1. Winter 2018 CISC101
11/22/2018
CISC101 Reminders
Prof’s office hours set to Thursdays 1:45 to 2:30 in GOO 550.
Quiz 1 next week. More info in last lecture.
Assn 1 due Friday, next week.
Exercise 2 is fair game now – work on console input and output techniques.
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

2. Today Continue Python Expressions. Operators and Precedence.
Winter 2018
CISC101 - Prof. McLeod

3. Arithmetic Operators As listed in Exercise 1: + addition
Winter 2013 CISC101
Arithmetic Operators
As listed in Exercise 1:
+ addition
- subtraction (and unary negation)
* multiplication
/ division
// "floor" division
% modulo or "remainder"
** exponentiation or "to the power of"
The first three make sense, how do the last four work?
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

4. / or Division For example, what is the value of 1 / 3 ?
In previous versions of Python the result of int divided by int would be an int. Not any more! Now the result is a float.
BTW, good thing there is a limit to the size of a float!
Winter 2018
CISC101 - Prof. McLeod

5. // or Integer or “Floor” Division
What is the value of 1 // 3?
The result is always truncated, not rounded. So, 99 // 100 is also zero.
You still get the truncated value, even if one or the other numbers are floats. So, 1.0 // 3.0 is still 0.0
Winter 2018
CISC101 - Prof. McLeod

6. % or “Modulo” Always gives the remainder after division.
For example 20 % 3 is 2
Also works with floats. Result is a float.
Winter 2018
CISC101 - Prof. McLeod

7. ** or “To the Power Of” Example 5 ** 2 is 25
If both numbers are ints, the result is an int. You can generate some pretty large int values this way!
For example 5 ** 100 is:
If at least one number is a float, the answer is a float.
So, 5.0 ** 100 is:
e+69
Winter 2018
CISC101 - Prof. McLeod

8. Mixed Numeric Types If you have an expression like: aVal = 4 + 5 – 2.0
the result will always be a float in this case.
So, if you get any float values in an expression that has integers the result will always be a float.
So something like / 5 gives 6.0
Winter 2018
CISC101 - Prof. McLeod

9. 5 * 'abc' is 'abcabcabcabcabc'
Strings and Numbers!
Multiplication:
5 * 'abc' is 'abcabcabcabcabc'
The + operator is the only other numeric operator that works with strings.
Winter 2018
CISC101 - Prof. McLeod

10. Addition and Strings
The + sign not only adds numbers but can also concatenate strings (and collections).
You can concatenate a number to a string, but have to convert it to a string first using the str() BIF.
Winter 2018
CISC101 - Prof. McLeod

11. Arithmetic Operators, Summary
+ - * / // % **
All of these work with numbers on both sides. Some will accept a string or even a collection on one side.
If an expression has a float number in it, then the result will always be a float.
Winter 2018
CISC101 - Prof. McLeod

12. Assignment Operator We’ve used this one already: =
Used to assign a value to a variable and to create that variable, if necessary.
Note that you can combine the other arithmetic operators with = as long as the variable already exists.
For example +=. Assuming aVar is 3, then
aVar += 5 stores 8 in aVar
This is the same as writing: aVar = aVar + 5
Winter 2018
CISC101 - Prof. McLeod

13. Precedence Rules Suppose I have an expression like: a = 5 * b + 27 / c
CISC101
Precedence Rules
Suppose I have an expression like:
a = 5 * b + 27 / c
How do we know the order of operations?
These rules are built-in to the interpreter.
= is always last (why?)
** first, then - (as unary negation), then *, /, // and %, then + and -, and then =
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

14. Precedence Rules, Cont. Rules again: ** - (unary) * / // % + - =
All operators on the same line have equal precedence.
Winter 2018
CISC101 - Prof. McLeod

15. Precedence Rules, Cont.
How can you take over control of the order of operations? Use the round brackets:
For example (4 + 5) // 3 is 3,
but // 3 is 5
If you have a series of operators that have equal precedence and no brackets to control things, then the expression is evaluated from left to right.
Winter 2018
CISC101 - Prof. McLeod

16. Example 4 + (5 * (6 - 3)) % 4 4 + (5 * 3) % 4 4 + 15 % 4 4 + 3 7
4 + (5 * (6 - 3)) % (5 * 3) % %
Winter 2018
CISC101 - Prof. McLeod

17. Another Example
aNum = 3 ((7 - 12) // 2) + 7 / 2 - aNum + 2 * 3 - (7 % 2) ((7 - 12) // 2) + 7 / * 3 - (7 % 2) (-5 // 2) + 7 / * 3 - (7 % 2) / * 3 - (7 % 2) / * 3 – * 3 – – – – – 1 2.5
Winter 2018
CISC101 - Prof. McLeod

18. CISC101
Boolean Operators
Also have a bunch of binary operators (and one unary operator) that yield a Boolean or a bool type result – a True or False.
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

19. Unary Boolean Operator
One unary boolean operator:
not
Sets True to False, and False to True (just like a NOT gate!)
Winter 2018
CISC101 - Prof. McLeod

20. 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 2018
CISC101 - Prof. McLeod

21. 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.
These must have a bool value on both sides.
and or
a logical AND
a logical OR
Winter 2018
CISC101 - Prof. McLeod

22. 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 2018
CISC101 - Prof. McLeod

23. 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 2018
CISC101 - Prof. McLeod

24. 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 2018
CISC101 - Prof. McLeod

25. 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 2018
CISC101 - Prof. McLeod

26. 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 2018
CISC101 - Prof. McLeod

27. 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 2018
CISC101 - Prof. McLeod

28. 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 2018
CISC101 - Prof. McLeod

29. 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 2018
CISC101 - Prof. McLeod