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Python Functional Python The Lambda Lambada Recursion

David Hernandez
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David Hernandez
Python Development Techdegree Graduate 12,180 Points
Posted by David Hernandez
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David Hernandez
Python Development Techdegree Graduate 12,180 Points

Need some help with this recursion problem, I am truly lost here. I know I want to find a base step...

I'm going to try a bit more, but more guidance would be appreciated. I'm thinking somehow dig deeper to get to prereqs and titles until there are no prereqs.

courses.py 
courses = {'count': 2,
           'title': 'Django Basics',
           'prereqs': [{'count': 3,
                     'title': 'Object-Oriented Python',
                     'prereqs': [{'count': 1,
                               'title': 'Python Collections',
                               'prereqs': [{'count':0,
                                         'title': 'Python Basics',
                                         'prereqs': []}]},
                              {'count': 0,
                               'title': 'Python Basics',
                               'prereqs': []},
                              {'count': 0,
                               'title': 'Setting Up a Local Python Environment',
                               'prereqs': []}]},
                     {'count': 0,
                      'title': 'Flask Basics',
                      'prereqs': []}]}


def prereqs(data, pres=None):
    pres = pres or set()

2 Answers

Jeff Muday
MOD
Jeff Muday
Treehouse Moderator 28,722 Points
Jeff Muday
Jeff Muday
Treehouse Moderator 28,722 Points

This one is pretty tough, but it can be broken down... kind of a divide-and-conquer.

We are interested in the prerequisites-- it is a list. We'll loop over the list and for each item in the list data.get('prereqs') we call the prerequisite function prereqs() recursively.

This can be done MORE efficiently, but I think it's more clear this way.

def prereqs(data, pres=None):
    pres = pres or set()
    # step 0. get the courses as a list
    courses = data.get('prereqs')
    for course in courses:
        # step 1. add the title only to the set
        # step 2. call the function recursively
    # outside the loop
    # step 3. return the set pres

SPOILER ALERT

def prereqs(data, pres=None):
    """ find prerequisites RECURSIVELY and return the set()"""
    pres = pres or set() # basically if pres is None, it instead makes it an empty set

    # want to loop over the (prerequisite) courses
    courses = data.get('prereqs')
    for course in courses:

        # now, we want to add the title to our set
        # the cool thing is a set won't repeat a course!
        title = course.get('title')
        pres.add( title )

        # now, the recursive call to see if there are prerequisites to
        # the particular course we just added
        prereqs(course, pres)

    # return the finished set
    return pres
David Hernandez
seal-mask
.a{fill-rule:evenodd;}techdegree seal-36
David Hernandez
Python Development Techdegree Graduate 12,180 Points
David Hernandez
.a{fill-rule:evenodd;}techdegree seal-36
David Hernandez
Python Development Techdegree Graduate 12,180 Points

This is what I was able to come up with. 

def prereqs(data, pres=None):
    pres = pres or set()

    # current instance could be a list or dictionary
    if isinstance(data, list):
        if not data:
            return pres

        # prereqs exist, grab title and dig deeper
        pres.add(data[0]['title'])
        prereqs(data[0]['prereqs'], pres)

        # iterate through the list of prereqs
        return prereqs(data[1:], pres)

    return prereqs(data['prereqs'], pres)
Jeff Muday
MOD
Jeff Muday
Treehouse Moderator 28,722 Points
Jeff Muday
Jeff Muday
Treehouse Moderator 28,722 Points

Awesome! I like that you handle the recursion in a fresh way. You nailed that Challenge!