Why none of the block giving the expected output with the “enumerator”?
=======================================================================
On Sunday 03 February 2013 Arup R. wrote
=> [11, 22, 31, 224, 44]
from (irb):22:in-' from (irb):22:inblock in irb_binding’
from (irb):22:ineach' from (irb):22:ineach’
from (irb):22
from C:/Ruby193/bin/irb:12:in `’
irb(main):023:0>–
Posted via http://www.ruby-forum.com/.
You create an enumerator from an array. When the enumerator’s each
method is
called, it’ll call the array’s each method, which calls the block for
every
element and returns the array itself.
In your first example, you execute the expression x%2==0 for each
element, but
you do nothing for the result. If you want to display the result, you’ll
need
something like
a.each{|x| puts(x%2==0)}
If you want to select only those elements you’ll want to use
Enumerable#select
rather than Enumerable#each:
a1 = a.select{|x| x%2==0}
I hope this helps
Stefano
Stefano C. wrote in post #1094918:
On Sunday 03 February 2013 Arup R. wrote
=> [11, 22, 31, 224, 44]
from (irb):22:in-' from (irb):22:inblock in irb_binding’
from (irb):22:ineach' from (irb):22:ineach’
from (irb):22
from C:/Ruby193/bin/irb:12:in `’
irb(main):023:0>
@Stefano why is it calling Array’s “each” method,instead of
“enum#each”,as i converted it to enumerator?
thanks
On Sunday 03 February 2013 Arup R. wrote
from C:/Ruby193/bin/irb:12:in `<main>'irb(main):023:0>
@Stefano why is it calling Array’s “each” method,instead of
“enum#each”,as i converted it to enumerator?thanks
–
Posted via http://www.ruby-forum.com/.
Because Enumerator#each, in turn, calls the each method of the object it
was
created from.
It is (more or less) if the enumerator was created this way:
class Enumerator
def initialize object, method = :each
@object = object
@method = method
@args = args
end
def each
@object.send :method, *args
end
end
Actually, Enumerator is written in C, so its implementation is different
from
this.
So, when you create an enumerator from an array, the #each method of the
Enumerator calls the #each method of the array.
Stefano
On Sunday 03 February 2013 Arup R. wrote
–
Posted via http://www.ruby-forum.com/.
Again, there’s no difference, since Enumerator#each (when the Enumerator
was
created from an array) calls Array.each.
To clarify things, imagine we create a new class, MyArray, which
contains an
array (to make things simple, we pass the array to the constructor).
This
class has an #each method which takes a block (using the &blk syntax),
then
displays the text “Calling MyArray#each”, then calls the #each method of
the
internal array:
class MyArray
def initialize a
@array = a
end
def each &blk
puts “Calling MyArray#each”
@array.each &blk
end
end
Now, we create an instance of MyArray, and create an enumerator from it.
ma = MyArray.new [1,2,3,4,5]
e = ma.to_enum
Now, try calling the each method once on ma and once on e, and you’ll
see that
they produce the same output:
ma.each{|i| puts i}
e.each{|i| puts i}
In both cases, the output is:
Calling MyArray#each
1
2
3
4
5
=> [1, 2, 3, 4, 5]
As you can see, there’s no difference between the two calls.
Stefano
Stefano C. wrote in post #1094920:
On Sunday 03 February 2013 Arup R. wrote
@Stefano ->
Could you simulate the same example with the enum#each method, just to
understand the difference between enum#each and array#each?
Thanks
Stefano C. wrote in post #1094924:
On Sunday 03 February 2013 Arup R. wrote
Perfect explanation and I understood. What I meant is that to give an
example which will call and work as enum#each.
Thanks
Am 02.02.2013 21:20, schrieb Arup R.:
Why none of the block giving the expected output with the “enumerator”?
They do, you have wrong expectations. (You really should read the docs
and google for examples/tutorials/… to build the right expectations.)
(BTW, next time please tell us what your expected output would have
been.)
=======================================================================
irb(main):017:0> a=[11,22,31,224,44].to_enum
=> #<Enumerator: [11, 22, 31, 224, 44]:each>
irb(main):018:0> a.each{|x| x%2 == 0}
=> [11, 22, 31, 224, 44]
irb(main):019:0> a.each{|x| x%4 == 0}
=> [11, 22, 31, 224, 44]
irb(main):020:0> a.each{|x| x > 22 }
=> [11, 22, 31, 224, 44]
First, you do not need to_enum here, simply use Array#each.
Second, the blocks behave exactly as should be expected.
What you probably meant was something like
1.9.3-p194 :001 > a = [11, 22, 31, 224, 44]
=> [11, 22, 31, 224, 44]
1.9.3-p194 :002 > a.each {|item| puts item if item % 2 == 0 }
22
224
44
=> [11, 22, 31, 224, 44]
You did not ‘do’ anything with the logical expression inside the block.
What you probably misinterpreted as the output (the part after ‘=>’)
is actually the value of the evaluated expresssion, which in this case
is the return value of the each method (-> the array itself, see docs).
Besides the output generated by puts statements etc., irb always
additionally outputs the value of the evaluated expression.
irb(main):021:0> a.each{|x,y| x-y }
TypeError: nil can’t be coerced into Fixnum
from (irb):21:in-' from (irb):21:inblock in irb_binding’
from (irb):21:ineach' from (irb):21:ineach’
from (irb):21
from C:/Ruby193/bin/irb:12:in `’
This cannot work, because #each yields exactly one parameter (see docs).
y therefore always is nil and nil cannot be subtracted from x.
unknown wrote in post #1094928:
Am 02.02.2013 21:20, schrieb Arup R.:
What you probably misinterpreted as the output (the part after ‘=>’)
is actually the value of the evaluated expresssion, which in this case
is the return value of the each method (-> the array itself, see docs).
Besides the output generated by puts statements etc., irb always
additionally outputs the value of the evaluated expression.irb(main):021:0> a.each{|x,y| x-y }
TypeError: nil can’t be coerced into Fixnum
from (irb):21:in-' from (irb):21:inblock in irb_binding’
from (irb):21:ineach' from (irb):21:ineach’
from (irb):21
from C:/Ruby193/bin/irb:12:in `’This cannot work, because #each yields exactly one parameter (see docs).
y therefore always is nil and nil cannot be subtracted from x.
+1 to you. You guided me perfectly! 