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INLS 560 – VARIABLES,
EXPRESSIONS, AND STATEMENTS
Instructor: Jason Carter
WHAT IS A PROGRAM?
Set of instructions that a computer follows to perform
a task or solve a problem
INSTRUCTIONS

29 keywords
and. def, exec, if, not, return
assert, del, finally, import, or, try
break, elif, for, in, pass, while
class, else, from, is, print, yield
continue, except, global, lambda, raise
HELLO WORLD EXAMPLE

print “Hello, World!”
Keyword
Output: Hello World
Value
ANOTHER EXAMPLE

print 1 + 1
Keyword
Output: 2
Value
VALUES HAVE TYPES

print “Hello, World!”
Keyword
The value “Hello World” is a type of string
 A string is a sequence of characters
 A string can be identified because it is enclosed in
quotes.

ANOTHER TYPE EXAMPLE

print 1 + 1
Keyword
The value 2 is a type of integer
 A non-decimal numerical value
 There are many different types

VARIABLES
A variable is a name that refers to a value
A name that represents a value stored in the
computer memory

Assignment statement: used to create a variable
and make it reference data
 General format is variable = expression

Example: age = 29
 Assignment operator: the equal sign (=)

ASSIGNMENT STATEMENT
In an assignment statement, variable receiving
value must be on left side
 You can only use a variable if a value is assigned to
it
 message = "What’s up, Doc?“
 n = 17
 pi = 3.14159

VARIABLE NAMES AND KEYWORDS

Rules for naming variables in Python:







Legal




Variable name cannot be a Python key word
Variable name cannot contain spaces
First character must be a letter or an underscore
After first character may use letters, digits, or underscores
Variable names are case sensitive
Variable name should reflect its use
message = "What’s up, Doc?“
n = 17
pi = 3.14159
Illegal



76trombones = ’big parade’
more$ = 1000000
class = ’Computer Science 101’
MANIPULATING VARIABLES

Variables can be:

Printed
 message
 print
message
 Output:
•
= "What’s up, Doc?”
What’s up Doc
Reassigned
 message
= "What’s up, Doc?”
 message
= “Hello?”
 print
message
 Output:
Hello?
print and assignment are two kinds of statements
STATEMENT
A statement is an instruction that the Python
interpreter can execute
 print 1
x=2
 print x
 Output:

1
 2


The assignment statement does not produce output.
REVIEW
Python has 29 keywords
 Variable



Assignment statement


A variable is a name that refers to a value
used to create a variable and make it reference data
Statement

an instruction that the Python interpreter can execute
EXPRESSIONS


A combination of values, variables, and operators
Operators are special symbols that represent computations





Addition
Subtraction
Division
Multiplication
Exponentiation







Performs division and returns the remainder
4%2=0, 5%2=1
Typically used to convert times and distances, and
to detect odd or even numbers
Evaluate the expression: 4 + 4


Raises a number to a power
x ** y = 𝑥 𝑦
Remainder (Modulus)

+
–
/
*
**
Output: ?
Evaluate the expression: (4 + 4) - 3
%
ORDER OF OPERATIONS
Follows same rule of precedence as mathematics
 PEMDAS
 Parentheses
 Exponentiation
 Multiplication and Division
 Addition and Subtraction


Evaluate the expression: (4 + 4) - 3
INPUT FROM KEYBOARD
Most programs need to read input from the user
 Built-in input function reads input from keyboard



Format: variable = input(prompt)


Returns the data as a string
prompt is typically a string instructing user to enter a
value
Does not automatically display a space after the
prompt
Example:
 name = input(“Please enter your name: ”)

READING NUMBERS WITH THE INPUT FUNCTION
input function always returns a string
 Built-in functions convert between data types

int(item) converts item to an int
 float(item) converts item to a float


Type conversion only works if item is valid numeric
value, otherwise, throws exception
INPUT FROM KEYBOARD

For integers:


For floats:


variable = int(input("Prompt"))
variable = float(input("Prompt"))
For strings:

variable = input("Prompt")
PRACTICE

Write a program that takes
Two numbers from a user
 Print out the sum, difference, product, and quotient of
the two numbers
