1. Variables, Expressions and Statements
Python – Part 2

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Values and Types
Values
Basic things program works with
e.g. letter, number
1, 2, ‘Hello World!’
Types
Values belong to different types
2 is an interger
‘Hello World!’ is a string
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Numbers
Integers: X1A2
Type ‘int’
Can’t be larger than 2**31(2 31)
Octal literals begin with 0 (0981 illegal!)
Hex literals begin with 0X, contain 0-9 and A-F
Floating point: E E-21
Type ‘float’
Same precision and magnitude as C double
Long integers: L
Type ‘long’
Any magnitude
Python usually handles conversions from int to long
Complex numbers: 1+3J
Type ‘complex’
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String
>>> print "Per's lecture“
Per's lecture
Single quotes or double quotes can be used for string literals
Produces exactly the same value
Special characters in string literals: \n newline, \t tab, others
Triple quotes useful for large chunks of text in program code
>>> print "One line.\nAnother line.“
One line.
Another line.
>>> print """One line,
another line.""“
One line,
another line.
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Values and Types
Print statement for integers
>>>print 4 4
Can check the value type
>>> type(‘Hello world!’)
<type ‘str’>
>>> type (17)
<type ‘int’>
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Values and Types
Strings belog to the type str
Integers belong to the type int
Numbers with a decimal point belong to the type float.
>>>type (3.2)
<type ‘float’>
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Values and Types
What about ’17’ and ‘3.2’?
>>>type (’17’)
<type ‘str’>
>>>type (‘3.2’)
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Values and Types
>>>print 1,000,000
Output ?
1 0 0
Semantic error
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Variables
Variable is a name that refers to a value
Assignment statement creates new variables and gives them values.
>>>message=‘New message’
>>>n=17
>>>pi=
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Variables
Use print statement to display the value of a variable
>>> print n 17
>>> print message
New message
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Variables
Type of variable is the type of value it refers to(The type of the variable is determined by Python) >>>type (pi)
<type ‘float’>
>>>type (n)
<type ‘int’>
>>>type (message)
<type ‘str’>
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Variable Names
Can contain both letters and numbers
Begin with a letter
Good idea to begin variable names with a lowercase letter
Underscore character (_) can appear in a name (often in names with multiple words), e.g. my_name
The variable name is case sensitive: ‘val’ is not the same as ‘Val’
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Variable Names
>>> 76trombones = 'big parade'
SyntaxError: invalid syntax
SytaxError: invalid syntax
>>>class =‘CS104’
Class -> one of Python’s keywords
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Python Keywords
And del from not while
As elif global or with
Assert else if pass yield
Break except import print
Class exec in raise
Continue finally is return
Def for lambda try
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Statements
Unit of code that Python interpreter can execute (print, assignment statement)
print 1
x=2
print x
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16. Operators and operands
Operators – special symbols that represent computations (e.g. addition, division)
Values the operator is applied to are called operands
+ addition
- subtraction
* multiplication
/ division
** exponentiation
% modulus
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17. Operators and operands
20+32
Hour-1
Hour*60+minute
5**2
(5+9)*(15-7)
7%3
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18. Operators and Operands
>>>minute=59 >>>minute/60 0 ?
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19. Operators and operands
If both operands are integers, result is also an integer
If either of the operands is a floating-point number Python performs floatin-point division; result is a float
>>>minute/60.0
In Python 3.0 or later the result is a float
// operator performs integer division
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Expressions
Combination of values, variables and operators 17 X
X+17
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Expressions
>>>1+1 2 In a script, expression by itself doesn’t do anything.
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Order of operations
Order of evaluation depends on rules of precedence.
Python follows mathematical convention
Parentheses – highest precedence
Exponentiation –next highest precedence
Multiplication, Division, Modulus (same precedence)
Addition and Subtraction (same prec.)
Same precedence operators – left to right
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Order of operations
2*(3-1)
(1+1)**(5-2)
2**1+1
3*1**3
2*3-1
6+4/2*3
7%3+8/2
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String operations
Concatenation operator +
first =‘CS’
second=‘104’
print first+second
Repitition operator *
‘spam’*3
‘spamspamspam’
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Comments
Notes that can be added to program to explain what the program is doing
Start with the # symbol
#compute the percentage of the hour that has elapsed
percentage=(minute*100)/60
percentage=(minute*100)/60 #percentage of an hour
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26. Reading Input from the Keyboard
Programs commonly need to read input typed by the user on the keyboard. We will use the Python functions to do this.
Python uses built-in functions to read input from the keyboard.
A function is a piece of prewritten code that performs an operation and then returns a value back to the program.
The input function can be used to read numeric data from the keyboard.

27. Reading Input from the Keyboard
Reading Numbers with the input Function
Use the input function in an assignment statement:
variable = input (prompt)
where,
variable name of the variable that will reference the data
= assignment operator
input name of the function
prompt string that is displayed on the screen
For example:
hours = input (‘How many hours did you work?’)

28. Reading Input from the Keyboard
Reading Strings with the raw_input Function
The raw_input function retrieves all keyboard input as a string.
>>> name = raw_input(‘Enter your name:’)
>>> print name
Enter your name: Ahmad

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Write scripts in IDLE
Now we need to write proper scripts, saved in files
In IDLE:
'File'
'New Window'
Do immediately 'Save as…'
Browse to directory 'Desktop'
Create a directory 'Python course'
Go down into it
Enter the file name 't1.py'
Save
Work in the window called 't1.py'
Enter the following code:
Save the file: Ctrl-S, or menu 'File', 'Save'
Run the script: F5, or menu 'Run', 'Run Module'
"file Ex1.py" # this is a documentation string
print "Hello world!"
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30. End
Part 2