LESSON 2. CONDITIONAL LOGIC, IF ELSE STATEMENTS, SELECTION, DEBUGGING Introduction to, with examples, conditional logic and the use of IF and ELSE statements. Look at SELECTION in game design. Learn about Debugging and Error Checking. Analyse the use of a flow chart and how to design before implementation. Discuss: Video gaming addiction! Create a password checker and a username and password (login) app. Learn about the use of ELIF. Learn about Boolean variables and their use. Learn about Multiple comparisons using and/or. Includes a suggested videos, ‘Big ideas’ discussion, and HW/research projects section. Discussion on Artificial Intelligence and Robotics.

1. www.teachingcomputing.com
Mastering Programming in Python
Lesson 2

2. • Introduction to the language, SEQUENCE variables, create a Chat bot
• Introduction SELECTION (if else statements)
• Introducing ITERATION (While loops)
• Introducing For Loops
• Use of Functions/Modular Programming
• Introducing Lists /Operations/List comprehension
• Use of Dictionaries
• String Manipulation
• File Handling – Reading and writing to CSV Files
• Importing and Exporting Files
• Transversing, Enumeration, Zip Merging
• Recursion
• Practical Programming
• Consolidation of all your skills – useful resources
• Includes Computer Science theory and Exciting themes for every
lesson including: Quantum Computing, History of Computing,
Future of storage, Brain Processing, and more …
Series Overview
*Please note that each lesson is not bound to a specific time (so it can be taken at your own pace)
Information/Theory/Discuss
Task (Code provided)
Challenge (DIY!)
Suggested Project/HW

3. In this lesson you will …
 Learn about conditional logic/control
flow/selection using IF ELSE statements
 Look at the use of Selection in Game Design
 Learn about Debugging/Error checking
 Analyse a Flow Chart (which has Selection in it)
 Discuss Video Gaming – Addiction
 Create a password checker
 Create a username and password (login) app
 Learn about the use of ELIF
 Learn about Boolean Variables
 Learn about Multiple Comparisons using and/or

4. Conditional Logic – we use it all the time!
IF Mr X is kind and helpful, THEN, we like him
ELSE we like him less!
Some philosophers will argue that all love is conditional. Others would say
nothing is truly unconditional(except God’s love – known as Agape in Greek) By
‘conditional’ we mean that an outcome depends on certain conditions.
In Python programming ELIF statements are
also used – short for ELSE AND IF
IF we are full (after dinner) THEN we go without
dessert, ELSE, we eat the apple pie!
In programming we can control the flow of a
program by using SELECTION (IF ELSE/ ELIF
statements, comparisons or Boolean variables)

5. Did you know?
A software bug is an error or flaw in a
computer program.
As a programmer, you are going to come
up a lot of these and it is important that
you are patient and persevere!
The first ever bug was an actual bug!
Operators traced an error in the Mark II to
a moth trapped in a relay, coining the
term bug. This bug was carefully removed
and taped to the log book
Stemming from this we now call glitches in
a computer a bug.
A page from the Harvard Mark IIelectromechanical computer's log,
featuring a dead moth that was removed from the device

6. Examples of IF…ELSE in game design.
It would be hard to think about creating a game without using SELECTION (if else
statements). You probably remember first using these in SCRATCH (if else blocks)
Pong (marketed as PONG) is one of the earliest arcade
video games and the very first sports arcade video game
What sort of if statements would be
needed in PONG?

7. An example of the use of Selection (IF)
Ifthe player has run out of lives, he…..dies!
https://youtu.be/GxL07giAC3M
Speaking of Video
games, the
following short
documentary on
Video Game
addition may be of
some interest.
What do you think?
Can Video Games
ruin people’s lives?

8. Can you follow the logic of this flow chart?
You will often need to create flow charts to show the logic behind your program.
Can you make any sense of the following flow chart? What is it trying to say?
If x < y
If y < x
Output: “Done”
Output “x is less
than y”
Output “y is less
than x”
Input x
Input y
5
7
Read more about Flow charts here: https://en.wikipedia.org/wiki/Flowchart

9. A few things to keep in mind …
Q1: If password !=“moose” then …. (In this code, what do you think “ != “ means?
!= is the
operator for
NOT equal to.
Q2: Also, do you know the difference between a = 2 and if a ==2 ?
Answer 1: Answer 2:
Use = when you are assigning
a value. (e.g. a = 2) and == is
used when you are CHECKING
to see if two values are equal.
(e.g. if a==2 then do x,y,z )
It’s very important not to get
the two confused!

10. Task 1: Create a password checker
1. Open a Python
Module
2. Copy and paste
the code on the
right into the
module
3. Run the program
to see what it
does
4. See if you can fix
the errors to get it
to work.
#This doesn't quite work - lots of errors. Fix them (mostly syntax and
indentation errors) and get the program to work!
#This doesn't quite work - lots of errors. Fix them (mostly syntax and
indent error) and get the program to work!
password="open123"
print('Enter password:')
answer=input()
if answer==password:
print("yes")
else
print("no")

11. Task 1: Solution
password="open123"
print('Enter password:')
answer=input()
if answer==password:
print("yes")
else:
print("no")
Common errors to look for.
Speech marks when referring to the data type STRING
In Python 3, don’t forget the brackets when printing text
It’s easy to forget to use the double equal signs in Python
Identation is key in Python. Make sure the IF and ELSE are
on the same indent line.
Similarlty the PRINT (within the IF and ELSE statements)
need to be indented.

12. Note: Sometimes we put bits of code in a function. In
Python a function is defined with the word: ‘def’ (see
below to call a function and run the code)
def login():
username="username1"
print('Enter username')
answer1=input()
print('Enter password:')
if answer1 == username and answer2 =
password:
print("Access Granted")
else:
print("Sorry, Access Denied")

13. Challenge 1: Username and psswd check.
1. Open a Python
Module
2. Copy and paste
the code on the
right into the
module
3. Here we use a
function so in the
shell type “login()”
to RUN the
program
4. Get it to work!
#Task - create a program which asks the user for their username AND
password.
#only if BOTH are correct, access is granted, else any other combination
results in access denied
def login():
username="username1"
print('Enter username')
answer1=input()
print('Enter password:')
if answer1 == username and answer2 = password:
print("Access Granted")
else:
print("Sorry, Access Denied")

14. Challenge 1: Solution
def login():
username="username1"
password="open123"
print('Enter username')
answer1=input()
print('Enter password:')
answer2=input()
if answer1 == username and answer2 == password:
print("Access Granted")
else:
print("Sorry, Access Denied")
Declare a password variable and assign it a value!
Declare ‘answer2’ as a variable for user input.
Don’t forget the double equals sign here.
Remember when you are checking equivalence a
“==“ is used, but when it is a simple assignment
operator, a single equals sign “=“ will suffice,

15. Task 2: Check number using ELIF
1. Open a Python
Module
2. Copy and paste
the code on the
right into the
module
3. Here we use a
function so in the
shell type
“checknum()” to
RUN the program
4. Analyse the code
def checknumber():
# In this program, we input a number
# check if the number is positive or
# negative or zero and display
# an appropriate message
num = float(input("Enter a number: "))
if num > 0:
print("Positive number")
elif num == 0:
print("Zero")
else:
print("Negative number")

16. Challenge 2: Create a grade checker app
1. Open a Python
Module
2. Using the previous
code in Task 2,
see if you can
create a function
called
‘gradechecker’
3. The pseudo code
for the program is
on the right.
4. Can you do it!?
1. Ask the user for their grade (which must be between 0 and 100)
2. Allow the user to input their response (an integer response between 0
and 100)
3. If the user enters anything under 50 (<50) then print “Fail”
4. If the user enters anything above 50 (>50) then print “Pass”
Extension: If you’ve done the above, see if you can define more speficic
grade boundaries.
e.g. 0 – 20 = F
20 – 40 = E
40 –60 = D
60 – 70 = C
And so on …*You may need to do some research to complete this task!

17. Challenge 2: Solution(s)
def gradechecker():
# In this program,
we input a number
# check if the number
is positive or
# negative or zero
and display
# an appropriate
message
num =
float(input("Enter a
number: "))
if num > 50:
print("Pass")
elif num < 50:
print("Failed”)
Challenge 2 Solution Extension Solution

18. Using And / Or with IF statements
You can check multiple conditions by chaining together comparisons with and /or
#Example of AND
If x < y and x < z:
print (“x is less than y and z)
Example 1 Example 2
#Non Exclusive Or
If x < y or x < z:
print(“x is less than either y or z”)

19. Boolean Variables with IF statements
Python supports Boolean variables. They are basically variables that can either
store the value “True” or “False”. When using an IF statement, the expression
needs to evaluate to either True or False
In the following
example, the
variable
seat_booked is set
to ‘True’. What
would happen if it
was changed to
‘False’ ?

20. ROBOTICS AND IF STATEMENTS
As an extension, check out the following website on Robotics and IF statements
http://arcbotics.com/lessons/if-statements/

21. Challenge: Extend your Chabot from
Lesson 1 using Selection and Functions.
Here are some
suggestions, but get
creative and implement
your own ideas!
**********Get the computer to
ask the user for an age. IF the
age is above 16, take them to
another function where the
program continues, ELSE, quit
the program and inform the user
that they are too young.
#This is a chatbot and this is a comment, not executed by the program
#Extend it to make the computer ask for your favourite movie and respond
accordingly!
print('Hello this is your computer...what is your favourite number?')
#Declaring our first variable below called 'computerfavnumber' and storing
the value 33
computerfavnumber=33
#We now declare a variable but set the variable to hold whatever the
*user* inputs into the program
favnumber=input()
print(favnumber + '...is a very nice number indeed. So...what is your
name?')
name=input()
print('what a lovely name: ' + name + '...now, I will reveal what my
favourite number is:')
print (computerfavnumber)

22. The official Python Site (very useful!)
Make a note of it and check out the tutorial! You can find what you’re looking for, in this
case, IF statements.

23. Use the glossary to reference something..

24. Useful Videos to watch on covered topics
https://www.youtube.com/watch?v=II5WTVvryvk
More on Robotics and AI today … Recommended video on Python Control Flow
https://www.youtube.com/watch?v=vMHfUWkELg0

25. Suggested Project / HW / Research
 Create a research information point on the history and future of AI
 Create a timeline of Artificial Intelligence
 What are the applications of Artificial Intelligence in today’s society?
 What are the advancements in Robotics today?
 What is the future of AI and Robotics?
 What countries are currently at the forefront in these crucial fields?
 Compare and contrast the use of SELECTION in four different high level
languages (suggested: Python, VB.Net, C++, Java)
 Give examples of the syntax of if statements in each language
 Look up ‘Case statements’ in VB.Net – what is the equivalent in Python
 Contrast and compare the advantages of Python and VB.Net as languages? Which
do YOU think is better and why?

26. Useful links and additional reading
http://anh.cs.luc.edu/python/hands-on/3.1/handsonHtml/index.html
http://www.python-course.eu/variables.php
http://www.programiz.com/python-programming/variables-datatypes
http://www.tutorialspoint.com/python/python_variable_types.htm
https://en.wikibooks.org/wiki/Python_Programming/Variables_and_Strings