1. Chapter 3 Instructor: Zhuojun Duan
CSC 2302/CSC 7352 , Fall 2017
Chapter 3
Instructor: Zhuojun Duan
The Department of Computer Science

2. Chapter 3 Imperative Programming
Python Programs
Interactive Input/Output
One-Way and Two-Way if Statements
for Loops
User-Defined Functions
Object and class
Assignments Revisited and Parameter Passing

3. Python program A Python program is a sequence of Python statements
line1 = 'Hello Python developer...'
A Python program is a sequence of Python statements
Stored in a text file called a Python module
Executed using an IDE or “from the command line”
A Python program is a sequence of Python statements
Stored in a text file called a Python module
Executed using an IDE or “from the command line”
line2 = 'Welcome to the world of Python!'
print(line1)
print(line2)
line1 = 'Hello Python developer...'
line2 = 'Welcome to the world of Python!'
print(line1)
print(line2)
line1 = 'Hello Python developer...'
line2 = 'Welcome to the world of Python!'
print(line1)
print(line2)
line1 = 'Hello Python developer...'
line2 = 'Welcome to the world of Python!'
print(line1)
print(line2)
line1 = 'Hello Python developer...'
line2 = 'Welcome to the world of Python!'
print(line1)
print(line2)
line1 = 'Hello Python developer...'
line2 = 'Welcome to the world of Python!'
print(line1)
print(line2)
Run hello.py
Hello Python developer…
Run hello.py
Hello Python developer…
Welcome to the world of Python!
Run hello.py
hello.py

4. Built-in function print()
Function print() prints its input argument to the IDLE window
The argument can be any object: an integer, a float, a string, a list, …
Strings are printed without quotes and “to be read by people”, rather than “to be interpreted by Python”,
The “string representation” of the object is printed
>>> print(0)
>>> print(0.0)
0.0
>>> print('zero')
zero
>>> print([0, 1, 'two'])
[0, 1, 'two']

5. Built-in function input()
Function input() requests and reads input from the user interactively
It’s (optional) input argument is the request message
Typically used on the right side of an assignment statement
>>> name = input('Enter your name: ')
Enter your name: Michael
>>> name
'Michael'
>>> ========= RESTART =============
>>>
Enter your first name: Michael
Enter your last name:
>>> name = input('Enter your name: ')
Enter your name: Michael
>>> name
'Michael'
>>> ========= RESTART =============
>>>
Enter your first name: Michael
Enter your last name: Jordan
Hello Michael Jordan...
Welcome to the world of Python!
>>> name = input('Enter your name: ')
>>> name = input('Enter your name: ')
Enter your name: Michael
>>>
>>> name = input('Enter your name: ')
Enter your name: Michael
>>> name
'Michael'
>>> ========= RESTART =============
>>>
Enter your first name:
>>> name = input('Enter your name: ')
Enter your name: Michael
>>> name
'Michael'
>>> name = input('Enter your name: ')
Enter your name:
>>> name = input('Enter your name: ')
Enter your name: Michael
When executed:
The input request message is printed
The user enters the input
The string typed by the user is assigned to the variable on the left side of the assignment statement
first = input('Enter your first name: ')
last = input('Enter your last name: ')
line1 = 'Hello’ + first + '' + last + '…'
print(line1)
print('Welcome to the world of Python!')
first = input('Enter your first name: ')
last = input('Enter your last name: ')
line1 = 'Hello’ + first + '' + last + '…'
print(line1)
print('Welcome to the world of Python!')
first = input('Enter your first name: ')
last = input('Enter your last name: ')
line1 = 'Hello’ + first + '' + last + '…'
print(line1)
print('Welcome to the world of Python!')
first = input('Enter your first name: ')
last = input('Enter your last name: ')
line1 = 'Hello’ + first + '' + last + '…'
print(line1)
print('Welcome to the world of Python!')
input.py

6. Built-in function eval()
Function input() evaluates anything
the user enters as a string
What if we want the user to interactively enter non-string input such as a number?
Function input() evaluates anything
the user enters as a string
What if we want the user to interactively enter non-string input such as a number?
Solution 1: Use type conversion
Solution 2: Use function eval()
Takes a string as input and evaluates it as a Python expression
>>> age = input('Enter your age: ')
Enter your age: 18
>>> age
'18'
>>> age = input('Enter your age: ')
Enter your age: 18
>>> age
'18'
>>> int(age) 18
>>> eval('18')
>>> eval('age')
>>> eval('[2,3+5]')
[2, 8]
>>> eval('x')
Traceback (most recent call last):
File "<pyshell#14>", line 1, in <module>
eval('x')
File "<string>", line 1, in <module>
NameError: name 'x' is not defined
>>>
>>> age = input('Enter your age: ')
Enter your age: 18
>>> age
'18'
>>> int(age) 18

7. Exercise Write a program that: Requests the user’s name
Requests the user’s age
Computes the user’s age one year from now and prints the message shown
>>>
Enter your name: Marie
Enter your age: 17
Marie, you will be 20 next year!
name = input('Enter your name: ')
age = int(input('Enter your age: '))
line = name + ', you will be ' + str(age+1) + ' next year!'
print(line)

8. Exercise Write a program that:
Requests the user’s name
Requests the user’s age
Prints a message saying whether the user is eligible to vote or not
Need a way to execute a Python statement
if a condition is true

9. print('Be sure to drink liquids.')
One-way if statement
if <condition>:
<indented code block>
<non-indented statement>
if temp > 86:
print('It is hot!')
print('Be sure to drink liquids.')
print('Goodbye.')
if temp > 86:
print('It is hot!')
print('Be sure to drink liquids.')
print('Goodbye.')
if temp > 86:
print('It is hot!')
print('Be sure to drink liquids.')
print('Goodbye.')
The value of temp is 90.
The value of temp is 50.
temp > 86:
True
print('It is hot!')
Rhombus 'rɑmbəs
False
print('Be sure to drink liquids.')
Print('Goddbye.')

10. Exercises Write corresponding if statements:
If age is greater than 62 then print 'You can get Social Security benefits’
If string 'large bonuses' appears in string report then print 'Vacation time!’
If hits is greater than 10 and shield is 0 then print "You're dead..."
>>> hits = 12
>>> shield = 0
>>> if hits > 10 and shield == 0:
print("You're dead...")
You're dead...
>>> hits, shield = 12, 2
>>>
>>> age = 45
>>> if age > 62:
print('You can get Social Security benefits')
>>> age = 65
You can get Social Security benefits
>>>
>>> report = 'no bonuses this year'
>>> if 'large bonuses' in report:
print('Vacation time!')
>>> report = 'large bonuses this year'
Vacation time!

11. Indentation is critical
if temp > 86:
print('It is hot!')
print('Drink liquids.')
print('Goodbye.')
if temp > 86:
print('It is hot!')
print('Drink liquids.')
print('Goodbye.')
temp > 86:
temp > 86:
True
True
print('It is hot!')
print('It is hot!')
False
False
print('Drink liquids.')
print('Drink liquids.')
print('Goddbye.')
print('Goddbye.')

12. print('Be sure to drink liquids.')
Two-way if statement
if <condition>:
<indented code block 1>
else:
<indented code block 2>
<non-indented statement>
if temp > 86:
print('It is hot!')
print('Be sure to drink liquids.')
else:
print('It is not hot.')
print('Bring a jacket.')
print('Goodbye.')
if temp > 86:
print('It is hot!')
print('Be sure to drink liquids.')
else:
print('It is not hot.')
print('Bring a jacket.')
print('Goodbye.')
if temp > 86:
print('It is hot!')
print('Be sure to drink liquids.')
else:
print('It is not hot.')
print('Bring a jacket.')
print('Goodbye.')
The value of temp is 90.
The value of temp is 50.
temp > 86:
False
True
print('It is not hot!')
print('It is hot!')
print('Bring a jacket.')
print('Be sure to drink liquids.')
print('Bring a jacket.')

13. Exercise Write a program that: Requests the user’s name
Requests the user’s age
Prints a message saying whether the user is eligible to vote or not
>>>
Enter your name: Marie
Enter your age: 17
Marie, you can't vote.
>>> ============RESTART================
Enter your age: 18
Marie, you can vote.
name = input('Enter your name: ')
age = eval(input('Enter your age: '))
if age < 18:
print(name + ", you can't vote.")
else:
print(name + ", you can vote.")

14. Execution control structures
The one-way and two-way if statements are examples of execution control structures
Execution control structures are programming language statements that control which statements are executed, i.e., the execution flow of the program
The one-way and two-way if statements are, more specifically, conditional structures
Iteration structures are execution control structures that enable the repetitive execution of a statement or a block of statements
The for loop statement is an iteration structure that executes a block of code for every item of a sequence

15. for loop
Executes a block of code for every item of a sequence
If sequence is a string, items are its characters (single-character strings)
>>> name = 'Apple'
>>> for char in name:
print(char) A p
>>> name = 'Apple'
>>> for char in name:
print(char)
>>> name = 'Apple'
>>> for char in name:
print(char) A p
>>> name = 'Apple'
>>> for char in name:
print(char) A p
>>> name = 'Apple'
>>> for char in name:
print(char) A p
>>> name = 'Apple'
>>> for char in name:
print(char)
>>> name = 'Apple'
>>> for char in name:
print(char) A p l
>>> name = 'Apple'
>>> for char in name:
print(char) A p l e
>>> name = 'Apple'
>>> for char in name:
print(char) A p l
>>> name = 'Apple'
>>> for char in name:
print(char) A
>>> name = 'Apple'
>>> for char in name:
print(char) A
name =
'A p p l e'
'A'
char =
'p'
char =
'p'
char =
'l'
char =
'e'
char =

16. for loop 'stop' 'desktop' 'post' 'top'
Executes a code block for every item of a sequence
Sequence can be a string, a list, …
Block of code must be indented
for <variable> in <sequence>:
<indented code block >
<non-indented code block>
for word in ['stop', 'desktop', 'post', 'top']:
if 'top' in word:
print(word)
print('Done.')
'stop'
word =
'desktop'
word =
>>>
stop
desktop
>>>
stop
desktop
top
Done.
>>>
stop
desktop
>>>
stop
>>>
stop
desktop
top
>>>
word =
'post'
word =
'top'

17. Built-in function range()
Function range() is used to iterate over a sequence of numbers in a specified range
To iterate over the n numbers 0, 1, 2, …, n-1
for i in range(n):
To iterate over the n numbers i, i+1, i+2, …, n-1
for i in range(i, n):
To iterate over the n numbers i, i+c, i+2c, i+3c, …, n-1
for i in range(i, n, c):
>>> for i in range(2, 16, 4):
print(i) 2 6 10 14
>>>
>>> for i in range(0, 16, 4):
print(i) 4 8 12
>>>
>>> for i in range(2, 16, 10):
print(i) 2 12
>>>
>>> for i in range(2, 6):
print(i) 2 3 4 5
>>>
>>> for i in range(0):
print(i)
>>>
>>> for i in range(2, 2):
print(i)
>>>
>>> for i in range(2, 3):
print(i) 2
>>>
>>> for i in range(4):
print(i) 1 2 3
>>>
>>> for i in range(1):
print(i)
>>>

18. Exercise Write for loops that will print the following sequences:
0, 1, 2, 3, 4, 5, 6, 7, 8 , 9, 10
1, 2, 3, 4, 5, 6, 7, 8, 9
0, 2, 4, 6, 8
1, 3, 5, 7, 9
20, 30, 40, 50, 60

19. Defining new functions
A few built-in functions we have seen:
abs(), max(), len(),
sum(), print()
>>> abs(-9) 9
>>> max(2, 4) 4
>>> lst = [2,3,4,5]
>>> len(lst)
>>> sum(lst) 14
>>> print()
>>> def f(x):
res = 2*x + 10
return x**2 + 10
>>> f(1) 11
>>> f(3) 19
>>> f(0) 10
>>> abs(-9) 9
>>> max(2, 4) 4
>>> lst = [2,3,4,5]
>>> len(lst)
>>> sum(lst) 14
>>> print()
>>>
>>> abs(-9) 9
>>> max(2, 4) 4
>>> lst = [2,3,4,5]
>>> len(lst)
>>> sum(lst) 14
>>> print()
>>> def f(x):
res = 2*x + 10
return x**2 + 10
>>>
New functions can be defined using def
def: function definition keyword
f: name of function
x: variable name for input argument
def f(x):
res = x**2 + 10
return res
return: specifies function output

20. print() versus return
def f(x):
res = x**2 + 10
return res
def f(x):
res = x**2 + 10
print(res)
>>> f(2) 14
>>> f(2) 14
>>> 2*f(2) 28
>>> f(2) 14
>>> 2*f(2)
Traceback (most recent call last):
File "<pyshell#56>", line 1, in <module>
2*f(2)
TypeError: unsupported operand type(s) for *: 'int' and 'NoneType'
>>> f(2) 14
Function returns value of res which can then be used in an expression
Function prints value of res
but does not return anything

21. Defining new functions
The general format of a function definition is a
def <function name> (<0 or more variables>):
<indented function body> b
Let’s develop function hyp() that:
Takes two numbers as input (side lengths a and b of above right triangle )
Returns the length of the hypotenuse c
>>> hyp(3,4)
5.0
>>>
[haɪ'pɑtənus]
import math
def hyp(a, b):
res = math.sqrt(a**2 + b**2)
return res
import math
def hyp(a, b):
res = math.sqrt(a**2 + b**2)
def hyp(a, b):

22. Exercise Write function hello() that:
takes a name (i.e., a string) as input
prints a personalized welcome message
Note that the function does not return anything
>>> hello('Julie')
Welcome, Julie, to the world of Python.
>>>
def hello(name):
line = 'Welcome, ' + name + ', to the world of Python.'
print(line)
def hello(name):
def hello(name):
line = 'Welcome, ' + name + ', to the world of Python.’

23. Exercise Write function rng() that: takes a list of numbers as input
returns the range of the numbers in the list
The range is the difference between the largest and smallest number in the list
>>> rng([4, 0, 1, -2]) 6
>>>
def rng(lst):
res = max(lst) - min(lst)
return res
def rng(lst):
def rng(lst):
res = max(lst) - min(lst)

24. Comments and docstrings
Python programs should be documented
So the developer who writes/maintains the code understands it
So the user knows what the program does
>>> help(f)
Help on function f in module __main__:
f(x)
>>> help(f)
Help on function f in module __main__:
f(x)
>>> def f(x):
'returns x**2 + 10'
res = x**2 + 10
return res
returns x**2 + 10
>>>
Comments
def f(x):
res = x** # compute result
return res # and return it
Docstring
def f(x):
'returns x**2 + 10'
res = x** # compute result
return res # and return it

25. Objects and classes 3 3.0 'three' [1, 2, 3]
>>> c = 'three'
>>> d = [1, 2, 3]
>>> type(a)
<class 'int'>
>>> type(b)
<class 'float'>
>>> type(c)
<class 'str'>
>>> type(d)
<class 'list'>
>>> a = []
>>> a = 3
>>> b = 3.0
>>>
>>> a = 3
>>> b = 3.0
>>> c = 'three'
>>>
>>> a = 3
>>> b = 3.0
>>> c = 'three'
>>> d = [1, 2, 3]
>>>
>>> a = 3
>>> b = 3.0
>>> c = 'three'
>>> d = [1, 2, 3]
>>> type(a)
<class 'int'>
>>> type(b)
<class 'float'>
>>> type(c)
<class 'str'>
>>> type(d)
<class 'list'>
>>>
>>> a = 3
>>>
In Python, every value, whether a simple integer value like 3 or a more complex value, such as the list ['hello', 4,  5]  is stored in memory as an object.
Every object has a value and a type;
It is the object that has a type, not the variable!
int
float
str
list 3
3.0
'three'
[1, 2, 3]
An object’s type determines what values it can have and how it can be manipulated
Terminology: object X is of type int = object X belongs to class int

26. Class and class methods
Once again: In Python, every value is stored in memory as an object, every object belongs to a class (i.e., has a type), and the object’s class determines what operations can be performed on it
We saw the operations that can be performed on classes int and float
>>> pets = ['goldfish', 'cat', 'dog']
>>> pets.append('guinea pig')
>>> pets.append('dog')
>>> pets
['goldfish', 'cat', 'dog', 'guinea pig', 'dog']
>>> pets.count('dog') 2
>>> pets.remove('dog')
['goldfish', 'cat', 'guinea pig', 'dog']
>>> pets.reverse()
['dog', 'guinea pig', 'cat', 'goldfish']
>>> fish = ['goldfish']
>>> myPets = ['cat', 'dog']
>>> fish * 3
['goldfish', 'goldfish', 'goldfish']
>>> pets = fish + myPets
>>> pets
['goldfish', 'cat', 'dog']
>>> 'frog' in pets
False
>>> pets[-1]
'dog'
>>>
The list class supports:
operators such as +, *, in, [], etc.
methods such as append(), count(), remove(), reverse(), etc.

27. <variable> = <expression>
Assignment statement: a second look a b c d
A variable does not exist before it is assigned 3
[1, 2, 3]
>>> a
Traceback (most recent call last):
File "<pyshell#66>", line 1, in <module> a
NameError: name 'a' is not defined
>>> a = 3
>>> b =
>>> a
Traceback (most recent call last):
File "<pyshell#66>", line 1, in <module> a
NameError: name 'a' is not defined
>>> a = 3
>>> b =
>>> c = 'three'
>>> a
Traceback (most recent call last):
File "<pyshell#66>", line 1, in <module> a
NameError: name 'a' is not defined
>>>
>>> a
Traceback (most recent call last):
File "<pyshell#66>", line 1, in <module> a
NameError: name 'a' is not defined
>>> a = 3
>>> b =
>>> c = 'three'
>>> d = [1, 2] + [3]
>>> a
Traceback (most recent call last):
File "<pyshell#66>", line 1, in <module> a
NameError: name 'a' is not defined
>>> a = 3
'three'
3.3
<variable> = <expression>
<expression> is evaluated and its value put into an object of appropriate type
The object is assigned name <variable>

28. Mutable and immutable typesc d
>>> a 3
>>> a 3
>>> a = 6 6
>>> d
[1, 2, 3]
>>> a 3
>>> a = 6 6
>>> a 3
>>> a = 6 6
>>> d
[1, 2, 3]
>>> d[1] = 7
[1, 7, 3] 3
[1, 7, 3]
[1, 2, 3]
'three'
3.3 6
The object (3) referred to by variable a does not change; instead, a refers to a new object (6)
Integers are immutable
The object ([1, 2, 3]) referred to by d changes
Lists are mutable

29. Assignment and mutabilityc d
>>> a 6
>>> b
3.3
>>> a 6
>>> b
3.3
>>> b = a
>>> a = 9
>>> c = d
>>> c
[1, 7, 3]
>>> a 6
>>> b
3.3
>>> b = a
>>> a = 9
>>> c = d
>>> c
[1, 7, 3]
>>> d[2] = 9
[1, 7, 9]
>>> a 6
>>> b
3.3
>>> b = a
>>> a = 9
>>> a 6
>>> b
3.3
>>> b = a 9 3
[1, 7, 9]
[1, 7, 3]
'three'
3.3 6
c and d refer to the same list object
a and b refer to the same integer object
The list that c refers to changes; d refers to the same list object, so it changes too
Because lists are mutable, a change to d affects c
a now refers to a new object (9); b still refers to the old object (6)
Because integers are immutable, a change to a does not affect the value of b

30. Swapping values a b tmp Want: a b Have: 3 6 3 6 >>> a 3
>>> tmp = b
>>> b = a
>>> a 3
>>> b 6
>>> tmp = b
>>> b = a
>>> a = tmp
>>> a 3
>>> b 6
>>> a, b = b, a
>>> a 3
>>> b 6
>>> b = a
>>> a 3
>>> b 6
>>> a 3
>>> b 6
>>> tmp = b

31. Immutable parameter passingx
shell
Function call g(a) 3 5
>>> a = 3
>>> g(a)
>>>
>>> a = 3
>>>
>>> a = 3
>>> g(a)
>>>
def g(x):
x = 5
def g(x):
x = 5
def g(x):
x = 5
Variable x inside g() refers to the object a refers to
As if we executed x = a
Function g() did not, and cannot, modify the value of a in the interactive shell.
This is because a refers to an immutable object.

32. Mutable parameter passing
lst l
shell
Function call h(lst)
[5,2,3]
[1,2,3]
>>> lst = [1,2,3]
>>> h(lst)
>>>
>>> lst = [1,2,3]
>>> h(lst)
>>>
>>> lst = [1,2,3]
>>>
def h(l):
l[0] = 5
def h(l):
l[0] = 5
def h(l):
l[0] = 5
Variable l inside h() refers to the object lst refers to
As if we executed l = lst
Function h() did modify the value of lst in the interactive shell.
This is because lst and l refer to an mutable object.

33. Exercise Write function swapFS() that: takes a list as input
swaps the first and second element of the list, but only if the list has at least two elements
The function does not return anything
>>> mylst = ['one', 'two', 'three']
>>> swapFS(mylst)
>>> mylst
['two', 'one', 'three']
>>> mylst = ['one']
['one']
>>>
def swapFS(lst):
if len(lst) > 1:
lst[0], lst[1] = lst[1], lst[0]
def swapFS(lst):
def swapFS(lst):
if len(lst) > 1:

34. Questions
Thank You!