Variables, Expressions, and Statements Chapter 2

Question 1

Variables

Answer 1

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.

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

x = 12.2
y = 14

Question 2

Python Variable Name Rules

Answer 2

1. Must start with a letter or underscore _
2. Must consist of letters and numbers and underscores
3. Case Sensitive
   • Good: spam eggs spam23 _speed
   • Bad: 23spam #sign var.12
   • Different: spam Spam SPAM

Question 3

Reserved Words

Answer 3

You cannot use reserved words as variable names / identifiers

Question 4

Reserved Words

Answer 4

and del for is raise assert elif
from lambda return break else
global not try class except if or
while continue exec import pass
yield def finally in print as with

Question 5

Sentences or Lines

Answer 5

x = 2

Question 6

Assignment Statements

Answer 6

• 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

Example of an Assignment Statements :
x = 3.9 * x * ( 1 - x )

Question 7

Numeric Expressions

Answer 7

• 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 from in math.

\+  Addition
-  Subtraction
*  Multiplication
/  Division
**  Power
%  Remainder

Question 8

Numeric Expressions

Answer 8

> > > xx = 2
xx = xx + 2
print xx
4

> > > yy = 440 * 12
print yy
5280

> > > zz = yy / 1000
print zz
5

>>> jj = 23
>>> kk = jj % 5
>>> print kk
3
>>> print 4 ** 3
64

Question 9

Order of Evaluation

Answer 9

• 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

Question 10

Operator Precedence Rules

Answer 10

Highest precedence rule to lowest precedence rule:
> Parenthesis 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  

> > > x = 1 + 2 ** 3 / 4 * 5
print x
11

Question 11

Operator Precedence

Answer 11

• Remember the rules top to bottom
• When writing code - use parenthesis
• 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
Exam Question: x = 1 + 2 * 3 - 4 / 5

Question 12

Python Integer Division is Weird!

This changes in Python 3.0

Answer 12

>>> print 10 / 2
5
>>> print 9 / 2
4
>>> print 99 / 100
0
>>> print 10.0 / 2.0
5.0
>>> print 99.0 / 100.0
0.99

Question 13

Mixing Integer and Floating

Answer 13

• When you perform an
operation where one
operand is an integer and
the other operand is a
floating point, the result is a
floating point
• The integer is converted to a
floating point before the
operation

>>> print 99 / 100
0
>>> print 99 / 100.0
0.99
>>> print 99.0 / 100
0.99
>>> print 1 + 2 * 3 / 4.0 - 5
-2.5
>>>

Question 14

What does “Type” Mean?

Answer 14

• 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

Question 15

Type Matters

Answer 15

• 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 “”, line 1, in

TypeError: cannot concatenate
‘str’ and ‘int’ objects
»> type(eee)

> > > type(‘hello’)

> > > type(1)

> > >

Question 16

Several Types of Numbers

Answer 16

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)

> > > temp = 98.6
type(temp)

> > > type(1)

> > > type(1.0)

> > >

Question 17

Type Conversions

Answer 17

• 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
0.99
>>> i = 42
>>> type(i)

> > > f = float(i)
print f
42.0
type(f)

> > > print 1 + 2 * float(3) / 4 - 5
-2.5

Question 18

String

Conversions

Answer 18

• 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)

>>> print sval + 1
Traceback (most recent call last):
File "", line 1, in 
TypeError: cannot concatenate 'str'
and 'int'
>>> ival = int(sval)
>>> type(ival)

>>> print ival + 1
124
>>> nsv = 'hello bob'
>>> niv = int(nsv)
Traceback (most recent call last):
File "", line 1, in 
ValueError: invalid literal for int()

Question 19

User Input

Answer 19

• We can instruct Python
to pause and read data
from the user using the
raw_input() function

• The raw_input() function
returns a string

nam = raw_input(‘Who are you?’)
print ‘Welcome’, nam

Who are you? Chuck

Welcome Chuck

Question 20

Converting User Input

Answer 20

• 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 = raw_input(‘Europe floor?’)
usf = int(inp) + 1
print ‘US floor’, usf

Europe floor? 0
US floor 1

Question 21

Comments in Python

Answer 21

• 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

Question 22

Comments in Python

Answer 22

# Get the name of the file and open it
name = raw_input('Enter file:')
handle = open(name, 'r')
text = handle.read()
words = text.split()

# Count word frequency
counts = dict()
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

Question 23

String Operations

Answer 23

• Some operators apply to strings
> + implies “concatenation”
> * implies “multiple concatenation”

• Python knows when it is dealing with
a string or a number and behaves
appropriately

>>> print 'abc' + '123’
abc123
>>> print 'Hi' * 5
HiHiHiHiHi
>>>

Question 24

Mnemonic Variable Names

Answer 24

• 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

Question 25

What is this bit of

code doing?

Answer 25

x1q3z9ocd = 35.0
x1q3z9afd = 12.50
x1q3p9afd = x1q3z9ocd * x1q3z9afd
print x1q3p9afd

What is this bit of
code doing?

Question 26

What are these

bits of code doing?

Answer 26

x1q3z9ocd = 35.0
x1q3z9afd = 12.50
x1q3p9afd = x1q3z9ocd * x1q3z9afd
print x1q3p9afd

a = 35.0
b = 12.50
c = a * b
print c

hours = 35.0
rate = 12.50
pay = hours * rate
print pay

Question 27

Exercise

Answer 27

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

Question 28

Summary

Answer 28

• Type
• Reserved words
• Variables (mnemonic)
• Operators
• Operator precedence
• Integer Division
• Conversion between types
• User input
• Comments (#)