If you have an enumerator and some callable (proc/lambda/method) it is sometimes handy to compose them to create a new enumerator, like this
class Enumerator
def compose func
Enumerator.new do |yielder|
each { |x| yielder << func[x] }
end
end
end
# make an enumerator that yields 0, 2, 4, 6...
(0..10).each.compose(proc { |x| x * 2 })
Is there no built-in method for doing this? Or no simpler syntax? I've been trawling the docs because we have Enumerator::+ and Proc::<< which are close but not quite right.
That sounds like Enumerable#map
(0..10).each.map { |x| x * 2 }
In fact, you don't even need the each, since map is a method on anything that mixes in Enumerable.
(0..10).map { |x| x * 2 }
If you want to get a lazy enumerator, you can call Enumerable#lazy before applying any transformations. This returns an object on which all of the Enumerable methods can be applied lazily.
(0..10).lazy.map { |x| x * 2 }
Related
tap yields self to the block passed to it, and returns self unaltered. I often find myself wishing there was a version of tap which returned the return value of the block, rather than self. For example:
[1,2].inject(:+).tap {|x| x * 3} #=> returns 3, but I want 9
Is there any builtin method that will accomplish this?
The typical solution -- create a temporarily local var to store the output of [1,2].inject(:+), and multiply that by 3 -- seems kludgy.
There is no built in method for that, but you can add Kernel#ergo from Ruby Facets:
"a".ergo.upcase #=> "A"
nil.ergo.foobar #=> nil
"a".ergo { |o| o.upcase } #=> "A"
nil.ergo { |o| o.foobar } #=> nil
This is like #tap, but #tap yields self and returns self, where as #ergo yields self but returns the result.
This is an older question, but you can now use yield_self to do this.
You could do this yourself.
class Object
def my_tap
return yield self if block_given?
self
end
end
[1,2].inject(:+).my_tap {|x| x * 3}
=> 9
There is a method called all? in Enumerable.
I'm trying to learn all the methods of Enumberable's library by writing them myself.
This is what I've come up so far for the all? method. I sorta understand it but I got stumped when trying to pass initialized values to my method.
EDIT for the record, I'm aware that enum method that I have is not the right way ie, it's hard-coded array. This is for self-learning purposes. I'm just trying to figure out how to pass the initialized values to my all? method. That's why I wrote enum in the first place, to see that it is working for sure. Please don't take this class as a literal gospel. Thank you.
class LearningMethods
def initialize(values)
#values = values
end
def enum
array = [10, 3, 5]
end
def all?(a)
yield(a)
end
end
c = LearningMethods.new([10, 3, 5])
p c.enum.all? {|x| x >= 3 } #this works
p c.all?(10) { |x| x >= 3 } #this works
p c.all?(#values) { |x| x >= 3 } #this doesn't work. Why not? And how do I pass the initialized values?
I'm not sure why you need enum at all? Enumerable is a module included in array, so if you're not familiar with this I recommend you read about "modules and mix-ins" in Ruby.
all? works simply by passing EACH of the array elements to the block. If there is ANY element (at least 1) for which the block returns false, then all? evaluates to false. Try analyzing this code:
class MyAllImplementation
def initialize(array)
#array = array
end
def my_all?
#array.each do |element| # for each element of the array
return true unless block_given? # this makes sure our program doesn't crash if we don't give my_all? a block.
true_false = yield(element) # pass that element to the block
return false unless true_false # if for ANY element the block evaluates to false, return false
end
return true # Hooray! The loop which went over each element of our array ended, and none evaluted to false, that means all elements must have been true for the block.
end
end
a = MyAllImplementation.new([1,2,3])
p a.my_all? { |x| x > 0 } #=> true
p a.my_all? { |x| x > 1 } # false, because 1 is not bigger than 1, it's equal to 1
How can I create an enumerator that optionally takes a block? I want some method foo to be called with some arguments and an optional block. If it is called with a block, iteration should happen on the block, and something should be done on it, involving the arguments given. For example, if foo were to take a single array argument, apply map on it with the given block, and return the result of join applied to it, then it would look like:
foo([1, 2, 3]){|e| e * 3}
# => "369"
If it is called without a block, it should return an enumerator, on which instance methods of Enumerator (such as with_index) should be able to apply, and execute the block in the corresponding way:
enum = foo([1, 2, 3])
# => Enumerator
enum.with_index{|e, i| e * i}
# => "026"
I defined foo using a condition to see if a block is given. It is easy to implement the case where the block is given, but the part returning the enumerator is more difficult. I guess I need to implement a sublass MyEnum of Enumerator and make foo return an instance of it:
def foo a, &pr
if pr
a.map(&pr).join
else
MyEnum.new(a)
end
end
class MyEnum < Enumerator
def initialize a
#a = a
...
end
...
end
But calling MyEnum.new raises a warning message: Enumerator.new without a block is deprecated; use Object#to_enum. If I use to_enum, I think it would return a plain Enumerator instance, not the MyEnum with the specific feature built in. On top of that, I am not sure how to implement MyEnum in the first place. How can I implement such enumerator? Or, what is the right way to do this?
You could do something like this.
def foo a, &pr
if pr
a.map(&pr).join
else
o = Object.new
o.instance_variable_set :#a, a
def o.each *y
foo #a.map { |z| yield z, *y } { |e| e }
end
o.to_enum
end
end
Then we have
enum = foo([1,2,3])
enum.each { |x| 2 * x } # "246"
or
enum = foo([1,2,3])
enum.with_index { |x, i| x * i } # "026"
Inspiration was drawn from the Enumerator documentation. Note that all of your expectations about enumerators like you asked for hold, because .to_enum takes care of all that. enum is now a legitimate Enumerator!
enum.class # Enumerator
I'm looking for something similar to #detect in enumerables, but not quite. This is what enumerable does:
[1, 2, 3].detect {|i| i > 1 } #=> 2
it returns the first instance of the array which matches the condition. Now, my purpose is to return the value of the block. Concern is not exactly the conditions, but for instance, the first which is not nil. Something like this:
[var1, var2, var3].wanted_detect {|var| another_function(var) }
in which the function would return the first result of another_function call which isn't nil.
Mapping the values of applying the method on the variables and then using detect is not an option. This one would ideally have to work in lazy enumerators, for which the early mapping of all possible values is a no-go
[var1, var2, var3].lazy.map { |var| another_function(var) }.reject(&:nil?).first
If you don't have access to Enumerable#lazy, it is easy enough to implement what you want:
module Enumerable
def wanted_detect
self.each do |obj|
val = yield obj
return val if val
end
end
end
Demo:
[1, 2, 3, 4].wanted_detect { |x| x*x if x > 2 }
# => 9
EDIT: Sorry, I missed the last paragraph till falsetru pointed it out.
Thanks for the comments, falsetru.
For example:
a = [1,2,3,4,5]
a.delete_if { |x| x > 3 }
is equivalent to:
a = [1,2,3,4,5]
a.delete_if.each.each.each.each { |x| x > 3 }
I know a.delete_if returns an enumerator. But how does it know it should delete object when the each block returns true? How to implement delete_if by hand(and in Ruby)?
You can take a look at the Rubinius source code: enumerable module
Here an example of the reject method:
def reject
return to_enum(:reject) unless block_given?
ary = []
each do |o|
ary << o unless yield(o)
end
ary
end
In the implementation of delete_if, the code can verify the value returned from yield to decide whether or not to delete the given entry from the array.
You can read Implementing Iterators in the Programming Ruby guide for more details, but it would looks something like:
class Array
def delete_if
reject { |i| yield i }.to_a
end
end
The above uses yield to pass each item in the array to the block associated with the call to delete_if, and implicitly returns the value of the yield to the outer reject call.