CSIT881
Programming and Data Structures
Decision Making Statements
Decision Making Statements
Objectives:
● Understanding the usage of Decision Making Statements
● Solving problems using:
○ Single-selection statement if
○ Double-selection statement if-else
○ Multiple-selection statement if-elif-...-else
2
Hello World example
3
# ask user to choose a language
print("Choose a language: (I)talian (W)elsh (Z)ulu")
language_option = input("Enter language selection: ")
# display Hello World, How are you, in different languages
if (language_option == "I"):
print("Ciao mondo.")
print("Come stai?")
elif (language_option == "W"):
print("Helo Byd.")
print("Sut wyt ti?")
elif (language_option == "Z"):
print("Sawubona Mhlaba.")
print("Unjani?")
else:
print("Hello World.")
print("How are you?")
Choose a language: (I)talian (W)elsh (Z)ulu
Enter language selection: W
Helo Byd.
Sut wyt ti?
Hello World example
4
if (language_option == "I"):
#{
print("Ciao mondo.")
print("Come stai?")
#}
elif (language_option == "W"):
#{
print("Helo Byd.")
print("Sut wyt ti?")
#}
elif (language_option == "Z"):
#{
print("Sawubona Mhlaba.")
print("Unjani?")
#}
else:
#{
print("Hello World.")
print("How are you?")
#}
If you are coming from C and
Java background, perhaps
using the comments with
curly brackets like this may
help you to see the beginning
and the end of your code block
better!
if - else
5
Double-selection statement if-else
is the most common decision making statement.
We use this control structure to specify two different
actions in our program.
We will need to provide a condition. If this condition is
true then our program will perform a certain action. And
if this condition is false then another action will be taken.
if - else
if (some condition):
block 1 statements
…
else:
block 2 statements
...
6
Example 1
Number of items Cost
1-50 $3 per item
Postage: $10
More than 50 $2 per item
Postage: free
7
Example 1
Number of items Cost
1-50 $3 per item
Postage: $10
More than 50 $2 per item
Postage: free
If the user buys 10 item:
Item cost = $3 x 10 = $30
Postage: $10
Total cost: $40
8
Example 1
Number of items Cost
1-50 $3 per item
Postage: $10
More than 50 $2 per item
Postage: free
If the user buys 100 item:
Item cost = $2 x 100 = $200
Postage: free
Total cost: $200
9
# get the number of items from the user
item_input = input("Enter the quantity: ")
item_count = int(item_input)
# calculate the cost
if (item_count <= 50):

else:


Example 1
10
Example 1
# get the number of items from the user
item_input = input("Enter the quantity: ")
item_count = int(item_input)
# calculate the cost
if (item_count <= 50):
unit_price = 3
postage = 10
total_cost = unit_price * item_count + postage
print("Total cost: ${0}".format(total_cost))
else:


11
Example 1
# get the number of items from the user
item_input = input("Enter the quantity: ")
item_count = int(item_input)
# calculate the cost
if (item_count <= 50):
unit_price = 3
postage = 10
total_cost = unit_price * item_count + postage
print("Total cost: ${0}".format(total_cost))
else:

unit_price = 2
total_cost = unit_price * item_count
print("Total cost: ${0}".format(total_cost))
12
# get the number of items from the user
item_input = input("Enter the quantity: ")
item_count = int(item_input)
# calculate the cost
if (item_count <= 50):
#{
unit_price = 3
postage = 10
total_cost = unit_price * item_count + postage
print("Total cost: ${0}".format(total_cost))
#}
else:
#{
unit_price = 2
total_cost = unit_price * item_count
print("Total cost: ${0}".format(total_cost))
#}
Example 1
13
if - elif - elif - … - else
14
Multiple-selection statement if-elif-...-else
is to be used when we have different actions in our code
to handle different conditions.
In each of the if and elif control structures, we will
need to provide a condition.
The last control structure else is optional.
If else is used then the block of codes that belong to
else will be executed if all the previous conditions (of
if and elif) are false.
if - elif - elif - … - else
if (condition1):
# condition1 is true
statement
statement
...
elif (condition2):
# condition1 is false and condition2 is true
statement
statement
...
elif (condition3):
# condition1 is false, condition2 is false, and condition3 is true
statement
statement
...
else:
# all the conditions are false
statement
statement
... 15
Example 2
Number of items Cost
1-50 $3 per item
Postage:
Standard post: $10
Registered post: $15
Express post: $20
More than 50 $2 per item
Postage:
Standard post: free
Registered post: $15
Express post: $20
10 items + Registered Post
Item cost = $3 x 10 = $30
Postage: $15
Total cost: $45 16
Example 2
Number of items Cost
1-50 $3 per item
Postage:
Standard post: $10
Registered post: $15
Express post: $20
More than 50 $2 per item
Postage:
Standard post: free
Registered post: $15
Express post: $20
100 items + Registered Post
Item cost = $2 x 100 = $200
Postage: $15
Total cost: $215 17
Example 2
Number of items Cost
1-50 $3 per item
Postage:
Standard post: $10
Registered post: $15
Express post: $20
More than 50 $2 per item
Postage:
Standard post: free
Registered post: $15
Express post: $20
100 items + Standard Post
Item cost = $2 x 100 = $200
Postage: free
Total cost: $200 18
# get the number of items from the user
item_input = input("Enter the quantity: ")
item_count = int(item_input)
# get the shipping method Standard/Registered/Express?
shipping = input("Shipping method (s/r/e): ")
# calculate the cost
Example 2
19
# calculate the cost
# determine the unit price
# determine the postage
# determine the total cost
Example 2
20
# determine the unit price
if (item_count <= 50):
unit_price = 3
else:
unit_price = 2
Example 2
21
# determine the postage
if (shipping == "s"):
# standard post




elif (shipping == "r"):
# registered post

else:
# express post

Example 2
22
# determine the postage
if (shipping == "s"):
# standard post $10 for 1-50 items, free for > 50 items
if (item_count <= 50):
postage = 10
else:
postage = 0
elif (shipping == "r"):
# registered post

else:
# express post

Example 2
23
# determine the postage
if (shipping == "s"):
# standard post $10 for 1-50 items, free for > 50 items
if (item_count <= 50):
postage = 10
else:
postage = 0
elif (shipping == "r"):
# registered post $15
postage = 15
else:
# express post

Example 2
24
# determine the postage
if (shipping == "s"):
# standard post $10 for 1-50 items, free for > 50 items
if (item_count <= 50):
postage = 10
else:
postage = 0
elif (shipping == "r"):
# registered post $15
postage = 15
else:
# express post $20
postage = 20
Example 2
25
# determine the total cost
total_cost = unit_price * item_count + postage
print("Total cost: ${0}".format(total_cost))
Example 2
26
# grade A: 100-80, B: 79-60, C: 59-40, D: 39-0
# ask user to enter the mark
# determine the grade based on mark
# display the mark and grade
Example 3
27
# grade A: 100-80, B: 79-60, C: 59-40, D: 39-0
# ask user to enter the mark
mark_input = input("Please enter mark: ")
mark = int(mark_input)
# determine the grade based on mark
# display the mark and grade
Example 3
28
# grade A: 100-80, B: 79-60, C: 59-40, D: 39-0
# determine the grade based on mark
if (mark >= 80):
grade = "A"
elif (mark >= 60):
grade = "B"
elif (mark >= 40):
grade = "C"
else:
grade = "D"
Example 3
29
# display the mark and grade
print("Mark {0}, Grade {1}".format(mark, grade))
Example 3
30
#grade A: 100-80, B: 79-60, C: 59-40, D: 39-0
mark_input = input("Please enter mark: ")
mark = int(mark_input)
if (mark >= 80):
grade = "A"
elif (mark >= 60):
grade = "B"
elif (mark >= 40):
grade = "C"
else:
grade = "D"
print("Mark {0}, Grade {1}".format(mark, grade))
Please enter mark: 90
Mark 90, Grade A
mark is greater than or equal to 80
31
#grade A: 100-80, B: 79-60, C: 59-40, D: 39-0
mark_input = input("Please enter mark: ")
mark = int(mark_input)
if (mark >= 80):
grade = "A"
elif (mark >= 60):
grade = "B"
elif (mark >= 40):
grade = "C"
else:
grade = "D"
print("Mark {0}, Grade {1}".format(mark, grade))
Please enter mark: 62
Mark 62, Grade B
32
#grade A: 100-80, B: 79-60, C: 59-40, D: 39-0
mark_input = input("Please enter mark: ")
mark = int(mark_input)
if (mark >= 80):
grade = "A"
elif (mark >= 60):
grade = "B"
elif (mark >= 40):
grade = "C"
else:
grade = "D"
print("Mark {0}, Grade {1}".format(mark, grade))
Please enter mark: 45
Mark 45, Grade C
33
#grade A: 100-80, B: 79-60, C: 59-40, D: 39-0
mark_input = input("Please enter mark: ")
mark = int(mark_input)
if (mark >= 80):
grade = "A"
elif (mark >= 60):
grade = "B"
elif (mark >= 40):
grade = "C"
else:
grade = "D"
print("Mark {0}, Grade {1}".format(mark, grade))
Please enter mark: 15
Mark 15, Grade D
34
if (alone)
if (some condition):
statements
...
35
The single-selection statement if is to be used when
we want to perform an action if a condition is true or skip
the action when the condition is false.
36
Example 4
Finding the maximum
number among 3 given
numbers
user_input = input("Enter the 1st integer: ")
number1 = int(user_input)
user_input = input("Enter the 2nd integer: ")
number2 = int(user_input)
user_input = input("Enter the 3rd integer: ")
number3 = int(user_input)
number_max = number1
if (number2 > number_max):
number_max = number2

if (number3 > number_max):
number_max = number3
print("Max of {0}, {1}, {2} is {3}"
.format(number1, number2, number3, number_max))
37
Enter the 1st integer: 12
Enter the 2nd integer: 3
Enter the 3rd integer: 5
Max of 12, 3, 5 is 12
number1
12
number2
3
senario 1
38
number3
5
number_max 12
user_input = input("Enter the 1st integer: ")
number1 = int(user_input)
user_input = input("Enter the 2nd integer: ")
number2 = int(user_input)
user_input = input("Enter the 3rd integer: ")
number3 = int(user_input)
number_max = number1
if (number2 > number_max):
number_max = number2

if (number3 > number_max):
number_max = number3
print("Max of {0}, {1}, {2} is {3}"
.format(number1, number2, number3, number_max))
Enter the 1st integer: 5
Enter the 2nd integer: 12
Enter the 3rd integer: 3
Max of 5, 12, 3 is 12
senario 2
39
number1
5
number2
12
number3
3
number_max 5
number_max 12
user_input = input("Enter the 1st integer: ")
number1 = int(user_input)
user_input = input("Enter the 2nd integer: ")
number2 = int(user_input)
user_input = input("Enter the 3rd integer: ")
number3 = int(user_input)
number_max = number1
if (number2 > number_max):
number_max = number2

if (number3 > number_max):
number_max = number3
print("Max of {0}, {1}, {2} is {3}"
.format(number1, number2, number3, number_max))
senario 3
40
number1
5
number2
3
number3
12
number_max 5
number_max 12
Enter the 1st integer: 5
Enter the 2nd integer: 3
Enter the 3rd integer: 12
Max of 5, 3, 12 is 12
user_input = input("Enter the 1st integer: ")
number1 = int(user_input)
user_input = input("Enter the 2nd integer: ")
number2 = int(user_input)
user_input = input("Enter the 3rd integer: ")
number3 = int(user_input)
number_max = number1
if (number2 > number_max):
number_max = number2

if (number3 > number_max):
number_max = number3
print("Max of {0}, {1}, {2} is {3}"
.format(number1, number2, number3, number_max))
senario 4
41
number1
3
number2
5
number3
12
number_max 3
number_max 5
number_max 12
Enter the 1st integer: 3
Enter the 2nd integer: 5
Enter the 3rd integer: 12
Max of 3, 5, 12 is 12
user_input = input("Enter the 1st integer: ")
number1 = int(user_input)
user_input = input("Enter the 2nd integer: ")
number2 = int(user_input)
user_input = input("Enter the 3rd integer: ")
number3 = int(user_input)
number_max = number1
if (number2 > number_max):
number_max = number2

if (number3 > number_max):
number_max = number3
print("Max of {0}, {1}, {2} is {3}"
.format(number1, number2, number3, number_max))
Equality
if (number1 == 5):
# number1 is equal to 5
if (number1 == number2):
# number1 is equal to number2
if (your_answer == “Y”):
# your_answer is equal to “Y”
if (student_name == “John”):
# student_name is equal to “John”
Remember the double equal sign ==
42
Inequality
if (number1 != 5):
# number1 is not equal to 5
if (number1 != number2):
# number1 is not equal to number2
if (your_answer != “Y”):
# your_answer is not equal to “Y”
if (student_name != “John”):
# student_name is not equal to “John”
43
Comparison
if (number1 < 5):
# number1 is less than 5
if (number1 <= 5):
# number1 is less than or equal to 5
if (number1 > 5):
# number1 is greater than 5
if (number1 >= 5):
# number1 is greater than or equal to 5
44
Logical And
if ((number1 > 5) and (number1 < 10)):
# number1 is greater than 5 AND less than 10
if ((age > 40) and (student_type == “Domestic”)):
# age is greater than 40
# AND student_type is equal to “Domestic”
45
Logical Or
if ((number1 < 1000) or (number1 > 5000)):
# number1 is less than 1000
# OR greater than 5000
if ((student_type == “Exchange”) or (student_type
== “Domestic”)):
# student_type is equal to “Exchange”
# OR is equal to “Domestic”
46
Logical Negation
if (not (number1 == 1000)):
# number1 is not equal to 1000
if (condition):
this is
a block
of codes
that is indented
by the same amount
of spaces
else:
usually
we use 2, 3 or 4 spaces for
indentation
In Python, all the continuous lines indented with same number
of spaces form a block.
All statements within the block must be indented the same
amount.
We usually use 2, 3 or 4 spaces for indentation. 47
Block and indentation
if (item_count <= 50):
#{
unit_price = 3
postage = 10
total_cost = unit_price * item_count + postage
print("Total cost: ${0}".format(total_cost))
#}
else:
#{
unit_price = 2
total_cost = unit_price * item_count
print("Total cost: ${0}".format(total_cost))
#}
48
Block and indentation
If you are coming from C, C++, Java, etc... background,
perhaps using the above coding style will help you!
if (condition):
this is
a block
of codes
that is indented
by the same amount
of spaces
else:
usually
we use 2, 3 or 4 spaces for
indentation
Common mistakes
Forget the colon :
Wrong indentation,
mix-up between spaces and tabs
Make your choice of indentation and use it consistently! 49