Finish the prereqs function so that it recursively finds all of the course titles in courses.

Ira Bradley, glad you were able to figure it out. I was also initially stumped on this one until I realized the desired solution is only the prerequisites and does not include the top-level course title. That is, the top course isn't a prerequisite of itself. Solution contains 5 courses.

Here is an alternative solution that might be more easily understood:

def prereqs(data, pres=None):
    pres = pres or set()
    # for each prereq in this courses' prereqs...
    for prereq in data['prereqs']:
        # add title of this prereq course, then...
        pres.add(prereq['title'])
        # use recursive call to find further prerequisites of this
        # course, if any
        prereqs(prereq, pres)
    # return current 
    return pres

Hey,

Yeah this wont be easy to explain so grab a beer and let's ponder this lol.

What is happening is that your code is building fine. EXCEPT you are calling a function in a function. What this means is that when the return statement is hit. It's ok, everything looks find. But since you called the function in a function, it actually calls the function one more time. Not the entire function, but the inner function. The inner function is the function that calls it'self.

def prereqs(data, pres=None):    <---------OUTER FUNCTION
    pres = pres or set()
    if (data['prereqs']):
        for course in data['prereqs']:
            pres.add(course['title'])
        data = data['prereqs'][0]
        prereqs(data, pres)    <---------------------INNER FUNCTION
    else:
        return (pres)

What this means is that the function is in two places, and essentially, when it returns data. It takes the most inner portion of it'self.

Once you hit the return statement, since you ran a function in a function. The return statement kicks the process flow to the most inner function. Than that is the function that returns it data up to the higher version or outer version of it'self. Which ultimately is the function we except to get our data from.

Like this

A = prereqs(courses)

I would expect the function to return to A the value of pres. But it doesn't. The outer function runs. Calls the inner which in turn calls the outer. When the else statement runs. pres is returned but it's returned to the function. Which is the inner function. Not the outter function. Since the inner function is what called the outer function that produced the return statement.

When the outer Function is ran it runs. But when the inner function calls the outer function. And the if statement doesn't run the inner fucntion because it returns a false. The else statement runs for the outter function, but the last funciton to call the outer function was the inner function. So the data is returned to the inner function. This cause what is known as func_closure. Must close the inner function, but the innerfunction didn't really do anything except call the outter function. So the inner functions not going to return our list to A because our list was already returned to the inner Function. SO what gets put into A is actually a NoneType: None from the inner function.

Does this make any sense lol. It's a tuff topic I will break it down more if you need. lol

Ira Bradley, glad you were able to figure it out. I was also initially stumped on this one until I realized the desired solution is only the prerequisites and does not include the top-level course title. That is, the top course isn't a prerequisite of itself. Solution contains 5 courses.

Here is an alternative solution that might be more easily understood:

def prereqs(data, pres=None):
    pres = pres or set()
    # for each prereq in this courses' prereqs...
    for prereq in data['prereqs']:
        # add title of this prereq course, then...
        pres.add(prereq['title'])
        # use recursive call to find further prerequisites of this
        # course, if any
        prereqs(prereq, pres)
    # return current 
    return pres

Hi Ryan,

Thanks for the help. I'm still confused however. My return object has the same titles in it that yours does including 'Python Collections'. We both have 5 titles.

As far as I can tell there is no difference between our functions. Removing the else statement doesn't change when the return gets run. I must be missing something but am still not seeing it.

def prereqs(data, pres=None):
    pres = pres or set()
    if (data['prereqs']):
        [pres.add(course['title']) for course in data['prereqs'] ]
        data = data['prereqs'][0]
        prereqs(data, pres)
    return (pres)

I ended up passing with the above code. I still don't understand fully while the else: was screwing it up? My thinking is that when the if statement is true we enter that code block and then recursively call prereqs. When the if statement isn't true we hit the return. I don't see how removing the 'else' changes the control flow but it obviously does.

I'd really appreciate if you can explain that one to me.

Ryan,

Thanks for this explanation. I "knew" this but somehow forgot it temporarily. I was struggling to figure out why the return wasn't kicking us out of the function entirely but now I understand.

Thanks again. I officially owe you a beer :)

As an alternative, you could implement the function with a list comprehension:

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

    [(pres.add(course['title']), prereqs(course, pres)) for course in data['prereqs'] if data['count'] > 0]

    return pres