Algorithm

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Contents

Definition of Program and Algorithm Syntax and Semantics Pseudocode Operator

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Definition Program: developed to help perform tasks (to meet

your needs)- a collection of statements or steps

- solve a problem - converted into a language the computer understands to perform tasks

Algorithm: - an ordered set of unambiguous, executablesteps that defines a terminating process

- logically ordered set of statements: used tosolve a problem

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What is a Program?

Interpreter (; )- Translates programs statements into a language

the computer understands- ;

Compiler ()- Application reading all the programs statements

- Converts them into computer language- Produces an executable file running

independently of an interpreter- =>=>

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Art of problem solving

The creativity of art can not be taught.()

We will talk about the representation of the art.

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Example: Taipei=>Kaohsiung

=>

=>=>

=>=>

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?

?

Contents

Definition of Program and Algorithm Syntax and Semantics Pseudocode Operator

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Different types of programming languages

I Speak Computer

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I Speak Computer

Low-level language- Use binary code for instructions- machine language, assembly language

Machine language - Lowest-level programming language- Consist of binary bit patterns

Assembly language ()- One step up from machine language- Assign letter codes to each machine-languageinstruction

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I Speak Computer Assembler ()

- Program that reads assembly language code andconverts it into machine language

High-level language- Written in a more natural language that humans can

read and understand- eg.: Java, C++, Delphi, Fortran

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Syntax and Semantics

Syntax ()- Symbolic representation- Rules for how a programming languagesstatements must be constructed

Semantics ()- Meaning

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How to fold a bird?????? ?

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Syntax and semantics of folding paper

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Structure of a Program

Before writing a program in any language: Know how the program should work Know the language syntax

Formal definition of how statements must be constructed in the programming language

Learning a programming language is similar to learning a foreign language

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How to start an algorithm or a program?(, !!)

Help describe the method used to solve a problem- Break down each task in the plan into smallersubtasks

- For many tasks, plan a series of logicalsteps to accomplish them

- Provide a logical solution to a problem- Consist of steps to follow to solve the problem- Convert algorithm into programming statements by representing the steps in some format- Pseudocode is often used ()

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Flow chart

Start / end

Process

Decision

Path

Input/output

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Take out all coins in your pocket

Start

Any coin in pocket?

Take one coin from the pocket

End

yes

no

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Contents

Definition of Program and Algorithm Syntax and Semantics Pseudocode Operator

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Pseudocode

flow chart: sometimes too complex Pseudocode: a notational system:

less formal and intuitive ()

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Pseudocode

Readable description of an algorithm written in human language- Template describing what needs converting into programming language syntax

- No formal rules for writing pseudocode- Information should explain the process to someone with little experience in solving this typeof problem

- Practice provides necessary skill

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Pseudocode: temperature conversion chart

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Pseudocode Start with the formulas needed in the algorithm:

- Fahrenheit to Celsius: Celsius temp = (5/9) * (Fahrenheit temp 32)- Celsius to Fahrenheit: Fahrenheit temp = ((9/5) * Celsius temp) + 32

After formulas are proved correct, begin outlining steps to write a program - Input from the user- Calculates the conversions- Displays results to the user

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Pseudocode

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Pseudocode

Assignmentname expression

Conditional selection- if condition then action

- if condition then action1 else action2

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Contents

Definition of Program and Algorithm Syntax and Semantics Pseudocode Operator

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Syntax of a Programming Language

After defining an algorithm and testing the logic thoroughly, begin translating the algorithm

May have many different ingredients:- Variables- Operators- Control structures- Objects

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Variables ()

A name used to identify a certain location and value in the computers memory- Program type determines variable types needed- When a variable is defined, the data type is specified

Advantages: - Access memory locations content

- Use its value in a program- Easy way to access computer memory

- No need to know actual hardware address Identifier: name of a variable

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Identifiers and Naming Conventions

Identifier: access memory contents associated with a variable

Items to consider when deciding on an identifier:- Name should describe data being stored

- Use variable-naming standards- Can use more than one word for a variables identifier:eg.: Sun standard

- Use meaningful names: XX? YY?

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Operators

Symbols used to indicate data-manipulation operations- Manipulate data stored in variables- Classified by data type- One may work on numbers, and another oncharacters (depending on definition)

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Standard mathematical operators(may depend on Programming Language)

Math Operators

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Relational Operators Main purpose is to compare values

Standard relational operators

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== = ?

Logical Operators

Main function is to build a truth table when comparing expressions- Expression: programming statement returning a value when its executed

- Usually use relational operators to compare variables

Standard logical operators32

Precedence and Operators

Precedence (,): order in which something is executed

Symbols with a higher precedence executed before those with a lower precedence- Have a level of hierarchy

Example: 2 + 3 * 4- Output = 14 (not 20)

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Order of relational and mathematical precedence

Precedence and Operators

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Module form

if (not raining)then ( if (temperature = hot)

then (go swimming)else (play golf)

)else (watch television)

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Module form

Repeated executionwhile condition do activity

Procedureprocedure name (generic names)

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Repeat 3 times

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Iterative Structures

Pretest loop:while (condition) do

(loop body)

Posttest loop:repeat (loop body)

until(condition)

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loop structure: while (pretest)

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loop structure: repeat (posttest)

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Repetition number: unknown

Loop structure: while- unknown repetition number

eg. While (the pH level is greater than 4) do

(add a drop of sulfuric acid)

We do not know how many drops are needed.

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Better algorithm or program

Correctness Efficiency

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