1. Barb Ericson ericson@cc.gatech.edu Georgia Tech
Recursion
Barb Ericson
Georgia Tech

2. Recursion When a function calls itself def downUp(word ): print word
Used to break problems into smaller problems
Used to solve recursion problems
def downUp(word ):
print word
if len(word )==1:
return
downUp(word [1:])
>>> downUp("Hello")

3. Print filenames in a directory tree
import os
import java.io.File as File
def printAllFiles(directory ):
files = os.listdir(directory)
for file in files:
fullname = directory+"/"+file
if isDirectory(fullname ):
printAllFiles(fullname)
else:
print fullname
def isDirectory(filename ):
filestatus = File(filename)
return filestatus.isDirectory ()

4. Decrease Red Recursively
def decreaseRedR(aList ):
if aList == []: # empty
return
setRed(aList [0], getRed(aList [0])*0.8)
decreaseRedR(aList [1:])

5. What is the output? def test(num): Call Stack if num > 0:
test(0) return 0
test(1) return test(0) + 1
test(2) return test(1) + 2
test(3) return test(2) + 3
def test(num):
if num > 0:
return test(num -1) + num
else:
return 0
test(0) returns 0, test(1) returns 1, test(2) returns 3, test(3) returns 6

6. Stacks Add and remove from the top of the stack
Last in first out type of structure
Like a stack of plates in a cafeteria
Or a Pez container
Or a childs toy

7. Queue Add to the end and remove from the front
First in and first out type of structure
Like a line at a store checkout
Or a printer queue