cs2370 Notes: 13 Iteration and Recursion

Directory Traversals

Let’s write a script that prints the path to all the .py files in a directory.

The first thing we need is to be able to list the stuff in a directory.

(Save this in scratch/cs2370/16/findpy.py. Run with python ../16/findpy.py)

from pathlib import Path

wd = Path.cwd()

for item in wd.iterdir():
    print(item)

Running that just gets us the stuff in the current directory, mostly more directories. We want to be able to look in those too. Let’s add a function.

from pathlib import Path

def search(path):
    for item in wd.iterdir():
        print(item)

wd = Path.cwd()
search(wd)

Now, if an item is a directory, we can search that too:

def search(path):
    for item in path.iterdir():
        if item.is_dir():
            search(item)
        else:
            print(item)

Now we’ve got a function that calls itself, or a “recursive” function. Functions like this come up a lot in programming and are extremely useful and powerful, but they can be a bit tricky to reason about when you’re learning to program.

Key ideas about recursive functions:

A recursive function can’t always call itself. That program would never finish.
The scenario when the function doesn’t call itself again is the “base case”. When you’re thinking about a recursive function, you want to start thinking from the base case.
- For search, this is when none of the items in the directory at path are a directory.
The scenario when the function does call itself is the “general case” or “recursive case”.
- For search, this when

To figure out the general case of a recursive function:

First, identify the base case and make sure the function makes sense in that scenario.
Next, we’ll look at the general case:
- We checked the base case, so we assume that’s good.
- So let’s look at a specific call to the function in the general case.

More steps:

Check if item is a .py file, and only print it then
Return a list instead of printing.
- How to combine results?

More recursion examples:

Sum the numbers from 1..xx
Sum a list of lists of numbers