AS ComputingAS Computing

Introduction to Introduction to ProgrammingProgramming

What is a Computer Program?

A list of instructions that a computer must work through, in a logical sequence, to carry out a specific task

Computer programs are written in a programming language

Programming Languages

Many different languages and many different versions of same language

Each language has a vocabulary and rules that define arrangement - SYNTAX

Fewer words to

learn than aforeign language

Rules are more

rigid than a foreign language

Generations of Programming LanguagesComputers have been around since 1940s

Early computers programmed by changing switch settings to represent binary data

Bletchley Park

Alan Turing

Difference Engine – Charles Babbage

0010100100100101001001110110101011010100111110110100001010101010110101010101011011011010

First Generation – Machine Code

Processors can only work with Binary Digits

All instructions given to a computer must be in a pattern of zeros and ones

Second Generation – Assembly Language

Use words rather than 0s and 1s

Simple translation of 1 word to 1 machine code instruction

Uses mnemonics:LDA Load AccumulatorSTA Store AccumulatorADD Adds to AccumulatorSUB Subtracts from Accumulator

Low Level Languages;// ADDRESS OPCODE ASM; ----------------------------------$0000 $2A82 GOTO hifi_main$0004 $ delays_delay_1us:$0004 $0000 nop$0005 $0000 nop$0006 $0008 RETURN$0007 $ delays_delay_5500us:$0007 $300F MOVLW 15$0008 $1303 BCF STATUS,RP1$0009 $1283 BCF STATUS,RP0$000A $00F0 MOVWF STACK_0$000B $30FF MOVLW 255$000C $00F1 MOVWF STACK_1$000D $ delays_L_8:$000D $0BF0 DECFSZ STACK_0, F$000E $2810 GOTO delays_L_9$000F $2813 GOTO delays_L_10$0010 $ delays_L_9:$0010 $0BF1 DECFSZ STACK_1,F$0011 $2810 GOTO delays_L_9$0012 $280D GOTO delays_L_8$0013 $ delays_L_10:$0013 $303E MOVLW 62$0014 $00F0 MOVWF STACK_0$0015 $ delays_L_11:$0015 $0BF0 DECFSZ STACK_0, F

Machine Code & Assembly language are:

Processor Processor DependantDependant

Collectively known as Low Level Languages

Third Generation - High Level Languages

Problem oriented languages, easy to understand

if Age >=17 Then Showmessage(‘Can drive a car’)

Based on what the program must do, rather than what the processor and memory do

PORTABLE – programs can be used on different types of computer BASIC JAVA PASCAL C

Many Different High Level Languages

Written to cope with the demands of specific types of problem:

Business Applications

Databases

Web Pages

High Level Language Classifications

Procedural or ImperativeUse a sequence of instructions, executed in a particular order

Object OrientedUse objects and inheritance

DeclarativeProlog, SQLFacts and rules based

Translating High Level Translating High Level LanguagesLanguages

Easy for people to use, but computers can not understand them

The translation from high level code to machine code is a more complex process than for assembly language

There are 2 types of Translator:

INTERPRETER

COMPILER

Programming Constructs

Most high level languages construct their statements in a similar way:

SEQUENCESEQUENCE

ASSIGNMENTASSIGNMENT

SELECTIONSELECTION

ITERATIONITERATION

Assignment

Age = 34Age = 34

Sets the variable Age to the value of 34

Assignment is fundamental to programming

Types of Variables

IntegerLong IntegerReal Number

Number with decimal point

StringAlphanumeric

BooleanYes/No or True/False

Data TypesCurrent Rate of VAT

Real

Todays DateString / Date?

Total takings at a shopReal

If a school has a sixth form or notBooleans

Number of Pupils in a SchoolInteger

Selection - If

Compares values and decides outcome

If Age < 17 Then

Showmessage(‘Under Age to drive’);

Else

Showmessage(‘Old enough to drive’);

End If

Iteration

Often useful to repeat a section of code

Counting the number of letters in a word

Keeping an object moving, until it reaches a barrier

Doing something until a condition is met

Doing something a certain number of times

Doing something while a condition is true

2 Methods of Iteration

Count Controlled Count Controlled

Condition ControlledCondition Controlled

Iteration

Do while x < 10

x = x + 1

MessageBox x

Loop

For x = 1 to 10

Messagebox x

Next x

Count ControlledCount ControlledCount ControlledCount Controlled

Condition ControlledCondition ControlledCondition ControlledCondition Controlled