How does Array's each method change its elements ?

[w wg]

Hi,

I found Array's each method (block) changes its elements sometimes,
but sometimes it don't.

Please look this:

a = [1,2,3,4,5]
b = a.each { |x| x + 1}

puts a
[1,2,3,4,5]
puts b
[1,2,3,4,5]

The array keeps no changed.


But, look another:

a = ["how","are","you"]
b = a.each { |x| x << "-" }
puts a
["how-","are-","you-"]
puts b
["how-","are-","you-"]

The array changed!

I want to know how does Array's each method (block) change its elements ?

Thank you.

 

[Sebastian Hungerecker]

I want to know how does Array's each method (block) change its elements ?

It doesn't. You do.

b = a.each { |x| x + 1}

This executes "x + 1" for every element in the array. Compare the following:
x = 5
x + 1
p x
x is still 5. The expression x+1 does not change the value of x.

b = a.each { |x| x << "-" }

x = "foo"
x << "-"
p x
Prints "foo-" because << does modify the receiver. I.e. after calling << "-"
the string will contain a "-", so that's what's happening to each element in
the array.

Another note: each returns the receiver, so if you do
b = a.each {...}
b and a will point to the exact same array no matter what happens in the block
(unless you use break inside the block).

HTH,
Sebastian

 

[Peña, Botp]

From: w wg [mailto:[email protected]]=20
#..
# a =3D ["how","are","you"]
# b =3D a.each { |x| x << "-" }
# puts a
# ["how-","are-","you-"]
# puts b
# ["how-","are-","you-"]
#=20
# The array changed!
#=20
# I want to know how does Array's each method (block) change=20
# its elements ?

each just iterates on each element/object of the array. an array is just =
a container of objects (wc includes also arrays). if you use << or =
concat method, you change the object, ergo, you change the array too.

try array#map and just use simple + for concatenation, eg

a

=3D> ["how", "are", "you"]

c =3D a.map{ |x| x + "-" }

=3D> ["how-", "are-", "you-"]

a

=3D> ["how", "are", "you"]

c

=3D> ["how-", "are-", "you-"]

also note, strings are mutable objects, but, numbers are not.
see,

a=3D[1,2,3]

=3D> [1, 2, 3]

b=3Da.each{|x| x+=3D1}

=3D> [1, 2, 3]

a

=3D> [1, 2, 3]

b

=3D> [1, 2, 3]

no change
the expression x+=3D1 seems to change the object pointed by var x. but =
no. a new object (eg 2) is now pointed by x, wc is outside of the array. =
ruby cannot change the object 1. for ruby, 1 is always 1. Since we did =
not do anything on the new objects 2,3,4, they just get lost/cleanuped. =
They were there before. You can print them if you're in doubt. But you =
can save them easily if you use array#map.

let's use again map

b=3Da.map{|x| x+=3D1}

=3D> [2, 3, 4]

a

=3D> [1, 2, 3]

b

=3D> [2, 3, 4]

hth
kind regards -botp

 

[w wg]

I have noticed that Number elements and String elements of an array
generate diffrent results in its EACH method if you change the
elements. But it is difficult to understand how ruby implements
arrays each method block.

From: w wg [mailto:[email protected]]
#..
# a =3D ["how","are","you"]
# b =3D a.each { |x| x << "-" }
# puts a
# ["how-","are-","you-"]
# puts b
# ["how-","are-","you-"]
#
# The array changed!
#
# I want to know how does Array's each method (block) change
# its elements ?

each just iterates on each element/object of the array. an array is just =

a container of objects (wc includes also arrays). if you use << or concat m=
ethod, you change the object, ergo, you change the array too.

try array#map and just use simple + for concatenation, eg

a

=3D> ["how", "are", "you"]

c =3D a.map{ |x| x + "-" }

=3D> ["how-", "are-", "you-"]

a

=3D> ["how", "are", "you"]

c

=3D> ["how-", "are-", "you-"]

also note, strings are mutable objects, but, numbers are not.
see,

a=3D[1,2,3]

=3D> [1, 2, 3]

b=3Da.each{|x| x+=3D1}

=3D> [1, 2, 3]

a

=3D> [1, 2, 3]

b

=3D> [1, 2, 3]

no change
the expression x+=3D1 seems to change the object pointed by var x. but no=

a new object (eg 2) is now pointed by x, wc is outside of the array. ruby=
cannot change the object 1. for ruby, 1 is always 1. Since we did not do a=
nything on the new objects 2,3,4, they just get lost/cleanuped. They were t=
here before. You can print them if you're in doubt. But you can save them e=
asily if you use array#map.

let's use again map

b=3Da.map{|x| x+=3D1}

=3D> [2, 3, 4]

a

=3D> [1, 2, 3]

b

=3D> [2, 3, 4]

hth
kind regards -botp

--=20

 

[Sebastian Hungerecker]

I have noticed that Number elements and String elements of an array
generate diffrent results in its EACH method if you change the
elements.

That's not true. You simply can not change a Numeric object.

But it is difficult to understand how ruby implements
arrays each method block.

Array's each method is implemented like this, basically:
class Array
def each()
0.upto(length-1) do |i|
yield self
end
return self
end
end
Except that in reality it's a) not implemented in ruby and b) not using upto.
But if you'd redefine each using the above code there should be no difference
in behaviour except if you also redefine upto.

HTH,
Sebastian

 

[Sebastian Hungerecker]

There is also "<<" for integers but it works
different and doesn't change objects because of fact that integers are
immutable in Ruby.

Well, I would assume that if Integers were mutable, << would still not be a
mutating method as that would really not make much sense.

=2D-=20
Jabber: (e-mail address removed)
ICQ: 205544826

 

[Rick DeNatale]

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