Why is my block only executed once? - ruby

Question

Why is it that:
array = (1..20).to_a
array.index.each_slice(5) do |slice|
puts slice.inspect
end
returns:
[1, 2, 3, 4, 5]
[6, 7, 8, 9, 10]
[11, 12, 13, 14, 15]
[16, 17, 18, 19, 20]
while:
other_array = []
array = (1..20).to_a
array.index.each_slice(5) do |slice|
puts slice.inspect
other_array.push(1)
end
returns only:
[1, 2, 3, 4, 5]
How does other_array.push(1) breaks the execution of the block? An obvious conclusion would be that I cannot access variables that are not in the scope of the block, but why is that?

Answer 1

I found the solution in the array documentation. I wondered why you used the index function for the array when it seems like you just want to iterate over the array. For this you can use array.each_slice without invoking index.
Index says the following: http://ruby-doc.org/core-2.2.0/Array.html#method-i-index
Returns the index of the first object in ary such that the object is == to obj.
If a block is given instead of an argument, returns the index of the first object for which the block returns true. Returns nil if no match is found
So your code evaluates the block and checks if the result is true.
In the first example you do only a puts which returns nil. Nil is false.
The second example returns an object of an array containing a single 1.
In ruby every condition is true, if it is not false or nil.
You can see this here:
if nil
puts "foo"
end
=> nil
other_array = [1]
if other_array
puts "foo"
end
=> "foo"
So the block in your second example returns something not-false so it will not run again, because it found a "valid" result.
For the return, you maybe should know that ruby returns the last expression in any scope, if no other return is given. So it returns other_array.
If you don't want to reformat your code you could to the following:
other_array = []
array = (1..20).to_a
array.index.each_slice(5) do |slice|
puts slice.inspect
other_array.push(1)
nil
end
This will force the block to return nil and the iteration will work.

Answer 2

It seems tricky at the first glance, but if you check the docs for
Array#index
you'll see that this method return Enum if no block given.
Then you call #each_slice(n) on that Enum object with a block:
in the first case:
do |slice|
puts slice.inspect
end
it returns nil each time to #index method. You'll get the same result if you call array.index.each_slice(5) { nil } or
in the second one:
do |slice|
puts slice.inspect
other_array.push(1)
end
value of this block getting evaluated inside block and returns [1] to the #index method, so it returns the first slice of the array. You'll get the same result if you call array.index.each_slice(5) { any_non_falsy_object }