lilian blot to programming & python introduction autumn 2012 tpop 1

Lilian BlotLilian Blot

TO PROGRAMMING&

PYTHON

Introduction

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Lilian Blot

Declarative vs. Imperative Knowledge

Declarative knowledge Statement of facts, does not tell you how to get to the

fact, but does enable you to check the result is correct

Imperative knowledge Tell you how to accomplish something, how to solve a

problem, a kind of a recipe. Recipe to approximate the solution for example

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What is an Algorithm?

An Algorithm is a description on how to perform a computation

Instructions Flow of Control Termination Condition Does it converged? Find a solution

A computation is the process of taking input and following a step-by-step algorithm to get a specific output.

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Computational Process

A computational process, in a correctly working computer, executes programs precisely and accurately

In other words:The computer will always do what you tell them to do. So if a program doesn’t work it is YOUR fault.

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Types of Errors

Crashes

Never stops => infinite loop

Runs to completion & produces the wrong answer http://www.devtopics.com/20-famous-software-disasters/

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http://www.devtopics.com/20-famous-software-disasters/

Lilian Blot

What is a Program?

Given a small set of instruction you can build any kind of program that you want Alan Turing showed that in fact there are 6 primitive instructions

that are needed to do anything (computable) with a computer http://

www.alanturing.net/turing_archive/pages/reference%20articles/Turing's%20O-Machines.html

A Programming language, such as Python, provides a set of primitive functions

Interpreter Is a program that can execute any legal set of instruction. Can be

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http://www.alanturing.net/turing_archive/pages/reference%20articles/Turing's%20O-Machines.html



Lilian Blot

What is a Program?

A program is a sequence of instructions that specifies how to perform a computation.

Few basic instructions are needed to write any kind of program: input: Get data from the keyboard, a file, or some other device. output: Display data on the screen or send data to a file or other

device. math: Perform basic mathematical operations like addition and

multiplication. conditional execution: Check for certain conditions and execute the

appropriate sequence of statements. repetition: Perform some action repeatedly, usually with some

variation.

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Lilian BlotLilian Blot

CORE ELEMENTSPART I

Python Language

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A Powerful Programming Language

Primitive expressions Represent the simplest entities with which the

language is concerned

Means of combination Compound expression are built from simpler ones

Means of abstraction Compound objects can be named and manipulated as

units

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Formal Language

Formal Languages are language designed for specific application

Python is a formal Language

Python is unambiguous

Python has strict syntax rules Syntax rules with regard to tokens, basic elements of the

language Syntax rules with regard to structure, the way tokens are

arranged in a statement

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Syntax & Semantic

Syntax Tell us which sequence of characters and symbols

constitute a well-formed stringStatic semantic

Which well-formed strings have a meaningSemantics

What that meaning is Programs have only one meaning, no ambiguity

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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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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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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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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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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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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 Expressions25

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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


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>>> 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’


Lilian Blot


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Python shell

firstname lastname

title

name

‘Lilian’

‘Dr’

‘Blot’

‘Lilian Blot’


Lilian Blot

Summary

What is a program

Python a programming language Formal language with syntactic rules

Values, Variables and Data Types

Operators and the ‘rules of precedence’

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lilian blot to programming & python introduction autumn 2012 tpop 1

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