1. 4. If Statements Let's Learn Python and Pygame
Aj. Andrew Davison, CoE, PSU Hat Yai Campus
4. If Statements
Have your program make choices.
Depending on a test have a program do different things.

2. If statement: test, action(s)
If Andrew got the right answer, add 1 point to his score.
If Jane hit the alien, make an explosion sound.
If the file isn’t there, display an error message.

3. "If" Flowchart
If Andrew got the right answer, add 1 point to his score.
executing program
Test:
Did Andrew get the right score? no
yes
Action: Add 1 point
to his score
execution continues...

4. age1.py

5. Code Details
test: is age > 16?
print("Enter your age: ", end =" ") age = int(input()) if age > 16: print("You are Old, ") print("Old, " * age)
actions must be
indented (add 4 spaces
at the start of each line)

6. Flowchart executing program Test: no age > 16? yes
Action: print old
execution continues...

7. Test Operations

8. Comparison of Strings Comparison operators work for strings
>>> "David" < "David Cameron"
True
>>> "Dave" < "David"
>>> "Tom" < "Thomas"
False
>>> "Bill" < "William"
>>> "AAA" < "AAB"
>>> "AAA" < "aaa"
>>> "aaa" < "AAA"
Uses A-Z a-z ordering, letter by letter

9. If and Else print("Enter your age: ", end =" ") age = int(input())
if age > 16:
print("You are Old, ")
print("Old, " * age)
else:
print("You are Young")

10. Flowchart executing program Test: no age > 16? yes
Action: print young
Action: print old
execution continues...

11. Not quite what we want! Try with 65
Many ifs No
Yes
grade>70
print "A"
if (grade > 70): print("A") if (grade > 60): print("B") if (grade > 50): print("C") else: print("D") No
Yes
grade>60
print "B" No
Yes
grade>50
print "D"
print "C"

12. Elifs ("else if"s) No
Yes
grade>70
print "A"
if (grade > 70): print("A") elif (grade > 60): print("B") elif (grade > 50): print("C") else: print("D") No
Yes
grade>60
print "B" No
Yes
grade>50
print "D"
print "C"
Yes! Try with 65, 72, 40, ...

13. age2.py

14. age2.py Flowchart Yes! Try with 14, 7, 10, 9, ... No Yes age>16
"old" No
Yes
age>=12
"not bad" No
Yes
age>=8
"baby"
"get lost"
Yes! Try with 14, 7, 10, 9, ...

15. age3.py

16. age3.py Flowchart Yes! Try with 14, 7, 10, 9, ... No Yes age>16
"old" No
Yes
age>=12 No
Yes
age>14
"get lost"
"not bad"
"perfect"
Yes! Try with 14, 7, 10, 9, ...

17. Complex Test Operations
Operator
Example
Result
and
(2 == 3) and (-1 < 5)
False or
(2 == 3) or (-1 < 5)
True
not
not (2 == 3)
and: True only if both tests are true
or: True if one or both tests are true
not: True if test is false

18. 1. Truth Table for or print True if one or both are true Answer? A B
rate = -1
rate < 0 or rate > 100 A B
A or B
True
False
Answer?

19. 2. Truth Table for or print True if one or both are true Answer? A B
rate = 160
rate < 0 or rate > 100 A B
A or B
True
False
Answer?

20. 3. Truth Table for or print True if one or both are true Answer? A B
rate = 50
rate < 0 or rate > 100 A B
A or B
True
False
Answer?

21. 1. Truth Table for and print True only if both are true Answer? A B
rate = -1
rate >= 0 and rate <= 100 A B
A and B
True
False
Answer?

22. 2. Truth Table for and print True only if both are true Answer? A B
rate = 50
rate >= 0 and rate <= 100 A B
A and B
True
False
Answer?

23. 3. Truth Table for and print True only if both are true Answer? A B
rate = 160
rate >= 0 and rate <= 100 A B
A and B
True
False
Answer?

24. Summary of or, and or: True if one or both tests are true
and: True only if both tests are true

25. age4.py
and: True only if both tests are true

26. age4.py Flowchart and: True only if both tests are true No Yes
"old"
age>=10 and
age <= 14
and: True only if both tests are true No
Yes
"get lost"
"perfect