# CMU 15-112: Fundamentals of Programming and Computer Science Class Notes: Writing Functions

1. Vocabulary
x = 5 def f(y, z): result = x + y + z return result print(f(1, 2)) # 8 print(f(3, 4)) # 12 # Vocabulary: # global variable # local variable # statement # expression # function definition (or declaration) # function call # parameter (or "formal parameter") # argument # return value # return type

2. Return Statements
# Basic example def isPositive(x): return (x > 0) print(isPositive(5)) # True print(isPositive(-5)) # False print(isPositive(0)) # False

Return ends the function immediately:
def isPositive(x): print("Hello!") # runs return (x > 0) print("Goodbye!") # does not run ("dead code") print(isPositive(5)) # prints Hello, then True

No return statement --> return None:
def f(x): x + 42 print(f(5)) # None

Another example:
def f(x): result = x + 42 print(f(5)) # None

3. Print versus Return
# This is a common early mistake (confusing print and return): def cubed(x): print(x**3) # Here is the error! cubed(2) # seems to work! print(cubed(3)) # sort of works (but prints None, which is weird) print(2*cubed(4)) # Error!

Once again (correctly):
def cubed(x): return (x**3) # That's better! cubed(2) # seems to be ignored (why?) print(cubed(3)) # works! print(2*cubed(4)) # works!

4. Different Parameter and Return Types
def hypotenuse(a, b): return ((a**2) + (b**2))**0.5 print(hypotenuse(3, 4)) # 5.0 (not 5) print("---------------------") def xor(b1, b2): return ((b1 and (not b2)) or (b2 and (not b1))) # same as (b1 != b2) print(xor(True, True)) # False print(xor(True, False)) # True print(xor(False, True)) # True print(xor(False, False)) # False print("---------------------") def isPositive(n): return (n > 0) print(isPositive(10)) # True print(isPositive(-1.234)) # False

5. Function Composition
def f(w): return 10*w def g(x, y): return f(3*x) + y def h(z): return f(g(z, f(z+1))) print(h(1)) # hint: try the "visualize" feature

6. Helper Functions
def onesDigit(n): return n%10 def largerOnesDigit(x, y): return max(onesDigit(x), onesDigit(y)) print(largerOnesDigit(134, 672)) # 4 print(largerOnesDigit(132, 674)) # Still 4

7. Test Functions
• A broken test function
def onesDigit(n): return n%10 def testOnesDigit(): print("Testing onesDigit()...", end="") assert(onesDigit(5) == 5) assert(onesDigit(123) == 3) assert(onesDigit(100) == 0) assert(onesDigit(999) == 9) print("Passed!") testOnesDigit() # Passed! Why is this bad?

• A better version
def onesDigit(n): return n%10 def testOnesDigit(): print("Testing onesDigit()...", end="") assert(onesDigit(5) == 5) assert(onesDigit(123) == 3) assert(onesDigit(100) == 0) assert(onesDigit(999) == 9) assert(onesDigit(-123) == 3) # Added this test print("Passed!") testOnesDigit() # Crashed! So the test function worked!

8. Local Variable Scope
def f(x): print("In f, x =", x) x += 5 return x def g(x): return f(x*2) + f(x*3) print(g(2))

Another example:
def f(x): print("In f, x =", x) x += 7 return round(x / 3) def g(x): x *= 10 return 2 * f(x) def h(x): x += 3 return f(x+4) + g(x) print(h(f(1)))

9. Global Variable Scope
# In general, you should avoid using global variables. # You will even lose style points if you use them! # Still, you need to understand how they work, since others # will use them, and there may also be some very few occasions # where you should use them, too! g = 100 def f(x): return x + g print(f(5)) # 105 print(f(6)) # 106 print(g) # 100

Another example:
g = 100 def f(x): # If we modify a global variable, we must declare it as global. # Otherwise, Python will assume it is a local variable. global g g += 1 return x + g print(f(5)) # 106 print(f(6)) # 108 print(g) # 102