Ruby Variable (Array) Assignment Misunderstanding (With Push Method)

Ruby variable (Array) assignment misunderstanding (with push method)

Ruby works with references! Keep that in mind. If you want a copy you'd have to do it like:

@instance_ar = [1,2,3,4]
local_ar = @instance_ar.clone
local_ar_2 = local_ar.clone

Edit:

Examples:

a = ["a", "b", "c"]
b = a[0]
b = "d" # We assign a new object to b!

a is:
=> ["a", "b", "c"]

but:

a = ["a", "b", "c"]
b = a[0]
b[0] = "d" # We are working with the reference!

a is:
=> ["d", "b", "c"]

a = "hello"
b = a
b += " world" 
# Is the same as b = b + " world", we assign a new object!

a is:
=> "hello"

but:

a = "hello"
b = a
b<<" world"
# We are working with the reference!

a is:
=> "hello world"

a = "abc"
b = a
b[0] = "d" # we are working with the reference

a is:
=> "dbc"

You can read everything about it here: http://ruby-doc.org/docs/ProgrammingRuby/. Scroll down to "Variables" almost at the bottom of the page.

Array asignment error

Because other_array is @arr.

......................

Accumulator for each_with_object keeps re-initializing

Each iteration gets the same object, so you either need to mutate the object inside the block, or use reduce.

def gcp(dims)
  first = dims.shift
  dims.reduce((0...first).to_a) do |v, dim|
    puts "\nv: #{v}, dim: #{dim}"
    p v.product((0...dim).to_a)
  end
end

gcp([3,2,4])

Results in:

v: [0, 1, 2], dim: 2
[[0, 0], [0, 1], [1, 0], [1, 1], [2, 0], [2, 1]]

v: [[0, 0], [0, 1], [1, 0], [1, 1], [2, 0], [2, 1]], dim: 4
[[[0, 0], 0], [[0, 0], 1], [[0, 0], 2], [[0, 0], 3], [[0, 1], 0], [[0, 1], 1], [[0, 1], 2], [[0, 1], 3], [[1, 0], 0], [[1, 0], 1], [[1, 0], 2], [[1, 0], 3], [[1, 1], 0], [[1, 1], 1], [[1, 1], 2], [[1, 1], 3], [[2, 0], 0], [[2, 0], 1], [[2, 0], 2], [[2, 0], 3], [[2, 1], 0], [[2, 1], 1], [[2, 1], 2], [[2, 1], 3]]

Reassigning Elements of Array: Syntax (Ruby)

The array[0,1] syntax is picking a slice of the array starting at 0 and of length 1. Longer slices make that more obvious.

> a = [1,2,3]
 => [1,2,3]
> a[0,2]
 => [1, 2]

To swap the way you want in your first example, you need to specify both indices independently.

> a[0], a[1] = a[1], a[0]
 => [2, 1]
> a
 => [2, 1, 3]

In your second example, Ruby replaces the array[0,1] slice with [1, 0], effectively removing the first element and inserting the new [1, 0]. Changing to array[0], array[1] = [1, 0] will fix that for you too.

erb idiom to handle undefined variable

I would do this:

<% if defined?(environment) %>
  <% Array(environment).each do |f| %>
  one line: <%= f %>
  <% end %>
<% end %>

I didn't understand why you joining on new lines and then splitting on them again, so I removed it from the example.

How do I modify an array while I am iterating over it in Ruby?

Use map to create a new array from the old one:

arr2 = arr.map {|item| item * 3}

Use map! to modify the array in place:

arr.map! {|item| item * 3}

See it working online: ideone

Cloning an array with its content

You need to do a deep copy of your array.

Here is the way to do it

Marshal.load(Marshal.dump(a))

This is because you are cloning the array but not the elements inside. So the array object is different but the elements it contains are the same instances. You could, for example, also do a.each{|e| b << e.dup} for your case

In Ruby, is there an Array method that combines 'select' and 'map'?

I usually use map and compact together along with my selection criteria as a postfix if. compact gets rid of the nils.

jruby-1.5.0 > [1,1,1,2,3,4].map{|n| n*3 if n==1}    
 => [3, 3, 3, nil, nil, nil] 


jruby-1.5.0 > [1,1,1,2,3,4].map{|n| n*3 if n==1}.compact
 => [3, 3, 3] 

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