cs2370 Notes: 16 Directory Trees and Recursion

Continuing on from last class.

File Handles

The open bif gives us a file object. By default files are opened in text mode to read a series of characters, but they also can be opened in binary mode to read a series of bytes.

Methods (on text file):

• close - When we’re done; open files are a limited resource.
• readline - Read one line
• write - write a string to the file
• read - read the whole file

Do some examples, including writing to file.

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