I expected row to be [0, 0, 0, 0] and row_index to be nil in the following:
img_array = [
[0, 0, 0, 0],
[0, 0, 0, 0],
[0, 0, 0, 0],
[0, 1, 0, 0],
[0, 0, 0, 0],
[0, 0, 0, 0]
]
img_array.each do |row, row_index|
...
end
Actually, row is 0 and row_index is 0. Can anyone explain this?
img_array = [
[0, 1, 2, 3],
[4, 5, 6, 7],
[8, 9, 0, 1]
]
We have:
enum = img_array.each
#=> #<Enumerator: [[0, 1, 2, 3], [4, 5, 6, 7], [8, 9, 0, 1]]:each>
Array#each therefore creates an enumerator that is an instance of the class Enumerator. The method Enumerator#each passes each element of enum to the block and assigns the block variables:
enum.each { |row, row_index| puts "row=#{row}, row_index=#{row_index}" }
# row=0, row_index=1
# row=4, row_index=5
# row=8, row_index=9
We can see what each of the elements of enum are by using the method Enumerator#next:
enum.next #=> StopIteration: iteration reached an end
Whoops! I forgot to reset the enumerator:
enum.rewind
#=> #<Enumerator: [[0, 1, 2, 3], [4, 5, 6, 7], [8, 9, 0, 1]]:each>
enum.next #=> [0, 1, 2, 3]
enum.next #=> [4, 5, 6, 7]
enum.next #=> [8, 9, 0, 1]
enum.next #=> StopIteration: iteration reached an end
Alternatively, we could convert the enumerator to an array (no need to rewind):
enum.to_a #=> [[0, 1, 2, 3], [4, 5, 6, 7], [8, 9, 0, 1]]
When the element ([0, 1, 2, 3]) is passed to the block, the block variables are assigned as follows:
row, row_index = [0, 1, 2, 3] #=> [0, 1, 2, 3]
row #=> 0
row_index #=> 1
If the variables were |row, row_index, col_index|, the assignment would be:
row, row_index, col_index = [0, 1, 2, 3] #=> [0, 1, 2, 3]
row #=> 0
row_index #=> 1
col_index #=> 2
If they were |row, row_index, *rest|, it would be:
row, row_index, *rest = [0, 1, 2, 3] #=> [0, 1, 2, 3]
row #=> 0
row_index #=> 1
rest #=> [2, 3]
This is called parallel (or multiple) assignment. You can read up on the rules for such assignments at the link I've given.
The method .each() on array in Ruby will call your block with only one argument.
So if you write your code like this:
img_array.each do |row, row_index|
# do something here
end
It will equivalent to this:
img_array.each do |element|
row, row_index = element
# and now
# row = element[0]
# row_index = element[1]
end
I made an example that easier to understand
img_array = [
['a', 'b', 'd', 'e'],
['f', 'g', 'h', 'j']
]
img_array.each do |row, row_index|
p row
p row_index
end
And the result will be:
"a"
"b"
"f"
"g"
Run it online here: https://ideone.com/ifDaVZ
