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Python – Part 2. Variables, Expressions and Statements. 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. Integers: 12 0 -12987 0123 0X1A2

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python part 2

Python – Part 2

Variables, Expressions and Statements

values and types
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

Prepared by Department of Preparatory year

numbers

Integers: 12 0 -12987 0123 0X1A2

    • 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: 12.03 1E1 -1.54E-21
    • Type ‘float’
    • Same precision and magnitude as C double
  • Long integers: 10294L
    • Type ‘long’
    • Any magnitude
    • Python usually handles conversions from int to long
  • Complex numbers: 1+3J
    • Type ‘complex’
Numbers

Prepared by Department of Preparatory year

string
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.

Prepared by Department of Preparatory year

values and types1
Values and Types
  • Print statement for integers

>>>print 4

4

  • Can check the value type

>>> type(‘Hello world!’)

<type ‘str’>

>>> type (17)

<type ‘int’>

Prepared by Department of Preparatory year

values and types2
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’>

Prepared by Department of Preparatory year

values and types3
Values and Types
  • What about ’17’ and ‘3.2’?

>>>type (’17’)

<type ‘str’>

>>>type (‘3.2’)

<type ‘str’>

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values and types4
Values and Types
  • >>>print 1,000,000
  • Output ?
  • 1 0 0
  • Semantic error

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variables
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=3.1415926535897931

Prepared by Department of Preparatory year

variables1
Variables
  • Use print statement to display the value of a variable

>>> print n

17

>>> print message

New message

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variables2
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’>

Prepared by Department of Preparatory year

variable names
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’

Prepared by Department of Preparatory year

variable names1
Variable Names
  • >>>[email protected]=100000
  • SytaxError: invalid syntax
  • >>>class =‘CS104’
  • SyntaxError: invalid syntax
  • Class -> one of Python’s keywords

Prepared by Department of Preparatory year

python keywords
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
Statements
  • Unit of code that Python interpreter can execute (print, assignment statement)

print 1

x=2

print x

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operators and operands
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

Prepared by Department of Preparatory year

operators and operands1
Operators and operands
  • 20+32
  • Hour-1
  • Hour*60+minute
  • 5**2
  • (5+9)*(15-7)
  • 7%3

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operators and operands2
Operators and Operands

>>>minute=59

>>>minute/60

0

?

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operators and operands3
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

0.98333333333333328

  • In Python 3.0 or later the result is a float
  • // operator performs integer division

Prepared by Department of Preparatory year

expressions
Expressions
  • Combination of values, variables and operators

17

X

X+17

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expressions1
Expressions

>>>1+1

2

In a script, expression by itself doesn’t do anything.

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order of operations
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

Prepared by Department of Preparatory year

order of operations1
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

Prepared by Department of Preparatory year

string operations
String operations
  • Concatenation operator +

first =‘CS’

second=‘104’

print first+second

  • Repitition operator *

‘spam’*3

‘spamspamspam’

Prepared by Department of Preparatory year

comments
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

Prepared by Department of Preparatory year

reading input from the keyboard
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.
reading input from the keyboard1
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?’)
reading input from the keyboard2
Reading Input from the Keyboard

Reading Strings with the raw_input Function

Theraw_inputfunction retrieves all keyboard input as a string.

>>> name = raw_input(‘Enter your name:’)

>>> print name

Enter your name: Ahmad

write scripts in idle
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!"

Prepared by Department of Preparatory year

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