1. Winter 2019 CISC101
4/16/2019
CISC101 Reminders
You have been assigned to two groups in onQ – the “Lab” group and the “Grader” group. You must write the quiz in the lab time as indicated by the Lab group you are in.
Quiz 1 next week, starting in the Tuesday lab. More info in Tuesday’s lecture. Today’s lecture material, up to but not including Conditionals, is on the quiz.
Assignment 1 due next Friday.
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

2. Today Continue Python Syntax by looking at Expressions:
Winter 2019 CISC101
4/16/2019
Today
Continue Python Syntax by looking at Expressions:
Boolean Operators, Precedence again.
Branching Algorithms – Using Conditionals.
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

3. CISC101
Boolean Operators
There are a bunch of binary operators (and one unary operator) that yield a Boolean or a bool type result – a True or False.
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

4. Unary Boolean Operator
One unary boolean operator:
not
Sets True to False, and False to True (just like a NOT gate!)
Winter 2019
CISC101 - Prof. McLeod

5. Binary Boolean Operators
>
<
>=
<= == !=
greater than
less than
greater than or equal to
less than or equal to
equals
not equals
All of these result in a True or False bool value.
and or
a logical AND
a logical OR
Winter 2019
CISC101 - Prof. McLeod

6. Binary Boolean Operators, Cont.
>
<
>=
<= == !=
greater than
less than
greater than or equal to
less than or equal to
equals
not equals
All of these must have a number on both sides or a string on both sides.
and or
a logical AND
a logical OR
These must have a bool value on both sides.
Winter 2019
CISC101 - Prof. McLeod

7. Binary Boolean Operators, Cont.
The interpreter is going to want both sides to be either numeric or both sides to be strings.
You will get an error if you try to compare a string to a number.
Winter 2019
CISC101 - Prof. McLeod

8. Example
How can you test a variable, aVal, to see if it is between two limits, inclusive? Call them low and high:
Check:
low <= aVal and aVal <= high
Unfortunately, in Python, this also works sometimes:
low <= aVal <= high
This is bad form and will not work in most other programming languages!
Winter 2019
CISC101 - Prof. McLeod

9. Example, Cont.
If aVal is between the limits the expression on the previous slide will evaluate to a True.
Suppose you want to see a True if aVal is outside the limits. How? Two ways:
not( low <= aVal and aVal <= high )
Or:
low > aVal or aVal > high
Which way do you like better?
Winter 2019
CISC101 - Prof. McLeod

10. Aside - Short-Circuit Evaluation
Python uses this to speed up the execution of Boolean expressions using and and or.
For an and, if the LHS is already False, then the RHS is not evaluated.
For an or, if the LHS is already True, the RHS is not evaluated.
Winter 2019
CISC101 - Prof. McLeod

11. Expanded Precedence Rules
Unary operations (not and negation) first.
Math operators in usual order: **
* / // %
+ -
Binary boolean comparison operators:
> >= < <= == !=
Logical operator: and
Logical operator: or
Assignment operator: =
Winter 2019
CISC101 - Prof. McLeod

12. Expanded Precedence Rules, Cont.
Round brackets are still used to control precedence.
And, function invocations and variable substitution take place before all of the operators listed on the previous slide.
Can brackets force the assignment operator to go first?
Winter 2019
CISC101 - Prof. McLeod

13. Boolean Expression Examples
5 > 2
4 < 3 or 7 > 1
7 != 8
6 + 2 > 9 or 4 == 3 + 1
7 == 7 and 5 > 2 and 6 != 3
5 * 5 >= 5 ** 2
128 % 2 == 0
Winter 2019
CISC101 - Prof. McLeod

14. Another Example – Evaluate:
not(5 * 4 > 3) or <= 5 and 6 == 2
not(20 > 3) or <= 5 and 6 == 2
not True or <= 5 and 6 == 2
False or <= 5 and 6 == 2
False or <= 5 and 6 == 2
False or True and 6 == 2
False or True and False
False or False
False
Winter 2019
CISC101 - Prof. McLeod

15. Expressions, Cont. We’ve discussed literals, variables and operators.
We still have function/method calls, keywords and punctuation to discuss.
I’ll talk about conditionals first – so you can work on the assignment and Exercise 3.
Winter 2019
CISC101 - Prof. McLeod

16. So Far… Without conditionals programs are linear:
CISC101
So Far…
Without conditionals programs are linear:
Obtain data from user
Calculations…
Display results
etc.
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

17. Branching Algorithms
If a program could test a condition, then it would have a choice as to its path of execution:
if true
if false
etc.
Winter 2019
CISC101 - Prof. McLeod

18. Conditionals or “Selection Statements”
Python has if, if-else and chained if (if-elif-else) statements.
if statement syntax:
if boolean_expression :
statement1_when_true
statement2_when_true
statement3_when_true …
Example:
if capacitance < 0 :
print("Illegal capacitance")
Winter 2019
CISC101 - Prof. McLeod

19. if-else Statement Syntax of if-else statement: Example:
if boolean_expression :
statement(s)_when_true
else :
statement(s)_when_false
Example:
if stress > maxStress / 1.5 :
result = "failure"
result = "pass"
Winter 2019
CISC101 - Prof. McLeod

20. Aside - Indentation
All of the statements inside the true part and inside the false part are indented once. As soon as code is “out-dented”, then it is outside the conditional.
Just like code in a function: All the code in a function is indented once.
Note that the semi-colon, :, tells the interpreter to get ready for some indented code. There always has to be at least one indented line of code after a :.
Python uses indentation to indicate containment of code. Indentation is an important part of Python syntax!
Winter 2019
CISC101 - Prof. McLeod

21. if Statement, Cont. You will often have to nest if statements: etc.
Winter 2019
CISC101 - Prof. McLeod

22. Nested if Statements
For example, if you have three numbers, a, b, & c and you want to print them to the screen in order: T F
a < b T F T F
b < c
b < c T F T F
a < c
a < c
abc
cba
acb
cab
bac
bca
See SortThree.py
Winter 2019
CISC101 - Prof. McLeod

23. if-elif Statements
In the code in SortThree.py, you might have noticed this construct:
else :
if a < c :
This kind of thing occurs so often that it can be shortened to:
elif a < c :
Winter 2019
CISC101 - Prof. McLeod

24. if-elif Statements, Cont.
This leads to a common code structure often called a “chained if” construct:
if condition1 :
statement(s)
elif condition2 :
elif condition3 :
else :
You can have as many elifs as you want.
The else part is optional.
Winter 2019
CISC101 - Prof. McLeod

25. if-elif Statements, Cont.
There is nothing in this construct that you could not make with normal if-else statements.
For some kinds of conditionals, the if-elif might be easier to put together.
See Part B of assignment 1, for example.
Winter 2019
CISC101 - Prof. McLeod

26. Nested if Statements - Demo
What to wear? Depends on time to destination and outdoor temperature:
If time < 5 minutes, wear shorts & tshirt
if time between 5 and 20 minutes:
if temp >= 5 wear shorts & tshirt
if temp between -10 and 5 wear pants & sweatshirt
if temp less than -10 wear pants & jacket
if time > 20 minutes:
if temp >= 10 wear shorts & tshirt
if temp between 0 and 10 wear shorts & sweatshirt
if temp between – 10 and 0 wear pants & jacket
if temp < -10 wear pants & parka
Winter 2019
CISC101 - Prof. McLeod

27. Nested if Statements – Demo, Cont.
See:
ClothesVersion0.py
ClothesVersion1.py
ClothesVersion2.py
ClothesVersion3.py
All work correctly and all do the same thing.
Which one is “best” and why?
Don’t forget that Python uses “short circuit” evaluation of logical operators – and & or.
Winter 2019
CISC101 - Prof. McLeod

28. Building Conditionals
Nesting conditionals leads to the best structure for this example.
If you have to nest conditionals, write the test with the fewest conditions for the outermost if statement.
Make sure you have covered every possible combination of inputs.
When testing, test every possible combination of inputs to make sure your logic is good.
Writing nested conditionals can be tricky!
Winter 2019
CISC101 - Prof. McLeod