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

2. THE ONLINE BOOK CHAPTER II BASIC PYTHON

3. Values Python programs manipulate values "Hello world!" "2+3" 5
Values have types

4. Value types (I) >>> type ("Hello world!") <class 'str'>
<class 'int'>
>>> type(3.1416)
<class 'float'>
Strings, integer and floating point are basic Python types

5. Value types (II)
>>> type("3.1416") <class 'str'> >>> type('3.1416') >>>
Anything between matching quotes is a string

6. Strings Delimited by matching single quotes (' …')
Can contain double quotes (")
'The subject was "touchy", she said'
Delimited by matching double quotes (' …')
"Let's go!"
Delimited by three matching double quotes ("""…""")
Can contain almost anything

7. Find the malformed strings
"Hello!"
'I'm busy!'
"I like sugar cones.'
"He asked for a "petit noir"."
"""I don't want a "petit noir"."""
'Cookies and cream'

8. Find the malformed strings
"Hello!"
'I'm busy!' single quote inside '…'
"I like sugar cones.' mismatched quotes
"He asked for a "petit noir"."
"""I don't want a "petit noir"."""
'Cookies and cream'

9. Integers 0, 1 ,2, 3, … and -1, -2, -3 , … No separating quotes
It is and not 13,000
Can handle really huge large numbers
Computations are always exact
Nice feature of Python 3

10. Floating-point numbers
0.1, , , …
But also
.1, .25, .001
You could also meet later
1E3 (for 1 ×103), 1E-3 (for 0.001), …
Limited precision
Fine for most applications

11. Find the malformed numbers
55,000
0.375
-32
3E3
-.5
3 000

12. Find the malformed numbers
55,000
0.375
-32
3E3
-.5
3 000

13. Numbers and strings '9' and 9 are two different values
'9' means the digit "9"
9 means the number 9
They are not equal
You cannot use one instead of the other

14. Converting numbers into strings
>>> str(9) '9' >>> str(3.1416) '3.1416'

15. Converting strings into int35
>>> int("six")
Does not work
>>> int("3.1416")

16. Converting strings into float

17. Converting numbers
>>> float(3) 3.0 >>> int(3.1416) 3 >>> int(2.90) 2 >>> int(-2.90) -2
Truncates
Does not round up

18. Variables
Names that refer to a value pi
3.1416

19. Assigning a value name = 'Alice' Value assigned to variable can change
result = 0 result = result = 'undefined'

20. This is Python, not algebra (I)
In Python, Ruby, C, C++, Java, Javascript, …
The = sign does not mean equality
It means an assignment
variable  value
When you see
shirt_price = 24.99
think of
shirt_price  24.99

21. This is Python, not algebra (II)
Why?
Using an equal sign for assignments started with FORTRAN in mid fifties
Equal sign remains easier to type than left arrow
The symbol for equality is the double equal sign
a == b

22. Variable names Can consist of multiple characters:
Must be letters, digits and underscore (“_”)
Cannot start with a digit
Case sensitive
Total, total and TOTAL are three different variable names

23. Prohibited variable names
Some possible variable names are already used by Python
Keywords
Here is the full list
and as
assert
break
class
continue
def
del
elif
else
except
exec
finally
for
from
global if
import in is
lambda
nonlocal
not or
pass
raise
return
try
while
with
yield
True
False
None

24. Variables have types
>>> a = "hello!" >>> type(a) <class 'str'> >>> a = 5 <class 'int'> >>> a = 3.5 <class 'float'>
The type of a variable is the type of the value it refers to
It can change over time

25. Find the wrong variable names
aleph
Fall16
easy_money
bigBrother
2step starts with a digit
Texas2step
raise is a reserved word

26. Find the wrong variable name
aleph
Fall16
easy_money
bigBrother
2step
Texas2step
raise

27. Picking good variable names
Using mnemonic names helps making programs more readable by humans
instructor_name or instructorName, student__name or instructorName
Instead of
nm1, nm2
But Python ignores them!

28. Statements and expressions (I)
A statement specifies an action that the Python interpreter can execute
print("Hello world!)
count = 0 …
Typically occupies a line
Basic element of a program

29. Statements and expressions (II)
An expression is Python's equivalent of an algebraic expression
Three differences
They are always evaluated
They use somewhat different conventions
𝑎2 + 2𝑎𝑏 +𝑏2 becomes a**2 + 2*a*b + b**2
They are more general

30. Statements or expressions?
>>> name = "Mike"
>>> print(name)
Mike
>>> len(name) 4
>>> nameLength = len(name)
>>> 13
>>> total =

31. Statements or expressions?
>>> name = "Mike" Statement
>>> print(name) Statement
Mike
>>> len(name) Expression 4
>>> nameLength = len(name) Statement
>>> Expression 13
>>> total = Statement

32. Operators and Operands
Most of the operators are the same as those you encountered in HS Algebra but
Multiplication sign is *
Multiplication is never explicit
Must distinguish between max and m*a*x
Exponentiation is **
Operators obey the same precedence rules as in HS algebra

33. Examples
>>> >>> 1*2*3 6 >>> 25/ >>> 16/2 8.0
In Python the division operator always returns a float

34. Integer division (//) >>> 5//4 1 (and not 1.25)
>>> 8//4
2 (and not 2.0)
>>> 6.0//4
1.0 (and not 1.5)
>>> 3.5//2
1.0 (and not 1.75)

35. The modulus operator (%)
a%b returns the remainder of the integer division of a by b
>>> 9//4 2
>>> 9 - (9//4)*4 1
>>> 9%4

36. The modulus operator (%)
Also work for floating-point numbers
>>> 4.6//2
2.0
>>> 4.6%2
Floating-point computation was
not accurate but fairly close

37. Our friend the input() function (I)
Specific to Python
xyz = input("Blablabla: ")
Will print Blablabla:_
Will store in xyz the characters you will enter

38. Our friend the input() function (II)
The value returned by input will always be a string
price = enter("Enter a price: ")
If you want an int or a float you must convert the string
price = float(enter("Enter a price: "))
Note the two closing parentheses

39. Wishing a happy birthday (I)
We want to print a personalized birthday wish
Happy birthday Michael
User will enter the name of the person
Program will do the rest
Will also ask the program to prompt the user for a name
Enter a name:

40. Wishing a happy birthday (II)
name = input("Enter a name:") print("Happy birthday", name)
When you specify several items as the arguments of a print function, print separates them with spaces
>>> print ("Hello", "world!")
Hello world!

41. The concatenation operator
The same + as for addition
"U" + "H" returns "UH"
"How" + "are" + you" + "?" returns "Howareyou?"
No spaces are added!
"How_" + "are_" + you_" + "?" returns "How are you?"

42. Order of operations Consider a*b**2 +c*d/4 – e/f
In which order are the operations performed?
In the same order as in algebra
First exponentiations right to left
Then multiplications and divisions left to right
Finally additions and subtractions left to right
Parentheses override these priorities

43. Precedence rules **
*, / and //
+ and -

44. With or without parentheses?
Which expressions are equivalent?
a + (b – c) a + b – c
(a*b)**2 a*b**2
(a + b)/2 a + b/2
a/(b*c) a/b*c
(a*b)/c a*b/c
(a**b)**c a**b**c
a**(b**c) a**(b**c)

45. The answers Which expressions are equivalent? a + (b – c) a + b – c
(a*b)**2 a*b**2 = a*(b**2)
(a + b)/2 a + b/2 = a + (b/2)
a/(b*c) a/b*c = (a/b)*c
(a*b)/c a*b/c
(a**b)**c a**b**c = a**(b**c)
a**(b**c) a**b**c

46. My take Know the rules For the quiz
For deciphering complex expressions
Use wisely parentheses in your code
When it is needed
When it clarifies dubious cases
(a*b)/c
a**(b**c)

47. Remember
Nobody ever failed a test because he or she used a few extra parentheses
The reverse is not true

48. Reassignment Including variables At the beginning of the season
shirt_price = 24.99
Then discounted
shirt_price = 12.99
Finally red-tagged
shirt_price = 7.99

49. Updating a variable (I)
Coffeehouse tab
Open tab:
tab = 0
Add a latte:
tab = tab
Add sales tax
tab = tab *0.0825

50. A short cut Rather than typing tab = tab + 4.95 you can type
It works for +, -, * and /
price /= 2

51. Updating a variable revisited
>>> tab = 0
>>> tab
>>> tab += 4.95
4.95
>>> tab += 4.95*0.0825
>>>

52. A warning Unlike C, C++, Java, …
Autoincrements (i++, ++i) and autodecrements (i--, --i) are not allowed
Cannot use them
No i++?

53. Review assignment statement n = n + 1 (means n  n +1)
assignment token
= (means )
data type
A class of values: int, float, str, …

54. Review
expression
An algebraic/string expression that produces a result are evaluated to give that result.
float
Stores floating-point numbers.
Only approximate values.
initialization (of a variable)
To give it an initial value.
Essential

55. Review int Holds positive and negative whole numbers. integer division
Yields only the whole number of times that its numerator is divisible by its denominator
keyword
A word that you cannot use as a variable names

56. Review rules of precedence
Tells in which order expressions are evaluated
string
Holds a string of characters.
type conversion function
int(), float(), str().

57. Review
value
A number or string (or other things to be discussed later) that can be stored in a variable or computed in an expression.
variable
A name that refers to a value.
variable name
Must start with a letter (a..z, A..Z, and _)
Can contain digits(0..9)