COMP90059 Semester 1, 2023
Introduction to Programming
School of Computing and Information Systems
Faculty of Engineering and Information Technology
Lecture 3: Conditionals
Dr Simon D’Alfonso
COMP90059 Introduction to Programming 2
Today
• Important themes of good programming
• Python libraries
• Conditionals:
• if
• elif
• else
• Nested if statements
• Boolean comparisons and operators: not,
and, or
COMP90059 Introduction to Programming 3
A curiosity from last week
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Solutions to Lecture 2 Challenges
1. Take four numbers from user input and print
their mean (average) value.
2. Take three digits (0 – 9) from user input and
print all possible combinations of the digits.
3. Take two numbers from the user, the length
of a rectangle and its height (in
centimeters), then calculate and print out
both the total perimeter of the rectangle and
the area.
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Important Themes
• ANALYSIS: first understand the problem, before
diving into a solution.
• HIGH-LEVEL DESIGN: next, design an abstract
solution expressed in simple words or diagrams.
• ELEGANT solutions: those which get the job done
with minimal complexity and effort.
• EXTENSIBLE solutions: those that can be easily
extended or modified to handle related problems
(often called MAINTAINABLE or RE-USABLE code);
helped by elegant solutions and readable code.
• READABLE code: that which a human will readily
understand and see how it solves the problem;
achieved through various techniques including:
commenting, appropriate variable naming, and
elegant solution design.
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Program Design - working out the high-level solution
Computational problem solving should start with a thorough analysis
of the problem. Once the problem is precisely understood then an
ordered set of instructions are produced to solve the problem
systematically.
Wikipedia Source:
http://commons.wikimedia.org/wiki/File:Wikipedia_article-creation-2.svg
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Python built-in functions
• https://www.w3schools.com/python/python_r
ef_functions.asp
• A couple of useful built-in functions:
• abs() - return the absolute value of the operand
• len() - return the length of the operand
• What data type(s) are acceptable as the
arguments for these functions?
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abs() and len() examples
• abs():
• abs(7)
• abs(-7)
• abs(-5.4)
• abs(-True)
• abs("aaa")
• len():
• len("hello")
• len(22)
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Exercise 1
Given the variable num containing an int,
calculate the number of digits in num
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Python Libraries
• The Python interactive shell has a number of built-in
functions. They are loaded automatically as a shell
starts and are always available, such as print() and
input() for I/O, and int(), float() and str() for type
conversion.
• In addition to built-in functions, a large number of
pre-defined functions are also available as a part of
libraries and their modules bundled with Python
distributions.
• You can also install additional libraries (such as the
Scikit-learn library for machine learning tasks) and
make use of its modules. Libraries are how we reuse
existing code solutions and don’t have to ‘reinvent
the wheel’.
• You can also create and import your own libraries.
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Python standard library set
• These are generally available with a Python
installation, but unlike being available
automatically like built-in functions, they
require an import statement.
• https://docs.python.org/3/library/
• A couple of examples:
• math - This module provides access to the
mathematical functions defined by the C standard.
• datetime - The datetime module supplies classes for
manipulating dates and times.
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math examples
import math
#math.ceil(x) - return the ceiling of x, the smallest integer greater
than or equal to x.
print(math.ceil(4))
print(math.ceil(5))
print(math.ceil(4.5))
print(math.ceil(4.7))
Alternatively
from math import ceil
print(ceil(4))
print(ceil(5))
print(ceil(4.5))
print(ceil(4.7))
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math examples
• The math library also contains select
mathematical constants, some examples
being:
• math.pi - The mathematical constant π =
3.141592…, to available precision.
• math.e - The mathematical constant e =
2.718281…, to available precision.
#calculate the circumference of a given circle
import math
circle_radius = 2
circumference = 2 * math.pi * circle_radius
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Datetime example
import datetime
x = datetime.datetime.now()
print(x)
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dir() and help()
• dir() displays the contents of a library and
help() provides information about a Python
function (both in-built functions and library
functions):
• help(round)
• import math
• dir(math)
• help(math.cos)
COMP90059 Introduction to Programming
Conditionals
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Conditionals
• Conditionals are a core technique in
programming languages, used to specify what
actions to take under different conditions
• if a parking fine has been paid, send a thank you
message
• if a parking fine has NOT been paid, send a reminder to
pay
• The following are reserved words in Python and
are used to construct conditional code
• if
• else
• elif
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Booleans, Comparisons and Expressions
• Recall that True and False are the two Boolean
values (named after George Boole:
https://en.wikipedia.org/wiki/George_Boole)
• Expressions return a Boolean value when they
make a comparison or test some condition,
such as the following mathematical
comparisons
• 1 == 2 (equality)
• 1 != 2 (inequality)
• 1 < 2 (less than)
• 1 > 2 (greater than)
• 1 <= 2 (less than or equals to)
• 1 >= 2 (greater than or equals to)
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Booleans, Comparisons and Expressions
• Some other examples:
• "hello" in "hello world"
• x is y [is is an object identity (memory location)
comparison. == is a value comparison]
• isinstance(6, int)
• return True (used in functions)
• Recall that integers, floats and strings
correspond to bools in certain ways
• 0 is False
• Non-0 numbers are True
• Empty strings are False
• Non-empty strings are True
• None is False
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is versus ==
>>> a = 1
>>> a == 1
True
>>> a is 1
True
>>> a = 256
>>> a == 256
True
>>> a is 256
True
>>> a = 257
>>> a == 257
True
>>> a is 257
False (???)
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Boolean Logic Truth Tables
A B A and B
True True True
True False False
False True False
False False False
A B A or B
True True True
True False True
False True True
False False False
A not A
True False
False True
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Logical operators and ints
>>> a = 3
>>> b = 4
>>> c = 0
>>> a and b
4
>>> a or b
3
>>> c and b
0
>>> not a and b
False
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Operator precedence (highest to lowest)
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If statement
The simplest form of a conditional construction is
the if statement, consisting of one or more
conditions that are Boolean expressions:
if condition1 and condition2 and … :
do something
• The ‘do something’ part falls within the scope of
the ‘if part’. In Python, when one line falls within
the scope of another, it must be indented by at
least one space relative to that line. The
standard in Python is 4 spaces (recommended
to be used in this subject).
• The ‘if part’ ends with a colon (:)
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If examples
x = 1
if x == 1:
print("x is 1")
#---
x = -8
if x < 0:
x = abs(x)
print(x)
#---
if True and True and not False:
print("All True")
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If-else statement
• For an if statement to succeed, the condition must
be true. In our previous examples, if the if condition
was False, then the code within it would not activate
and the program would just continue.
• However, we can include an ‘else part’ after the ‘if
part’, such that if the condition of the ‘if part’ fails
(i.e., it is a Boolean expression that equates to
False), then the program instead executes the code
that falls within the ‘else part’. The else part does not
include a condition (it can be considered a catch-all
at the end).
if condition1:
do something
else:
do something else
COMP90059 Introduction to Programming 27
Exercise 2
Write a program to accept two positive integers
and print out the maximum and the minimum of
the two input numbers.
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Exercise 2 Solution
num1 = int(input("Input first number: "))
num2 = int(input("Input second number: "))
if num1 >= num2:
#nested if
if num1 == num2:
max_num = min_num = num1
#nested else
else:
max_num = num1
min_num = num2
else:
max_num = num2
min_num = num1
print("Max is " + str(max_num))
print("Min is " + str(min_num))
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Exercise 2 Alternative Solution
num1 = int(input("Input first number: "))
num2 = int(input("Input second number: "))
max_num = num1
min_num = num2
if num1 > num2:
max_num = num1
min_num = num2
if num1 < num2:
max_num = num2
min_num = num1
print("Max is " + str(max_num))
print("Min is " + str(min_num))
COMP90059 Introduction to Programming 30
elif and cascading conditions
It is possible to test various mutually-exclusive
conditions by adding extra conditions with elif,
and possibly a catch-all final state with else
if condition1:
do first option
elif condition2:
do second option
elif condition3:
do third option
else:
do fourth option
COMP90059 Introduction to Programming 31
elif examples
x = int(input("Input a number 0 or greater: "))
if x == 0:
print("The number is 0")
elif x == 1:
print("The number is 1")
elif x == 2:
print("The number is 2")
else:
print("The number is greater than 2")
COMP90059 Introduction to Programming 32
elif examples
x = int(input("Input the result of rolling the 6-sided die"))
if x == 1:
print("The result is 1")
elif x == 2:
print("The result is 2")
elif x == 3:
print("The result is 3")
elif x == 4:
print("The result is 4")
elif x == 5:
print("The result is 5")
elif x == 6: #Could also use else here
print("The result is 6")
COMP90059 Introduction to Programming 33
elif
Note that for such conditional constructions to
be sound, the options should be mutually
exclusive. Take the following code:
prime_numbers = [1,2,3,5,7]
even_numbers = [2,4,6,8,10]
number = int(input("Enter a number between 1 and
10: "))
if number in prime_numbers:
print(str(number) + " is a prime number")
elif number in even_numbers:
print(str(number) + " is an even number")
How should this code be fixed?
COMP90059 Introduction to Programming 34
elif
And order can matter. Suppose that someone is
planning to visit either the zoo or the museum, with a
preference for the museum. If neither are open, then
they will just stay at home.
zoo_open = True
museum_open = True
if zoo_open:
destination = 'zoo'
elif museum_open:
destination = 'museum'
else:
destination = 'home'
What is wrong with this code?
COMP90059 Introduction to Programming 35
Exercise 3
Leap years have 29 days in February instead
of 28. Evaluate whether a given year
expressed as an int (yyyy) is a leap year. The
rules for determining leap years:
• With the exception of century years (years
ending with 00), all years divisible by 4 are
leap years.
• A century year is a leap year if and only if it
is divisible by 400.
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Exercise 3
Leap year determination diagram
COMP90059 Introduction to Programming 37
Exercise 3
if year % 400 == 0:
print("leap year")
elif year % 100 == 0:
print("not leap year")
elif year % 4 == 0:
print("leap year")
else:
print("not leap year")
COMP90059 Introduction to Programming 38
Simplify the solution
Simplify the preceding code into one if
statement and one else statement (and no elif
statements)
COMP90059 Introduction to Programming 39
Exercise 3 – Simplified Solution
if (year % 400 == 0) or (year % 4 == 0 and year % 100 != 0):
print("leap year")
else:
print("not leap year")
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One-line conditionals
Python provides the syntax to convert conditional structures
such as the following into two-line or one-line statements.
a = False
if x == 1:
if y == 2:
a = True
if z == 3:
a = True
---
a = False
if x == 1 and ((y == 2) or (z == 3)):
a = True
a = (x == 1 and ((y == 2) or (z == 3)))
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Lecture Overview
• Important themes of good programming
• Libraries, e.g., the math library
• Recap of bool
• Conditionals: if ... elif... else, including
nested statements
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Lecture 3 Challenges: Write a program to ...
1. Write code to determine if a given number x
is even
2. Write code to calculate the ceiling of a given
number x, without using math.ceil()
3. Write code to calculate the area of a circle
with a given radius r
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Lecture Identification and Acknowledgement
Coordinator / Lecturer: Simon D’Alfonso
Semester: Semester 1, 2023
© University of Melbourne
These slides include materials from 2020 - 2021
instances of COMP90059 run by Kylie McColl or
Wally Smith