Constants

• Fixed values such as numbers, letters, and strings, are called

“constants” because their value does not change
• Numeric constants are as you expect
>>> print(123)
• String constants use single quotes (') 123
or double quotes (") >>> print(98.6)
98.6
>>> print('Hello world')
Hello world
 Reserved Words
You cannot use reserved words as variable names / identifiers

False await else import pass

None break except in raise
True class finally is return
and continue for lambda try
as def from nonlocal while
assert del global not with
async elif if or yield
 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

x = 12.2 x 12.2
y = 14
y 14
 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

x = 12.2 x 12.2 100

y = 14
x = 100 y 14
 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
 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
x1q3z9ocd = 35.0
x1q3z9afd = 12.50
x1q3p9afd = x1q3z9ocd * x1q3z9afd
print(x1q3p9afd)

What is this bit of

code doing?
 a = 35.0
x1q3z9ocd = 35.0 b =
x1q3z9afd = 12.50 12.50
x1q3p9afd = x1q3z9ocd * x1q3z9afd c = a *
print(x1q3p9afd) b
print(c)

What are these

bits of code doing?
 a = 35.0
x1q3z9ocd = 35.0 b =
x1q3z9afd = 12.50 12.50
x1q3p9afd = x1q3z9ocd * x1q3z9afd c = a *
print(x1q3p9afd) b
print(c)

hours = 35.0
What are these rate = 12.50
bits of code doing? pay = hours * rate
print(pay)
 Sentences or Lines

x = 2 Assignment statement
x = x + 2 Assignment with expression
print(x) Print statement

Variable Operator Constant Function

 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 )
A variable is a memory location x 0.6
used to store a value (0.6)

0.6 0.6
x = 3.9 * x * ( 1 - x )

0.4

The right side is an expression.

0.936
Once the expression is evaluated,
the result is placed in (assigned to) x.
A variable is a memory location used to
store a value. The value stored in a x 0.6 0.936
variable can be updated by replacing the
old value (0.6) with a new value (0.936).
0.6 0.6
x = 3.9 * x * ( 1 - x )

0.4
The right side is an expression. Once the
expression is evaluated, the result is
0.936
placed in (assigned to) the variable on
the left side (i.e., x).
Expressions…
 Numeric Expressions
Operator Operation
• Because of the lack of mathematical
symbols on computer keyboards - we + Addition
use “computer-speak” to express the - Subtraction
classic math operations
* Multiplication
• Asterisk is multiplication / Division

• Exponentiation (raise to a power) looks ** Power

different than in math % Remainder
 Numeric Expressions
>>> xx = 2 >>> jj = 23
>>> xx = xx + 2 >>> kk = jj % 5 Operator Operation
>>> print(xx) >>> print(kk)
+ Addition
4 3
>>> yy = 440 * 12 >>> print(4 ** 3) - Subtraction
>>> print(yy) 64 * Multiplication
5280
>>> zz = yy / 1000 4R3 / Division

>>> print(zz) 5 23 ** Power

5.28 20 % Remainder

3
 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
 Operator Precedence Rules
Highest precedence rule to lowest precedence rule:

• Parentheses are always respected Parenthesis

Power
• Exponentiation (raise to a power) Multiplication
Addition
• Multiplication, Division, and Remainder
Left to Right
• Addition and Subtraction

• Left to right
 1 + 2 ** 3 / 4 * 5
>>> x = 1 + 2 ** 3 / 4 * 5
>>> print(x)
11.0 1 + 8 / 4 * 5
>>>
1 + 2 * 5
Parenthesis
Power
Multiplication 1 + 10
Addition
Left to Right
11
 Operator Precedence Parenthesis
Power
• Remember the rules top to bottom Multiplication
Addition
• When writing code - use parentheses Left to Right

• 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
 What Does “Type” Mean?
• In Python variables, literals, and
constants have a “type”
>>> ddd = 1 + 4
• Python knows the difference >>> print(ddd)
5
between an integer number and a >>> eee = 'hello ' + 'there'
string >>> print(eee)
hello there
• For example “+” means “addition” if
something is a number and
“concatenate” if something is a string
concatenate = put together
 Type Matters
>>> eee = 'hello ' + 'there'
• Python knows what “type” >>> eee = eee + 1
everything is Traceback (most recent call last):
File "<stdin>", line 1, in
• Some operations are <module>
prohibited TypeError: can only concatenate
str (not "int") to str
>>> type(eee)
• You cannot “add 1” to a string <class'str'>
>>> type('hello')
• We can ask Python what type <class'str'>
something is by using the >>> type(1)
<class'int'>
type() function >>>
 Several Types of Numbers
>>> xx = 1
• Numbers have two main types
>>> type (xx)
<class 'int'>
- Integers are whole numbers:
>>> temp = 98.6
-14, -2, 0, 1, 100, 401233
>>> type(temp)
<class'float'>
- Floating Point Numbers have
>>> type(1)
decimal parts: -2.5 , 0.0, 98.6, 14.0
<class 'int'>
>>> type(1.0)
• There are other number types - they
<class'float'>
are variations on float and integer
>>>
Type Conversions
>>> print(float(99) + 100)
• When you put an integer and 199.0
>>> i = 42
floating point in an >>> type(i)
expression, the integer is <class'int'>
implicitly converted to a float >>> f = float(i)
>>> print(f)

• You can control this with the

42.0
>>> type(f)
built-in functions int() and <class'float'>
float() >>>
 Integer Division
>>> print(10 / 2)
5.0
>>> print(9 / 2)
Integer division produces a floating 4.5
point result >>> 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
 String
>>> sval = '123'
>>> type(sval)
<class 'str'>

Conversions
>>> print(sval + 1)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can only concatenate str
• You can also use int() and (not "int") to str
>>> ival = int(sval)
float() to convert between >>> type(ival)
<class 'int'>
strings and integers >>> print(ival + 1)
124

• You will get an error if the >>> nsv = 'hello bob'

>>> niv = int(nsv)
string does not contain Traceback (most recent call last):
File "<stdin>", line 1, in <module>
numeric characters ValueError: invalid literal for int()
with base 10: 'x'
 User Input
• We can instruct Python to
pause and read data nam = input('Who are you? ')
print('Welcome', nam)
from the user using the
input() function

• The input() function

Who are you? Chuck
returns a string
Welcome Chuck
Converting User Input
• If we want to read a
number from the user, we
inp = input('Europe floor?')
must convert it from a usf = int(inp) + 1
string to a number using a print('US floor', usf)
type conversion function

• Later we will deal with bad Europe floor? 0

input data US floor 1
 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

# 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)
 Summary
• Type • Integer Division

• Reserved words • Conversion between types

• Variables (mnemonic) • User input

• Operators • Comments (#)

• Operator precedence
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