Why does each_slice with block and .map return nil

enumerator, ruby

Solution

Some methods expect a block, and if block is not provided, they return `Enumerable`.

In your example, you invoked `map` without providing any block.

But you can call enumerable methods on enumerators. This allows you to change the behaviour of the iterators.

array[0].split('').each_slice(9).map { |el| el }

=> [["0", "1", "5", "0", "0", "3", "0", "0", "2"],
 ["0", "0", "0", "1", "0", "0", "9", "0", "6"],
 ["2", "7", "0", "0", "6", "8", "4", "3", "0"],
 ["4", "9", "0", "0", "0", "2", "0", "1", "7"],
 ["5", "0", "1", "0", "4", "0", "3", "8", "0"],
 ["0", "0", "3", "9", "0", "5", "0", "0", "0"],
 ["9", "0", "0", "0", "8", "1", "0", "4", "0"],
 ["8", "6", "0", "0", "7", "0", "0", "2", "5"],
 ["0", "3", "7", "2", "0", "4", "6", "0", "0"]]

It's one of slightly more advanced topics in Ruby but there are lot of resources in the internet, f.i. http://www.sitepoint.com/guide-ruby-collections-iii-enumerable-enumerator/

EDIT:

Just to answer the questions in the comment: You have only one long string in `array`. I don't ask why, but just to make some example, I needed actual array of (string?) numbers. `array[0]` takes this first element in array (the long string of numbers). When I have a string I can call `String#split` to get an array like `["0", "1", "5", "..."].

`Array` includes `Enumerable` module, thanks to that you can call methods like `each_slice` on it. As the docs state (http://rubydoc.info/stdlib/core/Enumerable:each_slice):

If no block is given, returns an enumerator

So after calling `each_slice(9)` I get `Enumerator` instance (http://rubydoc.info/stdlib/core/Enumerator). It has some own methods, but also includes `Enumerable` module (with methods like `map`, `each_slice`. Here's where the fun begins. If I call any of those methods on our enumerator (like `map`), it won't receive each element of array, like in normal case, but each element "returned" by enumerator, i.e. each single element is now array of 9 elements.

As always I highly recommend the Well Grounded Rubyist, where such subjects are thoroughly explained. http://www.manning.com/black2/

Problem

Why does this return nil and what does line 2 mean? The array represents rows of a Sudoku puzzle which I'm trying to create a solution for. I'm trying different methods in irb to see how many different ways I can view the array. ``` array = ['015003002000100906270068430490002017501040380003905000900081040860070025037204600'] array.each_slice(9) { |e| puts e.map } #<Enumerator:0x0000010288a470> => nil ```

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