Let's say I have a function
def odd_or_even n
if n%2 == 0
return :even
else
return :odd
end
end
And I had a simple enumerable array
simple = [1,2,3,4,5]
And I ran it through map, with my function, using a do-end block:
simple.map do
|n| odd_or_even(n)
end
# => [:odd,:even,:odd,:even,:odd]
How could I do this without, say, defining the function in the first place? For example,
# does not work
simple.map do |n|
if n%2 == 0
return :even
else
return :odd
end
end
# Desired result:
# => [:odd,:even,:odd,:even,:odd]
is not valid ruby, and the compiler gets mad at me for even thinking about it. But how would I implement an equivalent sort of thing, that works?
edit
In reality, the solution to my problem matters to me a lot less than the motivation/reasoning behind it, to help me understand more how ruby blocks work :)
You're so close. Just remove the returns and you're golden.
This is because the block passed to map is a proc (i.e. created with Proc.new), and not a lambda. A return within a proc doesn't just jump out of the proc- it jumps out of the method that executed (i.e. called call on) the proc. A return within a lambda, on the other hand, jumps out of only the lambda.
The proc method returns a lambda in Ruby 1.8, and a Proc in Ruby 1.9. It's probably best to just not use this method and be explicit with which construct you want to use.
I'm guessing you were either in IRB or a plain ruby script when you were trying this out.
a = Proc.new { return }
a.call # fails. Nothing to return from.
def foobar
a = Proc.new { return }
a.call
puts 'hello' # not reached. The return within the proc causes execution to jump out of the foobar method.
end
foobar # succeeds, but does not print 'hello'. The return within the proc jumps out of the foobar method.
b = lambda { return }
b.call # succeeds. The return only returns from the lambda itself.
def bazquux
b = lambda { return }
b.call
puts 'hello' # this is reached. The lambda only returned from itself.
end
bazquux # succeeds, and prints 'hello'
The lesson to learn from this is to use implicit returns unless you can't, I guess.
I suspect this may be a duplicate question, but to give a value out of a block, use next
simple.map do |n|
if n%2 == 0
next :even
else
next :odd
end
end
Shortest variant using Andrew's answer:
simple.map { |n| next :even if n % 2 == 0; :odd }
Related
Example: LinkedList printing method.
For this object, you will find a printing method using block, proc, and lambda.
It is not clear to me what the advantages/disadvantages are (if any).
Thank you
What is a LinkedList?
A LinkedList is a node that has a specific value attached to it (which is sometimes called a payload), and a link to another node (or nil if there is no next item).
class LinkedListNode
attr_accessor :value, :next_node
def initialize(value, next_node = nil)
#value = value
#next_node = next_node
end
def method_print_values(list_node)
if list_node
print "#{list_node.value} --> "
method_print_values(list_node.next_node)
else
print "nil\n"
return
end
end
end
node1 = LinkedListNode.new(37)
node2 = LinkedListNode.new(99, node1)
node3 = LinkedListNode.new(12, node2)
#printing the linked list through a method defined within the scope of the class
node3.method_print_values(node3)
#---------------------------- Defining the printing method through a BLOCK
def block_print_value(list_node, &block)
if list_node
yield list_node
block_print_value(list_node.next_node, &block)
else
print "nil\n"
return
end
end
block_print_value(node3) { |list_node| print "#{list_node.value} --> " }
#---------------------------- Defining the printing method through a PROC
def proc_print_value(list_node, callback)
if list_node
callback.call(list_node) #this line invokes the print function defined below
proc_print_value(list_node.next_node, callback)
else
print "nil\n"
end
end
proc_print_value(node3, Proc.new {|list_node| print "#{list_node.value} --> "})
#---------------------------- Defining the printing method through a LAMBDA
def lambda_print_value(list_node, callback)
if list_node
callback.call(list_node) #this line invokes the print function defined below
lambda_print_value(list_node.next_node, callback)
else
print "nil\n"
end
end
lambda_print_value(node3, lambda {|list_node| print "#{list_node.value} --> "})
#---------------------------- Defining the printing method outside the class
def print_values(list_node)
if list_node
print "#{list_node.value} --> "
print_values(list_node.next_node)
else
print "nil\n"
return
end
end
print_values(node3)
Examples display how to use different things to do the same. So, there is no principal difference between them in this context:
my_proc = Proc.new { |list_node| print "#{list_node.value} --> " }
node3.block_print_values(node3, &my_proc)
node3.proc_print_value(node3, my_proc)
node3.lambda_print_value(node3, my_proc)
Also, there is possible to define a method by using any of them:
define_method(:my_method, p, &proc { puts p })
my_method 'hello' #=> hello
define_method(:my_method, p, &-> { puts p })
my_method 'hello' #=> hello
But Proc, Lambda, block are not the same. Firstly, need a bit more display how to works magic &. The great article can help with that:
&object is evaluated in the following way:
if object is a block, it converts the block into a simple proc.
if object is a Proc, it converts the object into a block while preserving the lambda? status of the object.
if object is not a Proc, it first calls #to_proc on the object and then converts it into a block.
But this does not show the differences between them. So, now let go to the ruby source:
Proc objects are blocks of code that have been bound to a set of local variables. Once bound, the code may be called in different contexts and still access those variables.
And
+lambda+, +proc+ and Proc.new preserve the tricks of a Proc object given by & argument.
lambda(&lambda {}).lambda? #=> true
proc(&lambda {}).lambda? #=> true
Proc.new(&lambda {}).lambda? #=> true
lambda(&proc {}).lambda? #=> false
proc(&proc {}).lambda? #=> false
Proc.new(&proc {}).lambda? #=> false
Proc created as:
VALUE block = proc_new(klass, FALSE);
rb_obj_call_init(block, argc, argv);
return block;
When lambda:
return proc_new(rb_cProc, TRUE);
Both are Proc. In this case, the difference is just in TRUE or FALSE. TRUE, FALSE - check the number of parameters passed when called.
So, lambda is like more strict Proc:
is_proc = !proc->is_lambda;
Summary of Lambda vs Proc:
Lambdas check the number of arguments, while procs do not.
Return within the proc would exit the method from where it is called.
Return within a lambda would exit it from the lambda and the method would continue executing.
Lambdas are closer to a method.
Blocks: They are called closures in other languages, it is a way of grouping code/statements. In ruby single line blocks are written in {} and multi-line blocks are represented using do..end.
Block is not an object and can not be saved in a variable. Lambda and Proc are both an object.
So, let do small code test based on this answer:
# ruby 2.5.1
user system total real
0.016815 0.000000 0.016815 ( 0.016823)
0.023170 0.000001 0.023171 ( 0.023186)
0.117713 0.000000 0.117713 ( 0.117775)
0.217361 0.000000 0.217361 ( 0.217388)
This shows that using block.call is almost 2x slower than using yield.
Thanks, #engineersmnky, for good references in comments.
Proc is an object wrapper over block. Lambda basically is a proc with different behavior.
AFAIK pure blocks are more rational to use compared to procs.
def f
yield 123
end
Should be faster than
def g(&block)
block.call(123)
end
But proc can be passed on further.
I guess you should find some articles with performance comparison on the toppic
IMO, your block_print_value method is poorly designed/named, which makes it impossible to answer your question directly. From the name of the method, we would expect that the method "prints" something, but the only printing is the border condition, which does a
print "nil\n"
So, while I would strongly vote against using this way to print the tree, it doesn't mean that the whole idea of using a block for the printing problem is bad.
Since your problem looks like a programming assignment, I don't post a whole solution, but give a hint:
Replace your block_print_value by a, say block_visit_value, which does the same like your current method, but doesn't do any printing. Instead, the "else" part could also invoke the block to let it do the printing.
I'm sure that you will see afterwards the advantage of this method. If not, come back here for a discussion.
At a high level, procs are methods that can be stored inside variables like so:
full_name = Proc.new { |first,last| first + " " + last }
I can call this in two ways, using the bracket syntax followed by the arguments I want to pass to it or use the call method to run the proc and pass in arguments inside of parentheses like so:
p full_name.call("Daniel","Cortes")
What I did with the first line above is create a new instance of Proc and assigned it to a variable called full_name. Procs can take a code block as a parameter so I passed it two different arguments, arguments go inside the pipes.
I can also make it print my name five times:
full_name = Proc.new { |first| first * 5 }
The block I was referring to is called a closure in other programming languages. Blocks allow you to group statements together and encapsulate behavior. You can create blocks with curly braces or do...end syntax.
Why use Procs?
The answer is Procs give you more flexibility than methods. With Procs you can store an entire set of processes inside a variable and then call the variable anywhere else in your program.
Similar to Procs, Lambdas allow you to store functions inside a variable and call the method from other parts of the program. So really the same code I had above can be used like so:
full_name = lambda { |first,last| first + " " + last }
p full_name["daniel","cortes"]
So what is the difference between the two?
There are two key differences in addition to syntax. Please note that the differences are subtle, even to the point that you may never even notice them while programming.
The first key difference is that Lambdas count the arguments you pass to them whereas Procs do not. For example:
full_name = lambda { |first,last| first + " " + last }
p full_name.call("Daniel","Cortes")
The code above works, however, if I pass it another argument:
p full_name.call("Daniel","Abram","Cortes")
The application throws an error saying that I am passing in the wrong number of arguments.
However, with Procs it will not throw an error. It simply looks at the first two arguments and ignores anything after that.
Secondly, Lambdas and Procs have different behavior when it comes to returning values from methods, for example:
def my_method
x = lambda { return }
x.call
p "Text within method"
end
If I run this method, it prints out Text within method. However, if we try the same exact implementation with a Proc:
def my_method
x = Proc.new { return }
x.call
p "Text within method"
end
This will return a nil value.
Why did this occur?
When the Proc saw the word return it exited out of the entire method and returned a nil value. However, in the case of the Lambda, it processed the remaining part of the method.
I've got some code:
def my_each_with_index
return enum_for(:my_each_with_index) unless block_given?
i = 0
self.my_each do |x|
yield x, i
i += 1
end
self
end
It is my own code, but the line:
return enum_for(:my_each_with_index) unless block_given?
is found in solutions of other's. I can't get why they passed the function to enum_for as a parameter. When I invoke my function without a block, it won't return anything with or without enum_for. I could left sth like:
return unless block_given?
and it has the same result. Or am I wrong?
Being called without a block, it will return an enumerator:
▶ def my_each_with_index
▷ return enum_for(:my_each_with_index) unless block_given?
▷ end
#⇒ :my_each_with_index
▶ e = my_each_with_index
#⇒ #<Enumerator: main:my_each_with_index>
later on you might iterate on this enumerator:
▶ e.each { |elem| ... }
This behavior is specifically useful in some cases, like lazy iteration, passing block to this enumerator later etc.
Just returning nil cuts this ability off.
Think you for very precise answer. I recived also very good example to understand this issue for other new developers:
def iterator
yield 1
yield 2
yield 3
puts "koniec"
end
iterator { |v| puts v }
it = enum_for(:iterator)
puts it.next
puts it.next
puts it.next
puts it.next
Just run and analyze this code.
For any method that accepts a block, a good method implementation should have a well-defined behavior when the block is not given.
In the example shared by you, each_for_index is being re-implemented by author, may be to provide additional semantics or may be just for academic purpose given that its behavior is same as Ruby's Enumerable#each_with_index.
The documentation has following for Enumerable#each_with_index.
Calls block with two arguments, the item and its index, for each item
in enum. Given arguments are passed through to each().
If no block is given, an enumerator is returned instead.
In order to stay consistent with highlighted line indicating what should be the behavior if block is not given, one has to use something like
return enum_for(:my_each_with_index) unless block_given?
enum_for is interesting method
enum_for creates a new Enumerator which will enumerate by calling method on obj.
Below is an example reproduced from documentation:
str = "xyz"
enum = str.enum_for(:each_byte)
enum.each { |b| puts b }
# => 120
# => 121
# => 122
So, if one does not pass block to my_each_with_index, they have a chance to pass it later - just like one would have done with each_with_index.
e = obj.my_each_with_index
...
e.each { |x, i| # do something } # `my_each_with_index` executed later
In summary, my_each_with_index tries to be consistent with each_with_index and tries to be a well-behaved API.
I call a method with a block;
method do
"Hello"
end
and the method is defined as;
def method
yield
end
and when defining method; i want to check if given block is empty (nil) or not, because the variable in the method may end up like this;
method do
""
end
So in definition, i want to check if the yield block is nil or not. Like;
def method
if yield ? yield : "Empty block? Seriously?"
end
I know the above does not work. Bu it is what i want to achieve.
Also keep in mind that block_given? will always be "true" since the block is given even if it is nil or empty string.
UPDATE: As most of the comments/answers state that the question is unclear; here is the problem simplified by #ndn:
I want to check if the result of executing a block is "empty"(nil or "") without
invoking it first.
It is unclear what you are asking, because a block itself can not be empty. Therefore, you might mean a few different things:
A missing block. You can check if a block is given
block_given?
Block with empty body (aka {} or do end). This is not impossible, but requires some advanced voodoo ruby metaprogramming magic. Generally, if this is what you are looking for, either you are writing something very interesting or your approach is completely wrong.
You want to check if the result of executing a block is "empty" without invoking it first. This is impossible. For example, consider the following block:
{ [nil, "", true].sample }
Obviously, there is no way to know in advance.
You are ok with calling the block. Then you can assign the result to a variable and make checks on it:
def some_method
evaluation_result = yield if block_given?
if evaluation_result.nil? or evaluation_result == ""
# do something if the block was not given or the result is nil/empty
puts "Empty block? Seriously?"
else
# do something if the block was given and the result is non nil/empty
puts evaluation_result
end
end
Now when you invoke some_method:
some_method { "something" } # => "something"
some_method { 3 + 5 } # => 8
some_method { nil } # => "Empty block? Seriously?"
some_method { "" } # => "Empty block? Seriously?"
some_method { } # => "Empty block? Seriously?"
some_method # => "Empty block? Seriously?"
EDIT:
A workaround for case #3 might be to create two procs, one with what you want to do if the block is "empty" and one - if it is not, then pass them around to the endpoint where you will finally invoke the block. This might or might not be applicable depending on your exact situation.
EDIT2:
Another workaround can be to redefine the Proc#call method for your proc instances. However, this doesn't work for yield:
def secure(&block)
insecure_call = block.method(:call)
block.define_singleton_method(:call) do
insecure_call_result = insecure_call.call
if insecure_call_result.nil? or insecure_call_result == ""
"<b>Bummer! Empty block...</b>"
else
insecure_call_result
end
end
end
x = proc { }
y = proc { "" }
z = proc { nil }
a = proc { 3 + 5 }
b = proc { "something" }
u = proc { [nil, "", true].sample }
[x, y, z, a, b, u].each { |block| secure &block }
# some method that uses the block
def user(&block)
"What I got is #{block.call}!"
end
user &x # => "What I got is <b>Bummer! Empty block...</b>!"
user &y # => "What I got is <b>Bummer! Empty block...</b>!"
user &z # => "What I got is <b>Bummer! Empty block...</b>!"
user &a # => "What I got is 8!"
user &b # => "What I got is something!"
user &u # => Different each time
EDIT3: Another alternative, which is sort of cheating, is to wrap the given proc in another proc. This way, it will work for yield too.
def wrap(&block)
proc do
internal_proc_call_result = block.call
if internal_proc_call_result.nil? or internal_proc_call_result == ""
"<b>Bummer! Empty block...</b>"
else
internal_proc_call_result
end
end
end
Now using the result of wrap and will get you behavior similar to secure.
If I understand correctly, you want to statically determine what the runtime value of a block is. This is one of the many known impossible problems resulting from the undecidability of the Halting Problem.
In other words: it can't be done.
Not "it can't be done in Ruby", not "it is hard", it simply can't be done, period. And it can be (and has been) mathematically proven that it can't be done. Ever.
UPDATED Answer
My last effort to simplify the answer based on comments..
You can check for block emptiness with block_given? and you need to explicitly check for yield output for emptiness like below
def method(&block)
# Below if condition is to prove that block can be accessed
if block_given?
p block
p block.yield
end
b = yield if block_given?
(b.nil? || b.empty?) ? "Empty block? Seriously?" : b
end
p method {"Hello"} # inline block
result = method do
"World"
end
p result
p method # No blocks provided
p method {""} # Block that returns empty string
Output of the program
"Hello"
"World"
"Empty block? Seriously?"
"Empty block? Seriously?"
I am currently using the following code in a ruby program to evaluate variable length arguments that are passed to a method. The program is running however I'm wondering if there is a short hand way to write this.
Should have been more specific in my original description, trying to rewrite the Inject method for the Array class (hence the witty name...)
Therefore it needs to be able to accept a maximum of two args, and a minimum 0 if a block is given.
array.inject(:+)
array.inject{ |output, num| output + num }
array.inject(arg, :+)
array.inject(arg) { |output, num| output + num }
The most difficult case/s to handle are the first and forth where the 1 arg can be either a Fixnum or a Symbol. As mentioned, the code works, just looking for ways to tidy it up.
class Array
def enjict(*args)
if args.length == 2 && args[0].is_a?(Fixnum) && args[1].is_a?(Symbol)
start, symbol = args
elsif args.length == 1
raise ArgumentError unless args.first.is_a?(Symbol) || args.first.is_a?(Fixnum)
symbol = args.first if args.first.is_a?(Symbol)
start = args.first if args.first.is_a?(Fixnum)
else
raise ArgumentError unless block_given?
end
copiedArray = dup
start = copiedArray.shift unless start
if block_given?
copiedArray.each { |num| start = yield(start, num) }
else
copiedArray.each { |num| start = start.send(symbol, num) }
end
start
end
end
The sad truth is: it's messy, and there's nothing you can do about it. Almost all Ruby implementations implement Enumerable#inject with privileged access to the interpreter internals, including introspection of the arguments. MRI, YARV, MRuby implement it in C, MacRuby and RubyMotion in Objective-C, XRuby and JRuby in Java, Ruby.NET and IronRuby in C#, Topaz in RPython, Cardinal in PIR, and so on.
This is something that is simply not available to Ruby code.
Only Rubinius implements it in Ruby.
You can use a similar trick by (ab)using the fact that the default argument expression for an optional parameter can be any arbitrarily complex Ruby expression and that local variables of those expressions become local variables of the method. This is a common trick for figuring out whether an argument was passed or not:
def inject(initial=(no_initial = true; nil), sym=(no_sym = true; nil))
sym, initial = initial, nil if !block_given && no_sym
# and so on …
end
Judging from the conditions, how about refactoring your method arguments to:
def enjict(start, symbol, *options, &block)
e = proc{ raise ArgumentError if options.length > 0 && !block_given? }
e.call
if start.is_a?(Fixnum) && symbol.is_a?(Symbol)
# do something you want
else
e.call
end
end
Can anyone help me to figure out the the use of yield and return in Ruby. I'm a Ruby beginner, so simple examples are highly appreciated.
Thank you in advance!
The return statement works the same way that it works on other similar programming languages, it just returns from the method it is used on.
You can skip the call to return, since all methods in ruby always return the last statement. So you might find method like this:
def method
"hey there"
end
That's actually the same as doing something like:
def method
return "hey there"
end
The yield on the other hand, excecutes the block given as a parameter to the method. So you can have a method like this:
def method
puts "do somthing..."
yield
end
And then use it like this:
method do
puts "doing something"
end
The result of that, would be printing on screen the following 2 lines:
"do somthing..."
"doing something"
Hope that clears it up a bit. For more info on blocks, you can check out this link.
yield is used to call the block associated with the method. You do this by placing the block (basically just code in curly braces) after the method and its parameters, like so:
[1, 2, 3].each {|elem| puts elem}
return exits from the current method, and uses its "argument" as the return value, like so:
def hello
return :hello if some_test
puts "If it some_test returns false, then this message will be printed."
end
But note that you don't have to use the return keyword in any methods; Ruby will return the last statement evaluated if it encounters no returns. Thus these two are equivelent:
def explicit_return
# ...
return true
end
def implicit_return
# ...
true
end
Here's an example for yield:
# A simple iterator that operates on an array
def each_in(ary)
i = 0
until i >= ary.size
# Calls the block associated with this method and sends the arguments as block parameters.
# Automatically raises LocalJumpError if there is no block, so to make it safe, you can use block_given?
yield(ary[i])
i += 1
end
end
# Reverses an array
result = [] # This block is "tied" to the method
# | | |
# v v v
each_in([:duck, :duck, :duck, :GOOSE]) {|elem| result.insert(0, elem)}
result # => [:GOOSE, :duck, :duck, :duck]
And an example for return, which I will use to implement a method to see if a number is happy:
class Numeric
# Not the real meat of the program
def sum_of_squares
(to_s.split("").collect {|s| s.to_i ** 2}).inject(0) {|sum, i| sum + i}
end
def happy?(cache=[])
# If the number reaches 1, then it is happy.
return true if self == 1
# Can't be happy because we're starting to loop
return false if cache.include?(self)
# Ask the next number if it's happy, with self added to the list of seen numbers
# You don't actually need the return (it works without it); I just add it for symmetry
return sum_of_squares.happy?(cache << self)
end
end
24.happy? # => false
19.happy? # => true
2.happy? # => false
1.happy? # => true
# ... and so on ...
Hope this helps! :)
def cool
return yield
end
p cool {"yes!"}
The yield keyword instructs Ruby to execute the code in the block. In this example, the block returns the string "yes!". An explicit return statement was used in the cool() method, but this could have been implicit as well.