2. C:usersHPpython
Python 3.10.8 (tags/v3.10.8:aaaf517, Oct 11 2022, 16:50:30)
[MSC v.1933 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license()" for more
information.
>>> x = 1
>>> print(x)
1
>>> x = x + 1
>>> print(x)
2
>>> exit()
This is a good test to make sure that you have
Python correctly installed. Note that quit() also
works to end the interactive session.
3. Constants
• Fixed values such as numbers, letters, and strings,
are called “constants” because their value does not
change
• Numeric constants are as you expect
• String constants use single quotes (')
or double quotes (")
>>> print(123)
123
>>> print(98.6)
98.6
>>> print('Hello world')
Hello world
4. Reserved Words
You cannot use reserved words as variable names / identifiers
False class return is finally
None if for lambda continue
True def from while nonlocal
and del global not with
as elif try or yield
assert else import pass
break except in raise
5. Variables
• A variable is a named place in the memory where a programmer can store
data and later retrieve the data using the variable “name”
• Programmers get to choose the names of the variables
• You can change the contents of a variable in a later statement
12.2
x
14
y
x = 12.2
y = 14
6. Variables
• A variable is a named place in the memory where a programmer can store
data and later retrieve the data using the variable “name”
• Programmers get to choose the names of the variables
• You can change the contents of a variable in a later statement
12.2
x
14
y
100
x = 12.2
y = 14
x = 100
7. Python Variable Name Rules
Must start with a letter or underscore _
Must consist of letters, numbers, and underscores
Case Sensitive
Good: spam eggs spam23 _speed
Bad: 23spam #sign var.12
Different: spam Spam SPAM
8. Mnemonic Variable Names
• Since we programmers are given a choice in how we choose our
variable names, there is a bit of “best practice”
• We name variables to help us remember what we intend to store
in them (“mnemonic” = “memory aid”)
• This can confuse beginning students because well-named
variables often “sound” so good that they must be keywords
http://en.wikipedia.org/wiki/Mnemonic
9. Sentences or Lines
x = 2
x = x + 2
print(x)
Variable Operator Constant Function
Assignment statement
Assignment with expression
Print statement
10. Assignment Statements
We assign a value to a variable using the assignment statement (=)
An assignment statement consists of an expression on the
right-hand side and a variable to store the result
x = 3.9 * x * ( 1 - x )
11. x = 3.9 * x * ( 1 - x )
0.6
x
The right side is an expression.
Once the expression is evaluated, the
result is placed in (assigned to) x.
0.6 0.6
0.4
0.936
A variable is a memory location
used to store a value (0.6)
12. x = 3.9 * x * ( 1 - x )
0.6 0.936
x
0.4
0.936
The right side is an expression. Once the
expression is evaluated, the result is
placed in (assigned to) the variable on the
left side (i.e., x).
A variable is a memory location used to
store a value. The value stored in a
variable can be updated by replacing the
old value (0.6) with a new value (0.936).
0.6 0.6
13. Numeric Expressions
• Because of the lack of mathematical
symbols on computer keyboards - we
use “computer-speak” to express the
classic math operations
• Asterisk is multiplication
• Exponentiation (raise to a power) looks
different than in math
Operator Operation
+ Addition
- Subtraction
* Multiplication
/ Division
** Power
% Remainder
15. Order of Evaluation
• When we string operators together - Python must know which one
to do first
• This is called “operator precedence”
• Which operator “takes precedence” over the others?
x = 1 + 2 * 3 - 4 / 5 ** 6
16. Operator Precedence Rules
Highest precedence rule to lowest precedence rule:
Parentheses are always respected
Exponentiation (raise to a power)
Multiplication, Division, and Remainder
Addition and Subtraction
Left to right
Parenthesis
Power
Multiplication
Addition
Left to Right
17. 1 + 2 ** 3 / 4 * 5
1 + 8 / 4 * 5
1 + 2 * 5
1 + 10
11
>>> x = 1 + 2 ** 3 / 4 * 5
>>> print(x)
11.0
>>>
Parenthesis
Power
Multiplication
Addition
Left to Right
18. Operator Precedence
• Remember the rules top to bottom
• When writing code - use parentheses
• When writing code - keep mathematical expressions simple enough
that they are easy to understand
• Break long series of mathematical operations up to make them
more clear
Parenthesis
Power
Multiplication
Addition
Left to Right
19. What Does “Type” Mean?
• In Python variables, literals, and
constants have a “type”
• Python knows the difference between
an integer number and a string
• For example “+” means “addition” if
something is a number and
“concatenate” if something is a string
>>> ddd = 1 + 4
>>> print(ddd)
5
>>> eee = 'hello ' + 'there'
>>> print(eee)
hello there
concatenate = put together
20. Type Matters
• Python knows what “type”
everything is
• Some operations are
prohibited
• You cannot “add 1” to a string
• We can ask Python what type
something is by using the
type() function
>>> eee = 'hello ' + 'there'
>>> eee = eee + 1
Traceback (most recent call last):
File "<stdin>", line 1, in
<module>TypeError: Can't convert
'int' object to str implicitly
>>> type(eee)
<class'str'>
>>> type('hello')
<class'str'>
>>> type(1)
<class'int'>
>>>
21. Several Types of Numbers
• Numbers have two main types
- Integers are whole numbers:
-14, -2, 0, 1, 100, 401233
- Floating Point Numbers have
decimal parts: -2.5 , 0.0, 98.6, 14.0
• There are other number types - they
are variations on float and integer
>>> xx = 1
>>> type (xx)
<class 'int'>
>>> temp = 98.6
>>> type(temp)
<class'float'>
>>> type(1)
<class 'int'>
>>> type(1.0)
<class'float'>
>>>
22. Type Conversions
• When you put an integer and
floating point in an
expression, the integer is
implicitly converted to a float
• You can control this with the
built-in functions int() and
float()
>>> print(float(99) + 100)
199.0
>>> i = 42
>>> type(i)
<class'int'>
>>> f = float(i)
>>> print(f)
42.0
>>> type(f)
<class'float'>
>>>
23. Integer Division
Integer division produces a floating
point result
>>> print(10 / 2)
5.0
>>> print(9 / 2)
4.5
>>> print(99 / 100)
0.99
>>> print(10.0 / 2.0)
5.0
>>> print(99.0 / 100.0)
0.99
This was different in Python 2.x
24. String
Conversions
• You can also use int() and
float() to convert between
strings and integers
• You will get an error if the string
does not contain numeric
characters
>>> sval = '123'
>>> type(sval)
<class 'str'>
>>> print(sval + 1)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Can't convert 'int' object
to str implicitly
>>> ival = int(sval)
>>> type(ival)
<class 'int'>
>>> print(ival + 1)
124
>>> nsv = 'hello bob'
>>> niv = int(nsv)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: invalid literal for int()
with base 10: 'x'
25. User Input
• We can instruct Python to
pause and read data from
the user using the input()
function
• The input() function
returns a string
nam = input('Who are you? ')
print('Welcome', nam)
Who are you? Chuck
Welcome Chuck
26. Converting User Input
• If we want to read a number
from the user, we must
convert it from a string to a
number using a type
conversion function
• Later we will deal with bad
input data
inp = input('Europe floor?')
usf = int(inp) + 1
print('US floor', usf)
Europe floor? 0
US floor 1
27. Comments in Python
• Anything after a # is ignored by Python
• Why comment?
- Describe what is going to happen in a sequence of
code
- Document who wrote the code or other ancillary
information
- Turn off a line of code - perhaps temporarily
28. # Get the name of the file and open it
name = input('Enter file:')
handle = open(name, 'r')
# Count word frequency
counts = dict()
for line in handle:
words = line.split()
for word in words:
counts[word] = counts.get(word,0) + 1
# Find the most common word
bigcount = None
bigword = None
for word,count in counts.items():
if bigcount is None or count > bigcount:
bigword = word
bigcount = count
# All done
print(bigword, bigcount)
29. Summary
• Type
• Reserved words
• Variables (mnemonic)
• Operators
• Operator precedence
• Integer Division
• Conversion between types
• User input
• Comments (#)
30. Exercise
Write a program to prompt the user for hours
and rate per hour to compute gross pay.
Enter Hours: 35
Enter Rate: 2.75
Pay: 96.25
32. Conditional Steps
Output:
Smaller
Finis
Program:
x = 5
if x < 10:
print('Smaller')
if x > 20:
print('Bigger')
print('Finis')
x = 5
x < 10 ?
print('Smaller')
x > 20 ?
print('Bigger')
print('Finis')
Yes
No
Yes
No
33. Comparison Operators
• Boolean expressions ask a
question and produce a Yes or No
result which we use to control
program flow
• Boolean expressions using
comparison operators evaluate to
True / False or Yes / No
• Comparison operators look at
variables but do not change the
variables
http://en.wikipedia.org/wiki/George_Boole
Remember: “=” is used for assignment.
Python Meaning
< Less than
<= Less than or Equal to
== Equal to
>= Greater than or Equal to
> Greater than
!= Not equal
34. Comparison Operators
x = 5
if x == 5 :
print('Equals 5')
if x > 4 :
print('Greater than 4')
if x >= 5 :
print('Greater than or Equals 5')
if x < 6 : print('Less than 6')
if x <= 5 :
print('Less than or Equals 5')
if x != 6 :
print('Not equal 6')
Equals 5
Greater than 4
Greater than or Equals 5
Less than 6
Less than or Equals 5
Not equal 6
35. One-Way Decisions
x = 5
print('Before 5')
if x == 5 :
print('Is 5')
print('Is Still 5')
print('Third 5')
print('Afterwards 5')
print('Before 6')
if x == 6 :
print('Is 6')
print('Is Still 6')
print('Third 6')
print('Afterwards 6')
Before 5
Is 5
Is Still 5
Third 5
Afterwards 5
Before 6
Afterwards 6
x == 5 ?
Yes
print('Still 5')
print('Third 5')
No print('Is 5’)
36. Indentation
• Increase indent indent after an if statement or for statement (after : )
• Maintain indent to indicate the scope of the block (which lines are affected
by the if/for)
• Reduce indent back to the level of the if statement or for statement to
indicate the end of the block
• Blank lines are ignored - they do not affect indentation
• Comments on a line by themselves are ignored with regard to indentation
37. Warning: Turn Off Tabs!!
Atom automatically uses spaces for files with ".py" extension (nice!)
• Most text editors can turn tabs into spaces - make sure to enable this
feature
- NotePad++: Settings -> Preferences -> Language Menu/Tab Settings
- TextWrangler: TextWrangler -> Preferences -> Editor Defaults
• Python cares a *lot* about how far a line is indented. If you mix tabs and
spaces, you may get “indentation errors” even if everything looks fine
39. x = 5
if x > 2 :
print('Bigger than 2')
print('Still bigger')
print('Done with 2')
for i in range(5) :
print(i)
if i > 2 :
print('Bigger than 2')
print('Done with i', i)
print('All Done')
increase / maintain after if or for
decrease to indicate end of block
40. x = 5
if x > 2 :
print('Bigger than 2')
print('Still bigger')
print('Done with 2')
for i in range(5) :
print(i)
if i > 2 :
print('Bigger than 2')
print('Done with i', i)
print('All Done')
Think About begin/end Blocks
41. x = 42
if x > 1 :
print('More than one')
if x < 100 :
print('Less than 100')
print('All done')
Nested
Decisions
x > 1
print('More than one’)
x < 100
print('Less than 100')
print('All Done')
yes
yes
no
no
42. Two-way Decisions
• Sometimes we want to
do one thing if a logical
expression is true and
something else if the
expression is false
• It is like a fork in the
road - we must choose
one or the other path but
not both
x > 2
print('Bigger')
yes
no
x = 4
print('Not bigger')
print('All Done')
43. Two-way Decisions
with else:
x > 2
print('Bigger')
yes
no
x = 4
print('All Done')
x = 4
if x > 2 :
print('Bigger')
else :
print('Smaller')
print('All done')
print('Not bigger')
44. Visualize Blocks
x = 4
if x > 2 :
print('Bigger')
else :
print('Smaller')
print('All done')
x > 2
print('Bigger')
yes
no
x = 4
print('All Done')
print('Not bigger')
46. Multi-way
if x < 2 :
print('small')
elif x < 10 :
print('Medium')
else :
print('LARGE')
print('All done')
x < 2 print('small')
yes
no
print('All Done')
x < 10 print('Medium')
yes
print('LARGE')
no
47. Multi-way
x = 0
if x < 2 :
print('small')
elif x < 10 :
print('Medium')
else :
print('LARGE')
print('All done')
x < 2 print('small')
yes
no
print('All Done')
x < 10 print('Medium')
yes
print('LARGE')
no
x = 0
48. Multi-way
x = 5
if x < 2 :
print('small')
elif x < 10 :
print('Medium')
else :
print('LARGE')
print('All done')
x < 2 print('small')
yes
no
print('All Done')
x < 10 print('Medium')
yes
print('LARGE')
no
x = 5
49. Multi-way
x = 20
if x < 2 :
print('small')
elif x < 10 :
print('Medium')
else :
print('LARGE')
print('All done')
x < 2 print('small')
yes
no
print('All Done')
x < 10 print('Medium')
yes
print('LARGE')
no
x = 20
50. Multi-way
# No Else
x = 5
if x < 2 :
print('Small')
elif x < 10 :
print('Medium')
print('All done')
if x < 2 :
print('Small')
elif x < 10 :
print('Medium')
elif x < 20 :
print('Big')
elif x < 40 :
print('Large')
elif x < 100:
print('Huge')
else :
print('Ginormous')
51. Multi-way Puzzles
if x < 2 :
print('Below 2')
elif x < 20 :
print('Below 20')
elif x < 10 :
print('Below 10')
else :
print('Something else')
if x < 2 :
print('Below 2')
elif x >= 2 :
print('Two or more')
else :
print('Something else')
Which will never print
regardless of the value for x?
53. Exercise
Rewrite your pay computation to give the
employee 1.5 times the hourly rate for hours
worked above 40 hours.
Enter Hours: 45
Enter Rate: 10
Pay: 475.0
475 = 40 * 10 + 5 * 15
54. Exercise
Rewrite your pay program using try and except so
that your program handles non-numeric input
gracefully.
Enter Hours: 20
Enter Rate: nine
Error, please enter numeric input
Enter Hours: forty
Error, please enter numeric input
Editor's Notes
#31:Note from Chuck. If you are using these materials, you can remove the UM logo and replace it with your own, but please retain the CC-BY logo on the first page as well as retain the acknowledgement page(s).
![C:usersHPpython
Python 3.10.8 (tags/v3.10.8:aaaf517, Oct 11 2022, 16:50:30)
[MSC v.1933 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license()" for more
information.
>>> x = 1
>>> print(x)
1
>>> x = x + 1
>>> print(x)
2
>>> exit()
This is a good test to make sure that you have
Python correctly installed. Note that quit() also
works to end the interactive session.](https://image.slidesharecdn.com/ppt19102501a-a7a1-493e-818f-cf699918bbf6-250620133339-2c52b653/85/PPT_1_9102501a-a7a1-493e-818f-cf699918bbf6-pptx-2-320.jpg)
![# Get the name of the file and open it
name = input('Enter file:')
handle = open(name, 'r')
# Count word frequency
counts = dict()
for line in handle:
words = line.split()
for word in words:
counts[word] = counts.get(word,0) + 1
# Find the most common word
bigcount = None
bigword = None
for word,count in counts.items():
if bigcount is None or count > bigcount:
bigword = word
bigcount = count
# All done
print(bigword, bigcount)](https://image.slidesharecdn.com/ppt19102501a-a7a1-493e-818f-cf699918bbf6-250620133339-2c52b653/85/PPT_1_9102501a-a7a1-493e-818f-cf699918bbf6-pptx-28-320.jpg)
