1. COSC 1306 COMPUTER SCIENCE AND PROGRAMMING
Jehan-François Pâris
Fall 2017 1

2. THE ONLINE BOOK CHAPTER VII SELECTIONS

3. Motivation We will learn how programs can make choices:
Boolean variables
Boolean expressions
if and if/else

4. Boolean type Two values: True and False Created as
Result of a comparison: ==, !=, >, >=, <, <=
Conversion using bool(…) function
Zero values and anything empty converts into a False
Anything else converts into a True value

5. Examples >>> a = 3 >>> b = 5
>>> p = a < b
>>> q = b <= a
>>> print("p =", p, "and q =", q)
p = True and q = False

6. A Boolean function def isDivisible(x, y): return x % y == 0
Returns True if x is divisible by y

7. Boolean operations (I)
AND:
True if both operands are true
This is called a truth table a b
a and b F T

8. Boolean operations (II)
OR:
True unless both operands are false a b
a or b F T

9. Boolean operations (III)
NOT:
True if operand is false a
not a F T

10. Boolean operations (IV)
Equality/Mutual Implication:
True when both operands are equal
Represented by a == sign a b
a==b F T

11. Boolean operations (V)
Exclusive OR (XOR):
True when operands are not equal
Represented by a != sign a b
a!=b F T

12. Notes
XOR is used for
Generating parity bits
Hardware hashing

13. Examples Assuming p is True and q is False >>> p or q True
>>> p and q
False
>>> not p
>>> p != q

14. Online Demo

15. Precedence Rules Level Category Operators 7(high) exponent **
6 multiplication *, /, //, %
5 addition +, -
4 relational ==, !=, <=, >=, >, <
3 logical not not
2 logical and and
1(low) logical or or

16. Examples a < b + c or c > 0 same as (a < (b+c)) or c >0)
a and b or c
same as (a and b) or c
a and not b or c
same as (a and (not b)) or c

17. Conversely We must write a = (p or q) and s b = (c and d) != (e or f)…

18. Online demo

19. Binary selection
if BOOLEAN EXPRESSION : STATEMENTS_1 # when condition is True else: STATEMENTS_2 # when condition is False
Observe
The columns after if and else
The indentation

20. Binary selection If condition: something else: other stuff False
True
other stuff
something
If condition: something else: other stuff

21. Flowchart conventions
Graphic representation of flow of control
Loops represented by loops
Ifs have two branches
???
False
True

22. Online demo

23. Omitting the else clause
if BOOLEAN EXPRESSION: STATEMENTS_1 # if condition is True # Continue normal flow
Observe
The column after if
The indentation

24. Omitting the else clause
If condition : something
If tank_empty : get_gas
True
condition
something
False

25. Main usage if + else To make a choice between two options if
To handle a special case
Zero value, negative value
Work is done

26. A problem with letter grades
Semester Average
Letter Grade
≥ 90 A
≥ 80 but < 90 B
≥ 70 but < 80 C
≥ 60 but < 70 D
< 60 F

27. An ugly solution
if average >= 90 : grade = 'A' if average >= 80 and average < 90 : grade = 'B' if average >= 70 and average < 80 : grade = 'C' if average >= 60 and average < 70 : grade = 'D' if average < 60 : grade = 'F'

28. The right way
if average >= 90 : grade = 'A' else : if average >= 80: grade = 'B' else : if average >= 70 : grade = 'C' else : if average >= 60 : grade = 'D' else: grade = 'F'

29. Using an elif (I) Too many nested ifs else …
if cond-1 : … else if cond-2 : … else if cond-3 : …
else …

30. Using an elif (II) With elif, lines align better
if cond-1 : … elif cond-2 : … elif cond-3 : … else : …

31. The right way
if average >= 90 : grade = 'A' elif average >= 80: grade = 'B' elif average >= 70 : grade = 'C' elif average >= 60 : grade = 'D' else: grade = 'F'

32. A beginner's trap
if average >= 90 : grade = 'A' elif average >= 80 and average < 90 : grade = 'B' elif average >= 70 and average < 80 : grade = 'C' elif average >= 60 and average < 70: grade = 'D' elif average < 60: grade = 'F'

33. Useless comparisons
if average >= 90 : grade = 'A' elif average >= 80 and average < 90 : grade = 'B' elif average >= 70 and average < 80 : grade = 'C' elif average >= 60 and average < 70: grade = 'D' elif average < 60: # use else instead grade = 'F'

34. Indenting advice Python attaches great importance to indenting
You cannot indent anything that is not inside
An if, a while, a for, …
A function declaration or a main function
Your indentation must be consistent
Do not mix spaces and tabs
Four spaces is neither too small nor too big

35. Floating point values Differ from integer values
Cannot handle extremely large or extremely small values
Limited precision
Can manipulate trillions of dollars but results are not exact to the cent
Must specify number of decimals in outputs

36. Limited precision # precision.py
""" floating-point computations can be inaccurate
"""
biggie = 5E9
epsilon = 5E-9
sum = biggie + epsilon
delta = sum - biggie
print ('epsilon =', epsilon, 'but delta =', delta)

37. The output epsilon = 5e-9 but delta = 0.0 Why?
Computations are carried out with a limited number of significant bits
Result has a limited number of significant digits
Be careful when computing very small differences between two numbers

38. Note Limitations on floating point number size and precision are
Hardware dependent
Shared by most programming languages
Unlike Python, most programming languages do not support arbitrary-size integers
Python is better

39. Displaying floating point numbers
Easiest way to specify number of decimal places is string interpolation
'Answer is %.nf ' %x does two things
It inserts the value of x inside the string
It specifies that the string representation of x should have n decimal places

40. Example (I)
pi = print('pi equals %.1f' % pi ) print('pi equals %.2f' % pi ) print('pi equals %.3f' % pi ) print('pi equals %.4f' % pi )

41. Example (II) Program prints
pi equals 3.1 pi equals pi equals pi equals
Values are nicely rounded