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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

Variables, Expressions and Statements

• 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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>>> 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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• 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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• 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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• What about ’17’ and ‘3.2’?

>>>type (’17’)

<type ‘str’>

>>>type (‘3.2’)

<type ‘str’>

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• >>>print 1,000,000

• Output ?

• 1 0 0

• Semantic error

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

>>> print n

17

>>> print message

New message

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• 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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• 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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• >>> 76trombones = 'big parade'

SyntaxError: invalid syntax

• >>>more@=100000

SytaxError: invalid syntax

• >>>class =‘CS104’

SyntaxError: invalid syntax

• Class -> one of Python’s keywords

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

print 1

x=2

print x

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• 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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• 20+32

• Hour-1

• Hour*60+minute

• 5**2

• (5+9)*(15-7)

• 7%3

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>>>minute=59

>>>minute/60

0

?

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• 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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• Combination of values, variables and operators

17

X

X+17

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>>>1+1

2

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

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• 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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• 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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• 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

• 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