Lilian Blot CORE ELEMENTS PART I Python Language Autumn 2013 TPOP 1.

Lilian BlotLilian Blot

CORE ELEMENTSPART I

Python Language

Autumn 2013TPOP

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Lilian Blot

Values and Data Types

A value is one of the fundamental things that a program manipulates

A value can be: a letter ‘L’ a word ‘Lilian’ a number 1, 3.14, 1.2e3 (e.g. 1200.0) and more complex data (seen later in the year)

Values have a type, 1 and 1.0 are not represented the same way

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Lilian Blot

Values and Data Types

Words and letters are strings (‘str’ in Python) ‘lilian blot’ or “lilian blot”

Numbers with a decimal point are called float (‘float’) 124.0, 124e2, 0.123

Integer (‘int’), number with no decimal point 1, 124, 0

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>>> type(“Lilian Blot”)<type 'str'>>>> type(25)<type 'int'>>>> type(190.0)<type 'float'>

Python shell

Lilian Blot

Variables

A critical aspect of programming is the means it provides for using names to refer to computational objects

We say that the name identifies a variable whose value is the object

Python uses the assignment statement =

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>>> name = “Lilian Blot”>>> age = 25>>> height_cm = 190.0

Python shell

Lilian Blot

Variables and Data Types

Variables have types

The type of a variable is the type of the value it refers to

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>>> type(name)<type 'str'>>>> type(age)<type 'int'>>>> type(height_cm)<type 'float'>

Python shell

Lilian Blot

State Diagram

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“Lilian Blot”

190.0

25

name

age

height_cm

>>> name = “Lilian Blot”>>> age = 25>>> height_cm = 190.0

Python shell

>>> print nameLilian Blot

Python shell

Lilian Blot

Variable Names

variable names must start with a letter or an underscore

variable names can contain letters, numbers and underscores

variable names can be arbitrary long

Python is case sensitive, so size and Size are two different variables

by convention uppercase letters are not used

multiple words names use underscore, e.g. book_title, book_price, …

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Lilian Blot

Variable Name and Keyword

Language’s rule and structure are defined by keywords

Keywords cannot be used as variable names

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and as assert break class continue def del

elif else except exec for finally from global

if import in is not lambda or pass

print raise return try while with yieldPython’s 31 keywords

Lilian Blot

Statements & Expressions

A statement is an instruction that the Python interpreter can execute

An expression is a combination of values, variables, and operators

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>>> “Dr ” + name‘Dr Lilian Blot’

Python shell

>>> name = “Lilian Blot”>>> print nameLilian Blot

Python shell

Lilian Blot

Operators

Operators are special symbols that represent computations like addition and multiplication.

The values the operator uses are called operands.

As in Mathematics, when more than one operator appears in an expression, the order of evaluation depends on the rules of precedence. the result of 3 + 4 * 2 is 11, not 14

Parentheses have the highest precedence the result of (3 + 4) * 2 is 14, not 11 use of parentheses is encouraged in long and complex expressions

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Lilian Blot

Operator Precedence

Operator Name

Precedence

+, - Unary plus and minus (e.g. -10)

** Exponentiation

*, /, //, % Multiplication, division, integer division, and remainder

+, - Binary plus and minus (e.g. 3-10)

= Assignment operators

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Lilian Blot

Operator Precedence

If operators with the same precedence are next to each other, their associativity determines the order of evaluation ALL binary operators are left-associative

If in doubt, use parenthesis for clarity

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Lilian Blot

Composition

Expression can be combined to create a more complex expression

For example:<expression_1> operator <expression_2> (age * 10) + (height_cm / 100)print (‘Dr’ + name) +(“>”* 3)

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Lilian Blot

Assignment Operator

However, the left-hand side of the assignment operator has to be a variable, not an expression

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>>> “Dr ” + name = title_nameSyntaxError: can't assign to operator>>>

Python shell

Lilian Blot

Assignment and Expressions15

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>>> firstname = ‘Lilian’>>>

Python shell

‘Lilian’

Lilian Blot


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>>> firstname = ‘Lilian’>>>

Python shell

firstname

‘Lilian’

Lilian Blot


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>>> firstname = ‘Lilian’>>> lastname = ‘Blot’>>> title = ‘Dr’>>>

Python shell

firstname lastname

title

‘Lilian’

‘Dr’

‘Blot’

Lilian Blot


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>>> firstname = ‘Lilian’>>> lastname = ‘Blot’>>> title = ‘Dr’>>> name = firstname + ‘ ‘ + lastname>>>

Python shell

firstname lastname

title

‘Lilian’

‘Dr’

‘Blot’

‘Lilian’ + ‘ ‘ + ‘Blot’

firstname lastname

Lilian Blot


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>>> firstname = ‘Lilian’>>> lastname = ‘Blot’>>> title = ‘Dr’>>> name = firstname + ‘ ‘ + lastname>>>

Python shell

firstname lastname

title

‘Lilian’

‘Dr’

‘Blot’

‘Lilian Blot’

‘Lilian Blot’

name

Lilian Blot


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>>> firstname = ‘Lilian’>>> lastname = ‘Blot’>>> title = ‘Dr’>>> name = firstname + ‘ ‘ + lastname>>> name‘Lilian Blot’>>>

Python shell

firstname lastname

title

name

‘Lilian’

‘Dr’

‘Blot’

‘Lilian Blot’

Lilian Blot


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>>> firstname = ‘Lilian’>>> lastname = ‘Blot’>>> title = ‘Dr’>>> name = firstname + ‘ ‘ + lastname>>> name‘Lilian Blot’>>> name = title + ‘ ‘ + name

Python shell

firstname lastname

title

name

‘Lilian’

‘Dr’

‘Blot’

‘Lilian Blot’

‘Dr Lilian Blot’

‘Dr’ + ‘ ‘ + ‘Lilian Blot’

Lilian Blot


Autumn 2013TPOP

>>> firstname = ‘Lilian’>>> lastname = ‘Blot’>>> title = ‘Dr’>>> name = firstname + ‘ ‘ + lastname>>> name‘Lilian Blot’>>> name = title + ‘ ‘ + name>>> name ‘Dr Lilian Blot’

Python shell

firstname lastname

title

name

‘Lilian’

‘Dr’

‘Blot’

‘Dr Lilian Blot’

Lilian Blot

Summary

Values, Variables and Data Types

Operators and the ‘rules of precedence’

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Lilian Blot CORE ELEMENTS PART I Python Language Autumn 2013 TPOP 1.

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