# Beginner Question on Ruby

Hi all,

I am beginning to learn ruby and cant seem to reason this behaviour. I
expect since empty or nill is the array, arr*n shouldnt change it.

Why is this behaviour so?

Code:

num_arr = Array.new
puts “Num Alpha: #{num_arr} and size: #{num_arr.size}”
puts “Num Alpha: #{num_arr << num_arr} and size: #{num_arr.size}”
num_arr.insert(-1, num_arr << num_arr)
puts “Num Alpha: #{num_arr} and size: #{num_arr.size}”

Output:

Num Alpha: and size: 0
Num Alpha: […] and size: 1
Num Alpha: […][…][…][…][…][…][…][…][…] and size: 3

Hi –

On Fri, 2 May 2008, Arun K. wrote:

puts “Num Alpha: #{num_arr} and size: #{num_arr.size}”
puts “Num Alpha: #{num_arr << num_arr} and size: #{num_arr.size}”
num_arr.insert(-1, num_arr << num_arr)
puts “Num Alpha: #{num_arr} and size: #{num_arr.size}”

Output:

Num Alpha: and size: 0
Num Alpha: […] and size: 1
Num Alpha: […][…][…][…][…][…][…][…][…] and size: 3

You’re inserting an array into itself. Ruby represents that kind of
recursive inclusion with dots.

irb(main):005:0> a = []
=> []
irb(main):006:0> a << a
=> [[…]]

David

Hello David,

But would you know why it is so? Is there something syntactic or
semantic
issue that I am making,

the reason i ask this is…

n = 10
x += n
does a perfect x = nil + n => n initialization, which is good and
explainable.

In the array case however,
One would expect [atleast i am ] the elements of the source array [if
any] be appended to the target. Now why is the reference going in making
it
a recursive :?

sorry if this question is lame or bozo but i feel a clear behaviour
enables
clear understanding or would require even more clearer reasoning.

Cheers
Arun

Hi –

On Fri, 2 May 2008, Arun K. wrote:

expect since empty or nill is the array, arr*n shouldnt change it.

irb(main):005:0> a = []

the reason i ask this is…

n = 10
x += n
does a perfect x = nil + n => n initialization, which is good and
explainable.

You can’t add 10 to nil. You’ll get an error if you try to do that,
because nil has no + method.

In the array case however,
One would expect [atleast i am ] the elements of the source array [if
any] be appended to the target. Now why is the reference going in making it
a recursive :?

sorry if this question is lame or bozo but i feel a clear behaviour enables
clear understanding or would require even more clearer reasoning.

You’ve using the << method:

num_arr = Array.new
num_arr << num_arr

which appends the object num_arr (not its elements, but it, itself) to
the object num_arr.

If you want to add the elements of an array to another array, you can
use concat:

irb(main):001:0> a = [1,2,3]
=> [1, 2, 3]
irb(main):002:0> a.concat(a)
=> [1, 2, 3, 1, 2, 3]

It’s all perfectly reasonable – you just have to know what the
methods you’re using actually do

David

And now i do thanks.