Lecture 9, Tue 02/06

Functions

Recorded Lecture: 2_6_24

Function return Statement

• As discussed previously, a function may or may not have a return statement
• A function can only execute a return statement ONCE
• When a return statement is executed, Python stops executing code in the function, and the return value (or None) is returned back to the caller
• Example:

def lotsOfReturns():
	print("before return 1")
	return 1
	print("after return 1") # will never print (might as well delete)
	return 2 # Will never execute (might as well delete)

print(lotsOfReturns()) # 1

• return statements don’t necessarily need to return a value (None is returned by default)
• But if a return statement with or without a value is executed, the function will stop executing

def lotsOfReturns():
	print("before return 1")
	return # None is returned
	print("after return 1") # will never print (might as well delete)
	return # Will never execute (might as well delete)

print(lotsOfReturns()) # None

Practice

• Let’s go back and consider the example of determining the class standing of a student, but in this case, we can put our logic into a function:

def getClassStanding(units):
	''' Returns a string of either Freshman, Sophomore, Junior,
		Senior depending on the units '''

	if units < 50:
		return "Freshman"
	elif units < 90:
		return "Sophomore"
	elif units < 135:
		return "Junior"
	else:
		return "Senior"

numUnits = int(input("Enter number of units: "))
print(getClassStanding(numUnits))

• Note that the function definition must be declared before it is called
  • Python runs our code from top to bottom
  • If Python doesn’t see a function definition before it tries to call the function, it won’t know that the function exists
  • Try moving the function definition below the print statement, and see Python give you an error
• Also note, there are many ways we can structure our code to do the same thing
• Since the getClassStanding(units) immediately returns a value when a return statement is executed, we could possibly rewrite the function without elifs as:

def getClassStanding(units):
	''' Returns a string of either Freshman, Sophomore, Junior,
		Senior depending on the units '''

	if units < 50:
		return "Freshman"
	if units < 90:
		return "Sophomore"
	if units < 135:
		return "Junior"
	else:
		return "Senior"

biggestInt Example

• As an exercise, consider writing the biggestInt(a, b, c, d) function that returns the largest parameter value
• Assume we do a (very reasonable) attempt:

def biggestInt(a, b, c, d):
	''' Function that will return the biggest value
		(a, b, c, or d) '''
	biggest = 0
	if a >= b and a >= c and a >= d:
		biggest = a
	if b >= a and b >= c and b >= d:
		biggest = b
	if c >= a and c >= b and c >= d:
		biggest = c
	else:
		biggest = d
	return biggest

assert biggestInt(1,2,3,4) == 4
assert biggestInt(1,2,4,3) == 4
assert biggestInt(1,4,2,3) == 4 # error
assert biggestInt(4,1,2,3) == 4 # error

• This above approach contained a logic error (try and see what went wrong)
  • Here, if c is not the biggest value, it will always assign d as the biggest value regardless of the value of d
• There are MANY ways we can correctly write this function
  • We could use elif appropriately to make sure conditions are checked correctly
  • Or we could immediately returning the largest value when one of the if statements returns True. For example:

def biggestInt(a, b, c, d):
	''' Function that will return the biggest value
		(a, b, c, or d) '''

	if a >= b and a >= c and a >= d:
		return a
	if b >= a and b >= c and b >= d:
		return b
	if c >= a and c >= b and c >= d:
		return c
	return d