4. functions

2
Find the sum of integers from 1 to 10, from 20 to 37, and
from 35 to 49, respectively.
ARULKUMAR V AP/CSE SECE

3
sum = 0
for i in range(1, 10):
sum += i
print("Sum from 1 to 10 is", sum)
sum = 0
sum += i
sum = 0
sum += i

4
sum = 0
sum += i
sum = 0
sum += i
sum = 0
sum += i

5
def sum(i1, i2):
result = 0
for i in range(i1, i2):
result += i
return result
def main():
print("Sum from 1 to 10 is", sum(1, 10))
main() # Call the main function

 To define functions (§6.2).
 To invoke value-returning functions (§6.3).
 To invoke functions that does not return a value (§6.4).
 To pass arguments by values (§6.5).
 To pass arguments by values (§6.6).
 To develop reusable code that is modular, easy to read,
easy to debug, and easy to maintain (§6.7).
 To create modules for reusing functions (§§6.7-6.8).
 To determine the scope of variables (§6.9).
 To define functions with default arguments (§6.10).
 To return multiple values from a function (§6.11).
 To apply the concept of function abstraction in
software development (§6.12).
 To design and implement functions using stepwise
refinement (§6.13).
 To simplify drawing programs using functions (§6.14).
6ARULKUMAR V AP/CSE SECE

7
A function is a collection of statements that are
grouped together to perform an operation.
def max(num1, num2):
if num1 > num2:
result = num1
else:
result = num2
return result
function name formal parameters
return value
function
body
function
header
Define a function Invoke a function
z = max(x, y)
actual parameters
(arguments)

if num1 > num2:
result = num1
else:
result = num2
return result
return value
function
body
function
header
z = max(x, y)
actual parameters
(arguments)
8
A function contains a header and body. The header begins with the
def keyword, followed by function’s name and parameters,
followed by a colon.

if num1 > num2:
result = num1
else:
result = num2
return result
return value
function
body
function
header
z = max(x, y)
actual parameters
(arguments)
9
The variables defined in the function header are known as
formal parameters.

if num1 > num2:
result = num1
else:
result = num2
return result
return value
function
body
function
header
z = max(x, y)
actual parameters
(arguments)
10
When a function is invoked, you pass a value to the parameter. This
value is referred to as actual parameter or argument.

if num1 > num2:
result = num1
else:
result = num2
return result
return value
function
body
function
header
z = max(x, y)
actual parameters
(arguments)
11
A function may return a value using the return keyword.

12
Testing the max function
This program demonstrates calling a function
max to return the largest of the int values
TestMax Run

13
animation
def main():
i = 5
j = 2
k = max(i, j)
print("The maximum between",
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()

def main():
i = 5
j = 2
k = max(i, j)
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()
14
animation
Invoke the main function

def main():
i = 5
j = 2
k = max(i, j)
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()
15
animation
i is now 5

def main():
i = 5
j = 2
k = max(i, j)
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()
16
animation
j is now 2

def main():
i = 5
j = 2
k = max(i, j)
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()
17
animation
invoke max(i, j)

def main():
i = 5
j = 2
k = max(i, j)
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()
18
animation
invoke max(i, j)
Pass the value of i to num1
Pass the value of j to num2

def main():
i = 5
j = 2
k = max(i, j)
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()
19
animation
(num1 > num2) is true
since num1 is 5 and num2
is 2

def main():
i = 5
j = 2
k = max(i, j)
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()
20
animation
result is now 5

def main():
i = 5
j = 2
k = max(i, j)
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()
21
animation
return result, which is 5

def main():
i = 5
j = 2
k = max(i, j)
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()
22
animation
return max(i, j) and assign
the return value to k

def main():
i = 5
j = 2
k = max(i, j)
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()
23
animation
Execute the print
statement

def main():
i = 5
j = 2
k = max(i, j)
i, "and", j, "is", k)
if num1 > num2:
result = num1
else:
result = num2
return result
pass int 5
pass int 2
main()
24
animation
Return to the caller

25
(a) The main function
is invoked.
Space required for
the main function
j: 2
i: 5
(b) The max
function is invoked.
(c) The max function
is being executed.
Space required for
the main function
j:
i:
Space required for
the max function
num2:
num1:
int object
5
int object
2
This is a heap for
storing objects
stack stack
int object
5
int object
2
This is a heap for
storing objects
Space required for
the main function
j:
i:
Space required for
the max function
result:
num2:
num1:
stack

26
Space required for
the main function
k:
j: 2
i: 5
(e) The main
function is finished.
int object
5
int object
2
This is a heap for
storing objects
stack stack
(d) The max function is
finished and the return
value is sent to k.
Stack is
now empty

27
This type of function does not return a value. The
function performs some actions.
ReturnGradeFunction Run
PrintGradeFunction Run

28
A function that does not return a value is known as
a void function in other programming languages
such as Python, C++, and C#. In Python, such
function returns a special None.
def sum(number1, number2):
total = number1 + number2
print(sum(1, 2))

def nPrintln(message, n):
for i in range(0, n):
print(message)
29
Suppose you invoke the function using
nPrintln(“Welcome to Python”, 5)
What is the output?
Suppose you invoke the function using
nPrintln(“Computer Science”, 15)
What is the output?
What is wrong
nPrintln(4, “Computer Science”)

def nPrintln(message, n):
for i in range(0, n):
print(message)
30
What is wrong
nPrintln(4, “Computer Science”)
Is this OK?
nPrintln(n = 4, message = “Computer Science”)

31
In Python, all data are objects. A variable for an object is actually a
reference to the object. When you invoke a function with a
parameter, the reference value of the argument is passed to the
parameter. This is referred to as pass-by-value. For simplicity, we
say that the value of an argument is passed to a parameter when
invoking a function. Precisely, the value is actually a reference value
to the object.
Increment Run
If the argument is a number or a string, the argument is not affected,
regardless of the changes made to the parameter inside the function.

Functions can be used to reduce redundant coding
and enable code reuse. Functions can also be used
to modularize code and improve the quality of the
program.
32
GCDFunction
Run
PrimeNumberFunction
Run
TestGCDFunction

33
Write a function that converts a decimal integer
to a hexadecimal.
Decimal2HexConversion RunDecimal2HexConversion Run

Scope: the part of the program
where the variable can be
referenced.
34
A variable created inside a function is referred to as a
local variable. Local variables can only be
accessed inside a function. The scope of a local
variable starts from its creation and continues to
the end of the function that contains the variable.
In Python, you can also use global variables. They
are created outside all functions and are accessible
to all functions in their scope.

globalVar = 1
def f1():
localVar = 2
print(globalVar)
print(localVar)
f1()
print(globalVar)
print(localVar) # Out of scope. This gives an error

x = 1
def f1():
x = 2
print(x) # Displays 2
f1()

x = eval(input("Enter a number: "))
if (x > 0):
y = 4
print(y) # This gives an error if y is not created

sum = 0
sum += i
print(i)

x = 1
def increase():
global x
x = x + 1
increase()

Python allows you to define functions with
default argument values. The default values are
passed to the parameters when a function is
invoked without the arguments.
40
DefaultArgumentDemo Run

Python allows a function to return multiple
values. Listing 5.9 defines a function that takes
two numbers and returns them in non-
descending order.
41
MultipleReturnValueDemo
Run

42
TestRandomCharacter
Run
RandomCharacter

You can think of the function body as a black box
that contains the detailed implementation for the
function.
43
Function Header
Function Body
Black Box
Optional arguments
for input
Optional return
value

44
• Write a function once and reuse it anywhere.
• Information hiding. Hide the implementation
from the user.
• Reduce complexity.

The concept of function abstraction can be
applied to the process of developing programs.
When writing a large program, you can use the
“divide and conquer” strategy, also known as
stepwise refinement, to decompose it into
subproblems. The subproblems can be further
decomposed into smaller, more manageable
problems.

Let us use the PrintCalendar example to
demonstrate the stepwise refinement
approach.
46
PrintCalendar Run

47
printCalendar
(main)
readInput printMonth
getStartDay
printMonthTitle printMonthBody
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear

48
printCalendar
(main)
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear

49
printCalendar
(main)
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear

50
printCalendar
(main)
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear

51
printCalendar
(main)
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear

52
printCalendar
(main)
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear

53
A Skeleton for printCalendar
Top-down approach is to implement one function in the
structure chart at a time from the top to the bottom. Stubs
can be used for the functions waiting to be implemented.
A stub is a simple but incomplete version of a function.
The use of stubs enables you to test invoking the function
from a caller. Implement the main function first and then
use a stub for the printMonth function. For example, let
printMonth display the year and the month in the stub.
Thus, your program may begin like this:

54
Bottom-up approach is to implement one function in the
structure chart at a time from the bottom to the top. For
each function implemented, write a test program to test it.
Both top-down and bottom-up functions are fine. Both
approaches implement the functions incrementally and
help to isolate programming errors and makes debugging
easy. Sometimes, they can be used together.

55
UseCustomTurtleFunctions
def drawLine(x1, y1, x2, y2):
def writeString(s, x, y):
def drawPoint(x, y):
def drawCircle(x = 0, y = 0, radius = 10):
def drawRectangle(x = 0, y = 0, width = 10, height = 10):
UsefulTurtleFunctions
Run

4. functions

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