. The Array#each implementation would iterate over itself and call your block multiple times with x = each_element
pseudocode
def each
#iterate over yourself
yield( current_element )
end
Hence it results
1
2
3
4
5
The *block_args is a Ruby way to accept an unknown number of parameters as an array. The caller can pass in blocks with different number of arguments.
Finally let's see what yield within a block does.
class MyClass
def print_table(array, &block)
array.each{|x| yield x}
end
end
MyClass.new.print_table( [1,2,3,4,5] ) { |array_element|
10.times{|i| puts "#{i} x #{array_element} = #{i*array_element}" }
puts "-----END OF TABLE----"
}
Here Array#each yields each element to the block given to MyClass#print_table...
It does not mean anything special. It's just a yield like any other yield.
def test_method
["a", "b", "c"].map {|i| yield(i) }
end
p test_method {|i| i.upcase }
# => ["A", "B", "C"]
In the code snippet from active record, the purpose is to yield every time the block of any? is called.
This helped me understand: yield is a way to insert blocks into a method you already wrote, which means "execute something here". For instance,
Without parameter
def original_method
puts "a"
puts "b"
end
What if you want to execute some unknown block between those two p?
def updated_method
puts "a"
yield
puts "b"
end
# these two are the same:
updated_method { puts "execute something here" }
updated_method do puts "execute something here" end
The result would be
a
execute something here
b
You can have as many yields in a method as you like.
With parameter
def updated_method
puts "a"
yield
puts "b"
end
What if you want to execute some unknown block on the string "Execute something on me!"?
def updated_method_with_argument
puts "a"
yield("Execute something on me!")
puts "b"
end
updated_method_with_argument do |argument|
puts "This argument gets put out: " << argument
end
The result would be
a
This argument gets put out: Execute something on me!
b
I'm not sure of the best idiom for C style call-backs in Ruby - or if there is something even better ( and less like C ). In C, I'd do something like:
void DoStuff( int parameter, CallbackPtr callback )
{
// Do stuff
...
// Notify we're done
callback( status_code )
}
Whats a good Ruby equivalent? Essentially I want to call a passed in class method, when a certain condition is met within "DoStuff"
The ruby equivalent, which isn't idiomatic, would be:
def my_callback(a, b, c, status_code)
puts "did stuff with #{a}, #{b}, #{c} and got #{status_code}"
end
def do_stuff(a, b, c, callback)
sum = a + b + c
callback.call(a, b, c, sum)
end
def main
a = 1
b = 2
c = 3
do_stuff(a, b, c, method(:my_callback))
end
The idiomatic approach would be to pass a block instead of a reference to a method. One advantage a block has over a freestanding method is context - a block is a closure, so it can refer to variables from the scope in which it was declared. This cuts down on the number of parameters do_stuff needs to pass to the callback. For instance:
def do_stuff(a, b, c, &block)
sum = a + b + c
yield sum
end
def main
a = 1
b = 2
c = 3
do_stuff(a, b, c) { |status_code|
puts "did stuff with #{a}, #{b}, #{c} and got #{status_code}"
}
end
This "idiomatic block" is a very core part of everyday Ruby and is covered frequently in books and tutorials. The Ruby information section provides links to useful [online] learning resources.
The idiomatic way is to use a block:
def x(z)
yield z # perhaps used in conjunction with #block_given?
end
x(3) {|y| y*y} # => 9
Or perhaps converted to a Proc; here I show that the "block", converted to a Proc implicitly with &block, is just another "callable" value:
def x(z, &block)
callback = block
callback.call(z)
end
# look familiar?
x(4) {|y| y * y} # => 16
(Only use the above form to save the block-now-Proc for later use or in other special cases as it adds overhead and syntax noise.)
However, a lambda can be use just as easily (but this is not idiomatic):
def x(z,fn)
fn.call(z)
end
# just use a lambda (closure)
x(5, lambda {|y| y * y}) # => 25
While the above approaches can all wrap "calling a method" as they create closures, bound Methods can also be treated as first-class callable objects:
class A
def b(z)
z*z
end
end
callable = A.new.method(:b)
callable.call(6) # => 36
# and since it's just a value...
def x(z,fn)
fn.call(z)
end
x(7, callable) # => 49
In addition, sometimes it's useful to use the #send method (in particular if a method is known by name). Here it saves an intermediate Method object that was created in the last example; Ruby is a message-passing system:
# Using A from previous
def x(z, a):
a.__send__(:b, z)
end
x(8, A.new) # => 64
Happy coding!
Explored the topic a bit more and updated the code.
The following version is an attempt to generalize the technique, although remaining extremely simplified and incomplete.
I largely stole - hem, found inspiration in - the implementation of callbacks of DataMapper, which seems to me quite complete and beatiful.
I strongly suggest to have a look at the code # http://github.com/datamapper/dm-core/blob/master/lib/dm-core/support/hook.rb
Anyway, trying to reproduce the functionality using the Observable module was quite engaging and instructive.
A few notes:
method added seems to be require because the original instance methods are not available at the moment of registering the callbacks
the including class is made both observed and self-observer
the example is limited to the instance methods, does not support blocks, args and so on
code:
require 'observer'
module SuperSimpleCallbacks
include Observable
def self.included(klass)
klass.extend ClassMethods
klass.initialize_included_features
end
# the observed is made also observer
def initialize
add_observer(self)
end
# TODO: dry
def update(method_name, callback_type) # hook for the observer
case callback_type
when :before then self.class.callbacks[:before][method_name.to_sym].each{|callback| send callback}
when :after then self.class.callbacks[:after][method_name.to_sym].each{|callback| send callback}
end
end
module ClassMethods
def initialize_included_features
#callbacks = Hash.new
#callbacks[:before] = Hash.new{|h,k| h[k] = []}
#callbacks[:after] = #callbacks[:before].clone
class << self
attr_accessor :callbacks
end
end
def method_added(method)
redefine_method(method) if is_a_callback?(method)
end
def is_a_callback?(method)
registered_methods.include?(method)
end
def registered_methods
callbacks.values.map(&:keys).flatten.uniq
end
def store_callbacks(type, method_name, *callback_methods)
callbacks[type.to_sym][method_name.to_sym] += callback_methods.flatten.map(&:to_sym)
end
def before(original_method, *callbacks)
store_callbacks(:before, original_method, *callbacks)
end
def after(original_method, *callbacks)
store_callbacks(:after, original_method, *callbacks)
end
def objectify_and_remove_method(method)
if method_defined?(method.to_sym)
original = instance_method(method.to_sym)
remove_method(method.to_sym)
original
else
nil
end
end
def redefine_method(original_method)
original = objectify_and_remove_method(original_method)
mod = Module.new
mod.class_eval do
define_method(original_method.to_sym) do
changed; notify_observers(original_method, :before)
original.bind(self).call if original
changed; notify_observers(original_method, :after)
end
end
include mod
end
end
end
class MyObservedHouse
include SuperSimpleCallbacks
before :party, [:walk_dinosaure, :prepare, :just_idle]
after :party, [:just_idle, :keep_house, :walk_dinosaure]
before :home_office, [:just_idle, :prepare, :just_idle]
after :home_office, [:just_idle, :walk_dinosaure, :just_idle]
before :second_level, [:party]
def home_office
puts "learning and working with ruby...".upcase
end
def party
puts "having party...".upcase
end
def just_idle
puts "...."
end
def prepare
puts "preparing snacks..."
end
def keep_house
puts "house keeping..."
end
def walk_dinosaure
puts "walking the dinosaure..."
end
def second_level
puts "second level..."
end
end
MyObservedHouse.new.tap do |house|
puts "-------------------------"
puts "-- about calling party --"
puts "-------------------------"
house.party
puts "-------------------------------"
puts "-- about calling home_office --"
puts "-------------------------------"
house.home_office
puts "--------------------------------"
puts "-- about calling second_level --"
puts "--------------------------------"
house.second_level
end
# => ...
# -------------------------
# -- about calling party --
# -------------------------
# walking the dinosaure...
# preparing snacks...
# ....
# HAVING PARTY...
# ....
# house keeping...
# walking the dinosaure...
# -------------------------------
# -- about calling home_office --
# -------------------------------
# ....
# preparing snacks...
# ....
# LEARNING AND WORKING WITH RUBY...
# ....
# walking the dinosaure...
# ....
# --------------------------------
# -- about calling second_level --
# --------------------------------
# walking the dinosaure...
# preparing snacks...
# ....
# HAVING PARTY...
# ....
# house keeping...
# walking the dinosaure...
# second level...
This simple presentation of the use of Observable could be useful: http://www.oreillynet.com/ruby/blog/2006/01/ruby_design_patterns_observer.html
So, this may be very "un-ruby", and I am not a "professional" Ruby developer, so if you guys are going to smack be, be gentle please :)
Ruby has a built-int module called Observer. I have not found it easy to use, but to be fair I did not give it much of a chance. In my projects I have resorted to creating my own EventHandler type (yes, I use C# a lot). Here is the basic structure:
class EventHandler
def initialize
#client_map = {}
end
def add_listener(id, func)
(#client_map[id.hash] ||= []) << func
end
def remove_listener(id)
return #client_map.delete(id.hash)
end
def alert_listeners(*args)
#client_map.each_value { |v| v.each { |func| func.call(*args) } }
end
end
So, to use this I expose it as a readonly member of a class:
class Foo
attr_reader :some_value_changed
def initialize
#some_value_changed = EventHandler.new
end
end
Clients of the "Foo" class can subscribe to an event like this:
foo.some_value_changed.add_listener(self, lambda { some_func })
I am sure this is not idiomatic Ruby and I am just shoehorning my C# experience into a new language, but it has worked for me.
If you are willing to use ActiveSupport (from Rails), you have a straightforward implementation
class ObjectWithCallbackHooks
include ActiveSupport::Callbacks
define_callbacks :initialize # Your object supprots an :initialize callback chain
include ObjectWithCallbackHooks::Plugin
def initialize(*)
run_callbacks(:initialize) do # run `before` callbacks for :initialize
puts "- initializing" # then run the content of the block
end # then after_callbacks are ran
end
end
module ObjectWithCallbackHooks::Plugin
include ActiveSupport::Concern
included do
# This plugin injects an "after_initialize" callback
set_callback :initialize, :after, :initialize_some_plugin
end
end
I know this is an old post, but I found it when tried to solve a similar problem.
It's a really elegant solution, and most importantly, it can work with and without a callback.
Let's say we have the Arithmetic class which implements basic operations on them — addition and subtraction.
class Arithmetic
def addition(a, b)
a + b
end
def subtraction(a, b)
a - b
end
end
And we want to add a callback for each operation which will do something with the input data and result.
In the below example we will implement the after_operation method which accepts the Ruby block which will be executed after an operation.
class Arithmetic
def after_operation(&block)
#after_operation_callback = block
end
def addition(a, b)
do_operation('+', a, b)
end
def subtraction(a, b)
do_operation('-', a, b)
end
private
def do_operation(sign, a, b)
result =
case sign
when '+'
a + b
when '-'
a - b
end
if callback = #after_operation_callback
callback.call(sign, a, b, result)
end
result
end
end
Using with callback:
callback = -> (sign, a, b, result) do
puts "#{a} #{sign} #{b} = #{result}"
end
arithmetic = Arithmetic.new
arithmetic.after_operation(&callback)
puts arithmetic.addition(1, 2)
puts arithmetic.subtraction(3, 1)
Output:
1 + 2 = 3
3
3 - 1 = 2
2
I often implement callbacks in Ruby like in the following example. It's very comfortable to use.
class Foo
# Declare a callback.
def initialize
callback( :on_die_cast )
end
# Do some stuff.
# The callback event :on_die_cast is triggered.
# The variable "die" is passed to the callback block.
def run
while( true )
die = 1 + rand( 6 )
on_die_cast( die )
sleep( die )
end
end
# A method to define callback methods.
# When the latter is called with a block, it's saved into a instance variable.
# Else a saved code block is executed.
def callback( *names )
names.each do |name|
eval <<-EOF
##{name} = false
def #{name}( *args, &block )
if( block )
##{name} = block
elsif( ##{name} )
##{name}.call( *args )
end
end
EOF
end
end
end
foo = Foo.new
# What should be done when the callback event is triggered?
foo.on_die_cast do |number|
puts( number )
end
foo.run
I know this is an old post, but others that come across this may find my solution helpful.
http://chrisshepherddev.blogspot.com/2015/02/callbacks-in-pure-ruby-prepend-over.html