Extra Topic: Recursion and Files

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Note: extra topics are not in CS Academy, but are required and can appear on any future homework, writing session, quiz, or exam.

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File System Navigation
Folders can contain folders or files. Since folders can contain other folders, they are a recursive data structure. In fact, they are a kind of recursive structure called a tree (where each value has exactly one parent, and there is a topmost or "root" value). Traversing such a recursive data structure is a natural use of a recursive algorithm!

These programs only run locally (not in the browser), and require that you first download and expand sampleFiles.zip in the same folder as the Python file you are running.

1. printFiles
   
   import os def printFiles(path): # Base Case: a file. Just print the path name. if os.path.isfile(path): print(path) else: # Recursive Case: a folder. Iterate through its files and folders. for filename in os.listdir(path): printFiles(path + '/' + filename) printFiles('sampleFiles') # Note: if you see .DS_Store files in the sampleFiles folders, or in the # output of your function (as often happens with Macs, in particular), # don't worry: this is just a metadata file and can be safely ignored.


2. listFiles
   
   import os def listFiles(path): if os.path.isfile(path): # Base Case: return a list of just this file return [ path ] else: # Recursive Case: create a list of all the recursive results from # all the folders and files in this folder files = [ ] for filename in os.listdir(path): files += listFiles(path + '/' + filename) return files print(listFiles('sampleFiles'))


3. removeTempFiles
   Note: Be careful when using os.remove(): it's permanent and cannot be undone!
   That said, this can be handy, say to remove .DS_Store files on Macs, and can be modified to remove other kinds of files, too. Just be careful.
   import os def removeTempFiles(path, suffix='.DS_Store'): if path.endswith(suffix): print(f'Removing file: {path}') os.remove(path) elif os.path.isdir(path): for filename in os.listdir(path): removeTempFiles(path + '/' + filename, suffix) removeTempFiles('sampleFiles') # removeTempFiles('sampleFiles', '.txt') # be careful

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