Last updated: Thursday 28th November 2013, 13:37 PT, AD

 

annedawson.net

 

 

 

Part 1

 

Introduction to Computer Science,

Programming, and Python

 

1.1 Computer Systems

1.2 Programming and Problem Solving

1.3 Introduction to Python

1.4 Testing and Debugging

 

Computer Systems = Hardware + Software

 

 

Hardware, e.g.:

 

PC

Workstation

Mainframe

Network

 

 

Hardware - Main Components

input devices (mouse, keyboard)

output devices (screen, printer)

CPU (microprocessor - the brain)

main memory

secondary memory

 

Main Memory (RAM)

*    consists of a long list of memory locations

*    each memory location has an address

*    each memory location contains 0s and 1s

*    memory holds chunks of 8 bits (bytes)

*    a single character, eg A is held in one byte

*    a large number may be held in 2 or 4 bytes

 

Main memory is used when a program is running

 

ASCII Code
Capital letter A has an ASCII code of 65

which in binary looks like: 01000001

 

ASCII codes

 

Binary numbering system

based on 2 digits, 0 and 1

a binary digit is known as a bit

a group of 8 bits is a byte

half of a byte is known as a nibble

 

Numbering Systems

 

Secondary Memory

 

Secondary memory (also known as secondary storage)

is used for permanent storage of data.

 

Examples of secondary storage:

hard disks, floppies, tapes, CDROMs, USB flash drives

 

Software

 

Operating system

You communicate to a computer via its operating system.

Examples of operating systems:

Windows XP / Windows 2000 / Unix / Linux

 

Operating System

Any operating system,

whether it is on a mainframe computer,

a PC or a Mac machine, is a complicated computer program.

 

What is a computer program? ...

 

A computer program is

a set of instructions for the computer to follow.

 

A program usually has

data input,

and the data is processed in some way by the program.

 

A program in operation is said to be running or executing.

 

A computer program is software.

 

Computer Programming Languages

 

High level languages:

VB, C, C++, Java, PHP, Python, C#

 

Intermediate level language:

Assembly

 

Low level language:

machine code i.e. binary: 01111001

 

Software to generate executable programs:

 

Compilers + Interpreters

 

 

A Compiler

- is a computer program which converts

a high level language program (source code)

to low level language (binary code)

which can then be executed (run).

 

 

An Interpreter

- is a computer program that translates

and executes source language statements

one line at a time.

 

 

Problem Solving and Computer Programming

 

Algorithms

Program Design

Software Lifecycle

 

Algorithms

An algorithm is a sequence of precise instructions

(written in English) that leads to the solution to a problem.

 

An algorithm can be compared to a recipe to bake a cake,

a set of directions to enroll for a class

or a set of instructions to install an operating system...

 

Example Algorithm:


Problem: Find the average of three of numbers

 

1. get the first number

2. get the second number

3. get the third number

4. add the three numbers together to calculate the total

5. divide the total by 3 to calculate the average

 

The following link contains more information on algorithms:

 

http://www.annedawson.net/algorithms.html

 

 

 

Computer Science

 

Computer science can be defined as the study of algorithms.

 

But not only the study of algorithms! - read these comments by the inventor of C++ (screen 11).

 

AD had the honour and privilege to meet with Dr Stroustrup at

the Western Canadian Conference on Computing Education

held at SFU, in May 2009.

 

An algorithm is a set of precise instructions to solve a problem.

 

An algorithm is converted into pseudocode

which is then converted into a computer program.

 

Program Design

 

problem solving phase

(results in an algorithm)

 

implementation phase

(results in the program developed from the algorithm)

 

Software Lifecycle

 

1. Problem definition

2. Algorithm design

3. Coding

4. Testing

5. Maintenance

 

Table 1 Relative costs of software systems

 

System Type                           Phase costs (%)

________________________________________

                                     Design         Coding         Testing

Control system             46              20              34

Spaceborne system       34              20              46

Operating system          33              17              50

Scientific system          44              26              30

Business system           44               28               28

 

 

Data from Table taken from:

Boehm,B.W. (1981) Software Engineering Economics, Prentice Hall

 

Note!

 

According to the data in Table 1,

when you're developing a computer program,

you should be spending only one quarter of the time

on actually typing in the code!

 

Most of the time should be spent on design and testing.

 

 

Design and Testing

 

The Design and Testing stages

in the development of any computer program

are the most expensive stages.

 

Tools for Software Design

 

Flowcharts

Pseudocode

Hierarchy Charts

Documentation

 

 

Flowcharts

 

 


 

 

 


Pseudocode

 

Pseudocode and algorithm example (for reference only)

 

 

Hierarchy Charts

 

 


 

 


Any paperwork associated with a computer program

is know as

external documentation.

 

Internal documentation are

any comments you make in the program

code itself...

 

Documentation:

 

Requirements Phase (what the program has to do)

Design Phase (how it will do it)

Coding Phase (the code itself)

Testing Phase (the test data and results)

Operation Phase (the user manual)

 

 

A Brief History of Python:

*    invented by Guido van Rossum in 1990

*    based on the language ABC

*    named after the BBC show "Monty Python's Flying Circus"

 


 

 

 


Python is:

*    free

*    easy to use and learn

*    interpreted

*    object oriented

*    small

*    portable

*    extensible

 

 

 

 

Some short Python (version 3.0) example programs:

www.annedawson.net/Python3Programs.txt

 

Some short Python (version 2.x) example programs:

www.annedawson.net/PythonPrograms.txt

 

 

 

Testing and Debugging

 

An error in a program is called a bug.

 

The process of removing errors is

known as debugging.

 

Types of Errors

 

Syntax error (e.g. incorrect spelling of a keyword)

 

Run-time error (e.g. trying to divide by zero)

 

Logic (semantic) error (e.g. multiplying when you really meant to divide)

 

Reference (Appendix A - "Debugging", page 197, ThinkPython (pdf)

 

 

 

Using Python

http://www.annedawson.net/Python_Editor_IDLE.htm

 

 

 

This Presentation uses the following program files:

 

Python version 2.x

www.annedawson.net/PythonPrograms.txt

 

 

 

Python version 3.0

www.annedawson.net/Python3Programs.txt