Recursion Ruby example

Question

I've seen this example whilst working through a non Treehouse exercise. Please could some take me through it. In #caplock I've tried to show my understanding but would really appreciate a walk through and guidance.

def mergesort(array) #NEW METHODS WHICH TAKE THE ARGUMENT ARRAY 
def merge(left_sorted, right_sorted) #A NEW METHOD, WHICH TAKES TO ARGUMENTS. 
res = [] # THE RESULT OF THIS METHOD WILL CREATE AN EMPTY STRING ASSIGNED TO VARIABLE CALLED RES. 
l = 0 #INITIAL STARTING VALUE 
r = 0 #INITIAL STARTING VALUE

loop do #LOOP 
  break if r >= right_sorted.length and l >= left_sorted.length # EXIT THE LOOP IF R >= R SORTED LENGTH. 

  if r >= right_sorted.length or (l < left_sorted.length and left_sorted[l] < right_sorted[r])
    res << left_sorted[l] #PLEASE CAN SOMEONE EXPLAIN THE STRUCTURING HERE AND THE PARENTHESIS. 
    l += 1 #INCREMENT 
  else
    res << right_sorted[r]
    r += 1 #INCREMENT 
  end
end

return res #METHOD RETURN RESULT
end

def mergesort_iter(array_sliced) #NEW METHOD WHICH TAKES THIS ARGUMENT. return array_sliced if array_sliced.length <= 1 #I DON'T GET THIS

mid = array_sliced.length/2 - 1 #OR THIS OR THE REST THE BELOW. IN PARTICULAR HOW ID MID USED BELOW? 
left_sorted = mergesort_iter(array_sliced[0..mid])
right_sorted = mergesort_iter(array_sliced[mid+1..-1])
return merge(left_sorted, right_sorted)
end

If someone could help with an idiot's guide walk though, i would be immensely grateful.

Thanks, Sara

[MOD: edited code block]

Answer 1 · 2016-01-10T11:52:31Z

January 10, 2016 11:52am

line 11 is the same as a push method that adds a variable to the end of an array.

the entire mergesort_iter method Though it looks like there might be some code missing:

  def mergesort_iter(array_sliced) #NEW METHOD WHICH TAKES THIS ARGUMENT.
    return array_sliced if array_sliced.length <= 1 # Returns the argument/array if the array has 0 or 1 items in it
    mid = array_sliced.length/2 - 1 # Take the array length(number of items) and devide it by 2 then subtract 1
    left_sorted = mergesort_iter(array_sliced[0..mid]) #uses the mergesort_iter method with an argument of the arrays items that are between 0 and whatever mid comes up with
    right_sorted = mergesort_iter(array_sliced[mid+1..-1]) #same thing but with different items in the array
    return merge(left_sorted, right_sorted) #return everything that just happened.
  end

try testing the mergesort_iter method by calling it with an argument. play around, you wont break anything. :)

Happy Coding

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Sara Crwly

Sara Crwly

Recursion Ruby example

1 Answer

Houston Erekson

Houston Erekson