Functions
Victor Norman
CS104
Calvin College
Reading Quiz
• Counts toward your grade.
Function Def’n vs. Call
• Definition:
def funcname(param_names):
statements to execute when funcname is called
• Call:
y = int(input(“Enter your age: “))
result = funcname(y)
– How many function calls in this code?
• Call must provide the same # of values as
the function definition requires.
Function Defn is an Abstraction
• A function abstracts and names lines of code.
– can be called repeatedly
– can take arguments. Consider: y = sin(x).
• Takes a value x, runs the sin function on it, producing a
result that y is made to refer to.
Non-fruitful Functions
• Non-fruitful functions produce no value for
the caller.
– Just execute their statements
– Not useful unless they do something like print
something or make the robot move, etc.
– Useful for turtle graphics. E.g.,
def drawSquare(size):
code here to draw a square of size.
nothing.
Returns
Fruitful Function
• Computes a value and returns it to the caller.
def squared(val):
newval = val * val
return newval
• Return sends value back to the caller and
exits the code in the function.
• Function will exit the code anyway at the end
of the function.
Mistakes CS104ers Make
• Provide literals as parameters in function
definition:
def xyz(p1, “hello”):
# code here
• Ignoring the parameter value.
def xyz(p1):
p1 = 3
# code here using p1
Clicker Questions
Given this code:
1 def foo(x):
2
return x + 1
3 def bar(x, y):
4
t = foo(x)
5
return t + y
6 def baz(z):
7
t = bar(z, z)
8
return foo(t)
9 n = 3
10 res = baz(n)
11 print(res)
Parameters and Local Variables
• Parameters hold the *values* being passed in
in the function call.
• Both are defined only within the code of the
function.
– That is their scope or lifetime.
• They can be altered within the code.
• Can think of them as "temporary variables" -places to hold values while you work on
computing the result or doing the work.
Return Statement
• Computes the result to be sent back to the
caller.
• Exits the code in the function, even if there is
code after it.
• E.g.,
def computeIt(x, y, z):
w = x + y / z
return w
print("Done!") # never executed
Return vs. print()
• Printing a value in a function does not return
it.
– print sends characters out to the output “stream”
– i.e., the screen.
• To send a value back to the caller you must
use return <val>
Clicker Questions
Given this code:
1 def beatHope(i):
2
3
4
i = i * i
print("Let's beat Hope", i, "times")
print(beatHope(3))
Assignment
• Do ~21 questions from the Functions section
in CodeLab for Saturday, 11:59:59 pm.
• Study for test on Thursday.