functions (more details)

motivation

Understanding stack diagrams helps you understand how the computer works, and why python does what it does. It will also be vary valuable in week 10 when we learn recursion (functions calling multiple copies of themselves).

examples

Take a look at the following program and see if you can figure out what it will do and what it will print to the screen.

def main():
  S = "we love comp sci!"
  L = list(S)
  numVowels = capitalizeVowels(L)
  newS = "".join(L)
  print(numVowels)
  print(newS)

def capitalizeVowels(mylist):
  """count and capitalize the vowels"""
  count = 0
  for i in range(len(mylist)):
    if isVowel(mylist[i]):
      mylist[i] = mylist[i].upper()
      count += 1
  return count

def isVowel(ch):
  """return True if ch is a vowel, False if not"""
  vowels = "aeiou"
  if ch.lower() in vowels:
    return True
  else: 
    return False

main()

After you think you know what it will do, copy the text of the program into the python tutor and visualize its execution. Here are some things to note as you run the program:

the first thing that happens is all functions are defined
as main() is run, a blue box is put on "top" of the stack, with any variables declared in main() shown in the box (or with arrows pointing to the data of the variable)
when capitalize() is called from main(), another blue box is put on "top" of the stack. The function on top of the stack is the function that is currently executing
in capitalize(), the mylist variable points to the same data that L in main() points to. This means any changes made to mylist[i] will affect L in main()
each time isVowel() is called, there is a blue box for it put on top of the stack. When isVowel() ends, the box (and all variables in the box!) disappears and we drop back to capitalizeVowels(), the function that called isVowel()

python tutor image

The final output of the program is this:

5
wE lOvE cOmp scI!

Do you understand how L was changed in main(), even though capitalizeVowels() did not return mylist?

challenge

Write a function called listmax(L,maxval) that has two parameters: a list (L) of numbers and a maximum value (maxval). The function should reset any values in L that are above maxval. For example:

calling listmax(L,255) with L=[100,200,300] should change L to be [100,200,255]
calling listmax(L,100) with L=[100,200,300] should change L to be [100,100,100]

CS21 Topics