1. Conditional and iterative statements
– Computer and Programming

2. Agenda
Boolean expressions
Conditional statements
Iterative statements

3. Boolean Expressions
A Boolean expression is an expression whose value is either True or False.
Examples:
Do you want the coffee? (yes/no)
Is x greater than 10? (yes/no)
Have you found the answer? (yes/no)
Does 5 divide 153? (yes/no)

4. Boolean values
Yes
True No
False

5. Watch out! Python is case sensitive. I.e., For Boolean constants, use:
False and false are not equal.
For Boolean constants, use:
True, or
False

6. How to get Boolean results
Comparison operators:
To get Boolean result, we compare two things
Equal ==
Not equal !=
Greater than
>
Greater than or equal
>=
Less than
<
Less than or equal
<=

7. How to get Boolean results
Boolean operators
To get Boolean results, we combine one or two Boolean results
Boolean operators
operators
And
and Or or
Not
not

8. Quick review True and True True True and False False False and True
False and False
not True
False
not False
True
True or True
True
True or False
False or True
False or False
False

9. Examples of Boolean expressions
Suppose that variable x is 4.
Expression
Value
x < 5
x > 5
x <= 5
5 == x
x != 5
(3!=4) and (7<5)
(4>4) or (5<=10)
True
False
True
False
True
False
True

10. More examples x*x + 9*x + 10 == 0 (y%2 == 0) (y%2 != 1)
To check if x is a root of equation X2 + 9X + 10 = 0
x*x + 9*x + 10 == 0
To check if y is an even number
(y%2 == 0) or
(y%2 != 1)

11. Source: http://www.ryt9.com/s/prg/774090
If-statement
MRT
Children whose height is no larger than 140cm can enjoy a free ride.
Source:

12. If-statement
if statement evaluates a condition, and controls if the following statements shall be executed.
The statements inside the block will be executed when the condition is True.

13. Example price = 40 False height<=140 True
print('Hello kids')
True
price = 0
False
print('price =',price)
price = 40
price = 40 if height <= 140: print('Hello kids!') price = 0 print('price =',price)

14. Syntax and meaning false bool-expr true statement 1 statement 2 :
statement n
false
Statement syntax
if bool-expr: statement statement : statement n

15. Program flow control Normal sequential program x = int(input())
y = int(input())
print(x+y)
print("Hello",x)
z = x * y + 10
print(z)

16. Program flow control A program with if-statement price = 40
height <= 140
True
False
price = 40
if height <= 140:
print('Hello kids!')
price = 0
print('price =',price)
When height = 160
When height = 120

17. Block The aligned indented statements are grouped into a block.
price = 40
if height <= 140:
print('Hello kids!')
price = 0
print('price =',price)
The aligned indented statements are grouped into a block.
A condition in the if-statement decides whether the statements inside the block will be executed.

18. Good Bad Be careful Python uses the concept of blocks extensively.
Thus, you must be very careful about indentation.
Fdskfjsdlkfslkdjfdsff
fdskfsdflksdlkfdsf:
fddslfldskf
fdsfkdsfdsfd
fdkfddfdfd
fdkfdlf
fdslkdskslkdjsld
Fdskfjsdlkfslkdjfdsff
fdskfsdflksdlkfdsf:
fddslfldskf
fdsfkdsfdsfd
fdkfddfdfd
fdkfdlf
fdslkdskslkdjsld
Good
Bad

19. pass-statement for empty blocks
In Python, we cannot have an empty block.
if height <= 140:
print("I'm here") X
If you want a block that does nothing, put the pass statement inside it.
if height <= 140:
pass
print("I'm here")

20. Block can be nested Guess the output for various values of x. 3 5 7 910
x = int(input())
if x > 5:
print("hello")
if x < 10:
print("foofoo")
print("barbar")
print("well")
print("cheers")

21. Final words Indented block is a distinctive feature of Python.
It is one of the reason people like Python.
Be careful about indentation and blocks.

22. Thinking Corner 0 x = y Y = x y = x x = y t = x x = y y = t
Given two variables x and y, can you swap their values?
E.g., if x = 10 and y = 25, after executing your program, x = 25 and y = 10.
x = y
Y = x
y = x
x = y
t = x
x = y
y = t
This again won't work. Why?
This won't work. Why?

23. Thinking Corner 1
A candy store sales a certain kind of candy. However, it has a condition that the number of candies must be at least some limit s.
Given that variable x equals the actual number of candies you want to buy, write a program that modifies x so that it is the number of candies you end up buying.
E.g.,
For s = 5 and x = 7, at the end x = 7.
For s = 10 and x = 5, at the end x = 10.

24. Thinking Corner 1 (solution)
if x < s:
x = s

25. Thinking Corner 1 (function)
def num_candies(want, limit):
if want < limit:
return limit
return want
limit = 20
x = int(input("How many?"))
print("You should buy",
num_candies(x,limit),
"candies.")

26. Source: http://splinedoctors.com/2009/02/hurry-up-and-choose/
if – else statements
Source:

27. If-else-statement
If-statement
If-else-statement

28. if - else statement
When condition is True, statements in group T will be executed.
When condition is False, statements in group F will be executed.
Syntax
if condition: statement T statement T : statement Tn
else :
statement F statement F : statement Fn

29. Meaning Syntax condition statement T1 true statement T2 : statement Tn
false
statement F1
statement F2
statement Fn
Syntax
if condition: statement T statement T : statement Tn
else :
statement F statement F : statement Fn

30. Example for the if – else statement
Check if n is an even number or an odd number.
Value in N
Output
Even Number
It is an even number.
Odd Number
It is an odd number.
if n%2 == 0: print('It is an even number')
else:
print('It is an odd number')

31. Program Flow Control n % 2 == 0 print('It is an even number')
True
False
print('It is an odd number')

32. Thinking Corner 2
Based on the following table, write a program that determines if your score will pass the exam.
score
Output
Less than 50
You failed.
Other
You passed.
if score<50:
print('You failed.')
else:
print('You passed.')

33. Thinking Corner 3
Write a program that reads an integer and then tells if it is a positive integer, a negative integer, or a zero.

34. Thinking Corner 3
x > 0
x == 0
x < 0

35. Thinking Corner 3 x > 0 x < 0 x == 0 x > 0 x < 0 True
False
x == 0
False

36. Thinking Corner 3
x = int(input("Enter an integer:"))
if x > 0:
print("A positive integer")
else:
if x < 0:
print("A negative integer")
print("A zero")
Note that we are using if-statement inside a block in another if-statement.

37. Nested if-statement
An if-statements is just a kind of statements, so we can write it in a block inside another if-statement

38. Example: evaluation (if)
You a given an exam:
If you score > 8, then you are good
If you score > 4 but <= 8, you pass
If you score <= 4, you fail.
if score > 8:
print("Good")
if score > 4 and score <= 8:
print("Passed")
If score <= 4:
print("Failed")

39. Example: evaluation (nested-if)
You a given an exam:
If you score > 8, then you are good
If you score > 4 but <= 8, you pass
If you score <= 4, you fail.
if score > 8:
print("Good")
else:
if score > 4:
print("Passed")
print("Failed")

40. Example: evaluation (nested-if)
When do you get to this line?
if score > 8:
print("Good")
else:
if score > 4:
print("Passed")
print("Failed")
score > 8
score <= 8 and score > 4
score <= 8 and score <= 4
score <= 4

41. Example: evaluation (elif)
if score > 8:
print("Good")
else:
if score > 4:
print("Passed")
print("Failed")
if score > 8:
print("Good")
elif score > 4:
print("Passed")
else:
print("Failed")
This flow structure can be made more clear, using elif

42. if – elif – else statements
Source

43. if – elif – else statements
Syntax
if – elif – else – statement can be used when there are more than two choices.
if condition1: statement 1
elif condition2:
statement 2 elif condition3:
statement 3
: : :
else:
statement n

44. if – elif – else statements
condition1 .
statement 1
true
false
condition2
statement 2
statement 3
Syntax
if condition1: statement 1
elif condition2:
statement 2 else:
statement 3

45. Thinking Corner 4
Write a function fun that computes the following function
def fun(x):
if x <= 5:
return 2*x + 10
elif x <= 20:
return x*x + 10
elif x < 30:
return x*x*x + 10
else:
return 3*x
f(x) =
2x+10, x ≤ 5
x2+10, 5 < x ≤ 20
x-10, < x < 30
3x, x ≥ 30

46. Iterations
Source:

47. Repetitive work Computers are very good at doing repetitive work.
But how can we "program" them to do so?

48. print('I like Bossanova')
Repetitive Work
What is the output of the following program
What should we do if we want a program that prints that sentence for 2553 times?
print('I like Bossanova')

49. Repetitive work You can do it pretty easily with while-statement.
What should we do if we want a program that prints that sentence for 2553 times?
count = 1
while count <= 8:
print('I like Bossanova')
count = count + 1

50. How does it work? count <= 8 print('I like Bossanova')
False
count <= 8
True
print('I like Bossanova')
count = count + 1

51. while-statement Syntax
While the condition is True, the statements in the block inside the while statement are executed.
The condition is checked before every round.
Syntax
while condition: statement statement : statement n

52. While-statement Syntax
condition
statement 1
True
statement 2 :
statement n
False
while condition: statement statement : statement n
Syntax

53. Example A program that prints integers from 1 to 100 n = 1 while :
print(n)
n = 1
n <= 100
n += 1

54. Thinking Corner 5
Write a program that prints all even number from 100 down to 2
n = 100
while :
while :
print(n)
n = 100
n >= 2
n >= 1 or
if n%2 == 0:
print(n)
n = n - 2
n = n - 1

55. Thinking Corner 6
Write a function sum_to(n) that computes … + n.
Please don't use the formula n*(n+1)/2.
You can start by writing a program that computes that and then turning it into a function.

56. Thinking Corner 6: hints
You want to add 1, 2,
up to n
In each round:
What do you want to keep?
What do you want to change?
condition
False
True

57. Thinking Corner 6: more hints
First, think about the values 1, 2, …, n.
We will use variable i to keep tracks of these number.
Write a loop that do just this:
________
while ________:
_________
i = 1
i <= n
i += 1

58. Thinking Corner 6: more and more hints
What do you want to do with i.
Keep the summation
You will need a variable for it.

59. Thinking Corner 6: program
n = int(input())
total = 0
i = 1
while i <= n:
total = total + i
i = i + 1
print(total)

60. Thinking Corner 6: solution
def sum_to(n):
total = 0
i = 1
while i <= n:
total = total + i
i = i + 1
return total

61. Thinking Corner 7: Password
Write a program that asks for a password until the user enters the correct one.
Let the password be 'happy204111'.
Enter password: sad111
Sorry.
Enter password: happy111
Enter password: happy204111
Correct.

62. Thinking Corner 7: hints
Answer the following questions:
What condition do keep your loop going?
What do you want to do in each iteration?
What do you have to do before checking the first condition?

63. Thinking Corner 7: more hints
pwd = input("Enter password: ")
while ______________________:
____________________
print("Correct.")
print("Sorry.")

64. Thinking Corner 7: solutions
pwd = input("Enter password: ")
while ______________________:
_______________
________________________________
print("Correct.")
pwd != 'happy204111'
print("Sorry.")
pwd = input("Enter password: ")

65. Loop control
Usually how we control the loops falls into two broad categories:
Counter-controlled repetition
Sentinel-controlled repetition
Note: This clearly is not exhaustive. There are other ways of controlling loop.

66. Counter-controlled repetition
Look closely on the structure of this loop.
def sum_to(n):
total = 0
i = 1
while i <= n:
total = total+i
i = i + 1
return total
Initialization
Condition
Updates

67. Thinking Corner 8
Consider the following program
What happen if we change the operator from >= to <= ?
n = 100
while n >= 2:
print(n)
n = n - 2
What happen if we change this line from n = n-2 to n = n + 2

68. Sentinel-controlled repetition
pwd = input("Enter password: ")
while pwd != 'happy204111':
print("Sorry.")
print("Correct.")
Look closely on the structure of this loop.
What is the loop waiting for?
It won't stop until pwd == 'happy204111'

69. Thinking Corner 9 total = 0 n = 0 while n >= 0:
n = input('Input n : ')
if n >= 0:
total = total + n
print('total =", total)
What is this loop waiting for?
It won't stop until n < 0

70. Thinking Corner 10 A factorial of n (denoted by n!) is defined as
1 x 2 x … x n,
or 1 when n = 0.
Write a function fact(n) that computes the value of n!.
For the first version, you can ignore the case when n=0.