Reimplementing Enumerable Map method in Ruby

Question

I'm practicing for an internship interview at a ruby shop. One of the job questions I'm expecting is to reimplement an enumerable method.

I'm trying to implement map right now and I'm having trouble figuring out how to implement the case where a block is not given.

class Array
    def mapp()
      out = []
      if block_given?
        self.each { |e| out << yield(e) }
      else
        <-- what goes here? -->
      end
   out
   end
 end

Using my current implementation. If I run:

[1,2,3,4,5,6].mapp{|each| each+1} #returns => [2,3,4,5,6,7]

However, I'm not sure how to get cases where a block isn't passed in:

[1,2,3,4].mapp("cat") # should return => ["cat", "cat", "cat", "cat"]

If someone could point me in the right direction. I'd really appreciate it. I tried looking through the source code but it seems to do things very differently than what i'm used to.

static VALUE
enum_flat_map(VALUE obj)
{
VALUE ary;

RETURN_SIZED_ENUMERATOR(obj, 0, 0, enum_size);

ary = rb_ary_new();
rb_block_call(obj, id_each, 0, 0, flat_map_i, ary);

return ary;
}

Answer 1

I suppose that by [1,2,3,4].mapp("cat") you mean [1,2,3,4].mapp{"cat"}.

That said, map without a block returns an enumerator:

 [1,2,3,4].map
 => #<Enumerator: [1, 2, 3, 4]:map>

That is the same output of to_enum

[1,2,3,4].to_enum
 => #<Enumerator: [1, 2, 3, 4]:each> 

So in your code, you just want to call to_enum:

class Array
    def mapp()
        out = []
        if block_given?
            self.each { |e| out << yield(e) }
        else
            out = to_enum :mapp
        end
        out
    end
end