I got a sample code that turns a block into an object using Proc.new so that it can be executed independently. There are two puts at the end to print the result, in this example, the array is brought into the block for execution. At this point, I have something that I don't understand.
In the following code, why is .iterate!(square) valid? They are the method defined in the class and the name of a separate block object. Why can they be written together?
Second, what is the working process of def iterate!(code)? Why is the value being carried into (code)? And what are self and code.call here?
square = Proc.new do |n|
n ** 2
end
class Array
def iterate!(code)
self.map {|n| code.call(n)}
end
end
puts [1, 2, 3].iterate!(square)
puts [4, 5, 6].iterate!(square)
I'm a beginner, please explain as detailed as possible, thank you.
square = Proc.new do |n|
n ** 2
end
This is a simple proc, which expect an integer argument, and return square of the argument.
eg:
square.call(5) => 25
square.(5) => 25
Now, You have opened Array class and added iterate! method which takes an argument. and the method just work on self (self here refers to same array on which you are calling the iterate method. here self = [1,2,3] in your first case and [4,5,6] in you second case.
in the iterate! method definition, you are mapping/looping the array elements and you are calling the square proc with each element as arguments.
So it is more like
square.call(1) => 1
square.call(2) => 4
square.call(3) => 9
Back for more help on my coursework at Bloc. Decided to bring you guys in on a problem I'm having with Monkey Patching the Array Class. This assignment had 8 specs to be met and I'm stuck now with one left and I'm not sure what to do.
I'm only going to give you the RSpecs and written requirements for the part that I'm having trouble w/ since everything else seems to be passing. Also I'll include the starter scaffolding that they gave me to begin with so there's no confusion or useless additions to the code.
Here are the written requirements for the Array Class Monkey Patch:
Write a new new_map method that is called on an instance of the Array class. It should use the array it's called on as an implicit (self) argument, but otherwise behave identically. (INCOMPLETE)
Write a new_select! method that behaves like the select, but mutates the array on which it's called. It can use Ruby's built-in collection select method. (COMPLETE)
Here are the RSpecs that need to be met regarding the Array class:
Note: "returns an array with updated values" Is the only Spec not passing.
describe Array do
describe '#new_map' do
it "returns an array with updated values" do
array = [1,2,3,4]
expect( array.new_map(&:to_s) ).to eq( %w{1 2 3 4} )
expect( array.new_map{ |e| e + 2 } ).to eq( [3, 4, 5, 6] )
end
it "does not call #map" do
array = [1,2,3,4]
array.stub(:map) { '' }
expect( array.new_map(&:to_s) ).to eq( %w{1 2 3 4} )
end
it "does not change the original array" do
array = [1,2,3,4]
expect( array.new_map(&:to_s) ).to eq( %w{1 2 3 4} )
expect( array ).to eq([1,2,3,4])
end
end
describe '#new_select!' do
it "selects according to the block instructions" do
expect( [1,2,3,4].new_select!{ |e| e > 2 } ).to eq( [3,4] )
expect( [1,2,3,4].new_select!{ |e| e < 2 } ).to eq( [1] )
end
it "mutates the original collection" do
array = [1,2,3,4]
array.new_select!(&:even?)
expect(array).to eq([2,4])
end
end
end
Here is the scaffolding they started me with:
class Array
def new_map
end
def new_select!(&block)
end
end
Lastly, here is my code:
class Array
def new_map
new_array = []
self.each do |num|
new_array << num.to_s
end
new_array
end
def new_select!(&block)
self.select!(&block)
end
end
Ruby Array Class:
map { |item| block } → new_ary
A block is like a method, and you specify a block after a method call, for example:
[1, 2, 3].map() {|x| x*2} #<---block
^
|
method call(usually written without the trailing parentheses)
The block is implicitly sent to the method, and inside the method you can call the block with yield.
yield -> calls the block specified after a method call. In ruby, yield is equivalent to yield(), which is conceptually equivalent to calling the block like this: block().
yield(x) -> calls the block specified after a method call sending it the argument x, which is conceptually equivalent to calling the block like this: block(x).
So, here is how you can implement new_map():
class Array
def new_map
result = []
each do |item|
result << yield(item)
end
result
end
end
arr = [1, 2, 3].new_map {|x| x*2}
p arr
--output:--
[2, 4, 6]
This comment is a bit advanced, but you don't actually have to write self.each() to call the each() method inside new_map(). All methods are called by some object, i.e. the object to the left of the dot, which is called the receiver. For instance, when you write:
self.each {....}
self is the receiver of the method call each().
If you do not specify a receiver and just write:
each {....}
...then for the receiver ruby uses whatever object is assigned to the self variable at that moment. Inside new_map() above, ruby will assign the Array that calls the new_map() method to self, so each() will step through the items in that Array.
You have to be a little bit careful with the self variable because ruby constantly changes the value of the self variable without telling you. So, you have to know what ruby has assigned to the self variable at any particular point in your code--which comes with experience. Although, if you ever want to know what object ruby has assigned to self at some particular point in your code, you can simply write:
puts self
Experienced rubyists will crow and cluck their tongues if they see you write self.each {...} inside new_map(), but in my opinion code clarity trumps code trickiness, and because it makes more sense to beginners to write self there, go ahead and do it. When you get a little more experience and want to show off, then you can eliminate explicit receivers when they aren't required. It's sort of the same situation with explicit returns:
def some_method
...
return result
end
and implicit returns:
def some_method
...
result
end
Note that you could write new_map() like this:
class Array
def new_map(&my_block) #capture the block in a variable
result = []
each do |item|
result << my_block.call(item) #call the block
end
result
end
end
arr = [1, 2, 3].new_map {|x| x*2}
p arr
--output:--
[2, 4, 6]
Compare that to the example that uses yield(). When you use yield(), it's as if ruby creates a parameter variable named yield for you in order to capture the block. However, with yield you use a different syntax to call the block, namely (), or if their are no arguments for the block, you can eliminate the parentheses--just like you can when you call a method. On the other hand, when you create your own parameter variable to capture a block, e.g. def new_map(&my_block), you have to use a different syntax to call the block:
my_block.call(arg1, ...)
or:
myblock[arg1, ...]
Note that #2 is just like the syntax for calling a method--except that you substitute [] in place of ().
Once again, experienced rubyists will use yield to call the block instead of capturing the block in a parameter variable. However, there are situations where you will need to capture the block in a parameter variable, e.g. if you want to pass the block to yet another method.
Looking at the spec here:
it "returns an array with updated values" do
array = [1,2,3,4]
expect( array.new_map(&:to_s) ).to eq( %w{1 2 3 4} )
expect( array.new_map{ |e| e + 2 } ).to eq( [3, 4, 5, 6] )
end
It looks like they just want you to re-write Array.map so that it will work with any given block. In your implementation you are telling the method to work in a very specific way, namely to call .to_s on all of the array elements. But you don't want it to always stringify the array elements. You want it to do to each element whatever block is provided when the method is called. Try this:
class Array
def new_map
new_array = []
each do |num|
new_array << yield(num)
end
new_array
end
end
Notice in my example that the method definition doesn't specify any particular operation to be performed on each element of self. It simply loops over each element of the array, yields the element (num) to whatever block was passed when .new_map was called, and shovels the result into the new_array variable.
With that implementation of .new_map and given array = [1,2,3,4], you can call either array.new_map(&:to_s) (the block is where the arbitrary operation to be performed on each element of the array is specified) and get ["1","2","3","4"] or you can call array.new_map { |e| e + 2 } and get [3,4,5,6].
I would like to say a few words about new_select!.
select vs select!
You have:
class Array
def new_select!(&block)
self.select!(&block)
end
end
which you could instead write:
class Array
def new_select!
self.select! { |e| yield(e) }
end
end
This uses the method Array#select!. You said Array#select could be used, but made no mention of select!. If you cannot use select!, you must do something like this:
class Array
def new_select!(&block)
replace(select(&block))
end
end
Let's try it:
a = [1,2,3,4,5]
a.new_select! { |n| n.odd? }
#=> [1, 3, 5]
a #=> [1, 3, 5]
Explicit vs implicit receivers
Notice that I have written this without any explicit receivers for the methods Array#replace and Array#select. When there is no explicit receiver, Ruby assumes that it is self, which is the a. Therefore, Ruby evaluates replace(select(&block)) as though it were written with explicit receivers:
self.replace(self.select(&block))
It's up to you to decide if you want to include self.. Some Rubiests do, some don't. You'll notice that self. is not included in Ruby built-in methods implemented in Ruby. One other thing: self. is required in some situations to avoid ambiguity. For example, if taco= is the setter for an instance variable #taco, you must write self.taco = 7 to tell Ruby you are referring to the setter method. If you write taco = 7, Ruby will assume you want to create a local variable taco and set its value to 7.
Two forms of select!
Is your method new_select! a direct replacement for select!? That is, are the two methods functionally equivalent? If you look at the docs for Array#select!, you will see it has two forms, the one you have mimicked, and another that you have not implemented.
If select! is not given a block, it returns an enumerator. Now why would you want to do that? Suppose you wish to write:
a = [1,2,3,4,5]
a.select!.with_index { |n,i| i < 2 }
#=> [1, 2]
Has select! been given a block? No, so it must return an enumerator, which becomes the receiver of the method Enumerator#with_index.
Let's try it:
enum0 = a.select!
#=> #<Enumerator: [1, 2, 3, 4, 5]:select!>
Now:
enum1 = enum0.with_index
#=> #<Enumerator: #<Enumerator: [1, 2, 3, 4, 5]:select!>:with_index>
You can think of enum1 as a "compound enumerator". Look carefully at the description (above) of the object Ruby returns when you define enum1.
We can see the elements of the enumerator by converting it to an array:
enum1.to_a
# => [[1, 0], [2, 1], [3, 2], [4, 3], [5, 4]]
Each of these five elements is passed to the block and assigned to the block variables by Enumerator#each (which calls Array#each).
Enough of that, but the point is that having methods return enumerators when no block is given is what allows us to chain methods.
You do not have a test that ensures new_select! returns an enumerator when no block is given, so maybe that's not expected, but why not give it a go?
class Array
def new_select!(&block)
if block_given?
replace(select(&block))
else
to_enum(:new_select!)
end
end
end
Try it:
a = [1,2,3,4,5]
a.new_select! { |n| n.odd? }
#=> [1, 3, 5]
a #=> [1, 3, 5]
a = [1,2,3,4,5]
enum2 = a.new_select!
#=> #<Enumerator: [1, 2, 3, 4, 5]:new_select!>
enum2.each { |n| n.odd? }
#=> [1, 3, 5]
a #=> [1, 3, 5]
a = [1,2,3,4,5]
enum2 = a.new_select!
#=> #<Enumerator: [1, 2, 3, 4, 5]:new_select!>
enum2.with_index.each { |n,i| i>2 }
#=> [4, 5]
a #=> [4, 5]
I struggle a little bit with understanding how the Enumerator.new method works.
Assuming example from documentation:
fib = Enumerator.new do |y|
a = b = 1
loop do
y << a
a, b = b, a + b
end
end
p fib.take(10) # => [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
Where's the loop break condition, how does it know how many times loop should to iterate (as it doesn't have any explicit break condition and looks like infinite loop) ?
Enumerator uses Fibers internally. Your example is equivalent to:
require 'fiber'
fiber = Fiber.new do
a = b = 1
loop do
Fiber.yield a
a, b = b, a + b
end
end
10.times.map { fiber.resume }
#=> [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
Edit I think I understand your question now, I will still keep my original answer below.
y << a is an alias for y.yield(a), which is basically a sleep with return value. Each time a value is requested from the enumerator with next, the execution is continued until another value is yielded.
Enumerators do not need to enumerate a finite number of elements, so they are infinite. For example, fib.to_a will never terminate, because it tries to build an array with an infinite number of elements.
As such, enumerators are great as a representation of infinite series such as the natural numbers, or in your case, the fibonacci numbers. The user of the enumerator can decide how many values it needs, so in your example take(10) determines the break condition if you will.
The break condition itself is in the implementation of Enumerator#take. For demonstration purposes, we can make our own implementation called my_take:
class Enumerator
def my_take(n)
result = []
n.times do
result << self.next
end
result
end
end
Where you could of course "mentally substitute" your n.times loop with your classical C style for (i=0; i<n; i++). There's your break condition. self.next is the method to get the next value of the enumerator, which you can also use outside of the class:
fib.next
#=> 1
fib.next
#=> 1
fib.next
#=> 2
fib.next
#=> 3
That said, you can of course build an enumerator that enumerates a finite number of values, such as the natural numbers in a given range, but that's not the case here. Then, the Enumerator will raise a StopIteration error when you try to call next, but all values already have been enumerated. In that case, you have two break conditions, so to speak; the one that breaks earlier will then win. take actually handles that by rescuing from the error, so the following code is a bit closer to the real implementation (however, take is in fact implemented in C).
class Enumerator
def my_take(n)
result = []
n.times do
result << self.next
end
result
rescue StopIteration
# enumerator stopped early
result
end
end
I am using Ruby on Rails 3 and I would like to know what means the presence of a * operator near a function argument and to understand its usages in others scenarios.
Example scenario (this method was from the Ruby on Rails 3 framework):
def find(*args)
return to_a.find { |*block_args| yield(*block_args) } if block_given?
options = args.extract_options!
if options.present?
apply_finder_options(options).find(*args)
else
case args.first
when :first, :last, :all
send(args.first)
else
find_with_ids(*args)
end
end
end
This is the splat operator, which comes from ruby (and is thus not rails specific). It can be applied in two ways depending on where it is used:
to "pack" a number of arguments into an array
to split up an array into an argument list
In your function, you see the splat operator used in the function definition. The result is that the function accepts any number of arguments. The complete argument list will be put into args as an array.
def foo(*args)
args.each_with_index{ |arg, i| puts "#{i+1}. #{arg}" }
end
foo("a", "b", "c")
# 1. a <== this is the output
# 2. b
# 3. c
The second variant would be when you consider the following method:
def bar(a, b, c)
a + b + c
end
It requires exactly three arguments. You can now call this method like follows
my_array = [1, 2, 3]
bar(*my_array)
# returns 6
The splat applied in this case to the array will split it and pass each element of the array as an individual parameter to the method. You could do the same even by calling foo:
foo(*my_array)
# 1. 1 <== this is the output
# 2. 2
# 3. 3
As you can see in your example method, these rules do apply to block parameters in the same way.
This is a splat argument, which basically means that any 'extra' arguments passed to the method will all be assigned to *args.
Why does
a = [].tap do |x|
x << 1
end
puts "a: #{a}"
work as expected
a: [1]
but
b = [].tap do |x|
x = [1]
end
puts "b: #{b}"
doesn't
b: []
?
The reason why the second snippet does not change the array is the same why this snippet:
def foo(x)
x = [1]
end
a = []
foo(a)
does not change variable a. Variable x in your code is local to the scope of the block, and because of that you can assign anything to it, but the assignment won't be visible outside (Ruby is a pass-by-value language).
Of course, blocks have also closures on the local variables where they were declared, so this will work:
def foo(x)
yield(x)
end
b = []
foo(123) do |x|
b = [1]
end
p b # outputs [1]
The first method put 1 on the end of an empty array. In the same way you cant say that an empty array is equal to 1. Rather you would try and replicate it...
b = [].tap do |x|
x.unshift(1)
end
This is just an example yet have a look at the method call you can use on an Array by typing.
Array.methods.sort
All the best and Good luck
This is slightly unrelated -- but that [].tap idiom is horrible. You should not use it. Even many of the people who used it in rails code now admit it's horrible and no longer use it.
Do not use it.