knerr cs21 notes...

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WEEK10: recursion
 M: more recursive functions


 - can you write a recursive function to reverse a string?

$ python 
  string: we love computer science
  reversed: ecneics retupmoc evol ew

 Hint: if you have a string, s, and a working recursive revstr 
 function, what will revstr(s[1:]) + s[0] give you??

 What is the base case/stop-the-recursion condition for this one??

def revstr(str):
  """ Use recursion to reverse a given string.  """
  if len(str) <= 1:
    return str
    return revstr(str[1:]) + str[0]

 and just for fun, here is the non-recursive version:

def NRrevstr(str):
  """ Non-Recursive version """
  rs = ""
  for i in range(len(str)-1, -1, -1):
    rs = rs + str[i]
  return rs


 - can you write a recursive function to draw concentric circles?
   here's how it would be called from main: 

    reccircle(cenpt, size, color, win)

   your function should draw a circle at the given center point,
   with the given size and color, then call itself again with a
   smaller size.

   when should the recursion end? 

def reccircle(p, radius, color, w):
  """ draw recursive circles """
  if radius > 1:
    c = Circle(p, radius)
    reccircle(p, radius*0.45, color, w)

So far all of the recursive functions we have looked at could easily be written without recursion. How about the following image? Can you think of ways to create it with and without recursion?

circles pic