Python â Part 2

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# Python â Part 2 - PowerPoint PPT Presentation

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

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

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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

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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’)

<type ‘str’>

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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=3.1415926535897931

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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
• >>>more@=100000
• SytaxError: invalid syntax
• >>>class =‘CS104’
• SyntaxError: invalid syntax
• 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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Operators and operands
• Operators – special symbols that represent computations (e.g. addition, division)
• Values the operator is applied to are called operands

- subtraction

* multiplication

/ division

** exponentiation

% modulus

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

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Operators and Operands

>>>minute=59

>>>minute/60

0

?

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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

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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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• Notes that can be added to program to explain what the program is doing

#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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• 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 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 Strings with the raw_input Function

Theraw_inputfunction retrieves all keyboard input as a string.

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

>>> print name

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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End