GNVQ Advanced IT (Languages)

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Unlike low-level languages, high-level languages are relatively easy to learn because the instructions bear a close resemblance to everyday language, and because the programmer does not require a detailed knowledge of the internal workings of the computer. Each instruction in a high-level language is equivalent to several machine-code instructions. High-level programs are therefore more compact than equivalent low-level programs. However, each high-level instruction must be translated into machine code - by either a compiler or an interpreter program - before it can be executed by a computer. High-level languages are designed to be portable - programs written in a high-level language can be run on any computer that has a compiler or interpreter for that particular language.

Features of High Level Languages
Most high level languages have similar features.

• Reserved words
• Identifiers
• Data structures
• Operations on data
• Input/Output operations
• Control structures
• File handling
• Functions and procedures.

Identifiers are names created by the programmer.  These are such things as names given to variables, constants, sub programs etc.  These are given the group name of Identifiers.

Data Structures
The most common of the data types are:

• Integer  - whole numbers
• Real -  numbers that might have a fractional part
• Character -  any character on the keyboard
• String -  a number of characters connected together
• Boolean -  Value representing a logical state of TRUE or FALSE
• Array -  set of data items of a specific type, individual elements are addressable
• Record -  a group of variables of different types
• File -  collection of identically structured records

• Arithmetic - add, subtract, multiplty, divide and etc.
• Logical -  such as AND, OR, NOT and etc.

Control structures
The normal top to bottom sequential program operation can be altered in two main ways:

Iteration - eg. FOR..........NEXT, REPEAT........UNTIL, DO..........WHILE  etc.allowing instructions to be repeated.

Selection - eg  IF........THEN.........ELSE, CASE allowing alternative courses of action to be taken dependant upon a condition existing.

File handling
Most high level languages allow file handling and incorporate instructions such as:

• opening files
• closing files
• reading records from a sequential file
• writing records to a sequential file
• reading records from a random file
• writing records to a random file

Variables can be declared in the main program to work globally throughout the programme (including the sub programmes).  When a variable is declared in a sub programme the variable is described as a local variable

Functions and procedures

Languages such as Pascal and C allow the programmer to create subprograms (subroutines or modules) These sub programmes are given a name and can be called when required by the program code.  Subprograms are also called procedures or functions.  Each leaves the main program to carry out a sub routine and return to the point in program that they were called from to carry on with the sequential program. There is a slight difference with a function returning a value to the calling program.

Most versions make use of an interpreter, which translates BASIC into machine code and allows programs to be entered and run with no intermediate translation. Some more recent versions of BASIC allow a compiler to be used for this process.

Because each instruction must be translated each time the source program is run, interpreted programs run far more slowly than do compiled programs. However, unlike compiled programs, they can be executed immediately without waiting for an intermediate compilation stage.

Different compilers are needed for different high-level languages and for different computers. In contrast to using an interpreter, using a compiler adds slightly to the time needed to develop a new program because the machine-code program must be recompiled after each change or correction. Once compiled, however, the machine-code program will run much faster than an interpreted program.

Programs in high-level languages are translated into machine code by either a compiler or an interpreter program. Low-level programs are translated into machine code by means of an assembler program. The program, before translation, is called the source program; after translation into machine code it is called the object program.

A mnemonic-based low-level language replaces binary machine-code instructions, which are very hard to remember, write down, or correct, with short codes chosen to remind the programmer of the instructions they represent. For example, the binary-code instruction that means ‘store the contents of the accumulator’ may be represented with the mnemonic STA.

In contrast, high-level languages are designed to solve particular problems and are therefore described as problem-oriented languages.

There are several ways of writing programs:

procedural programming, in which programs are written as lists of instructions for the computer to obey in sequence, is by far the most popular. It is the ‘natural’ style, closely matching the computer's own sequential operation; declarative programming, as used in the programming language PROLOG, does not describe how to solve a problem, but rather describes the logical structure of the problem. Running such a program is more like proving an assertion than following a procedure; functional programming is a style based largely on the definition of functions. There are very few functional programming languages, HOPE and ML being the most widely used, though many more conventional languages (for example C) make extensive use of functions; object-oriented programming, the most recently developed style, involves viewing a program as a collection of objects that behave in certain ways when they are passed certain ‘messages’. For example, an object might be defined to represent a table of figures, which will be displayed on screen when a ‘display’ message is received.

• productivity this covers such software as spreadsheets, word processor, database, e-mail, graphics, desktop publishing and scheduling software
• business this can be sub categorised into horizontal market software, such as accounting and payroll applications which can be used by a range of business organisations; or, vertical market software such as costing/estimating software designed for specific industries.
• education software designed to train individuals under the control of a tutor or self teaching packages.  CBT (computer based training) and tutorials are examples in this area.
• reference electronic encyclopaedias
• entertainment

There are three main types (or ‘models’): hierarchical, network, and relational, of which relational is the most widely used. A free-text database is one that holds the unstructured text of articles or books in a form that permits rapid searching.

A collection of databases is known as a databank. A database-management system (DBMS) program ensures that the integrity of the data is maintained by controlling the degree of access of the applications programs using the data. Databases are normally used by large organizations with mainframes or minicomputers.

A telephone directory stored as a database might allow all the people whose names start with the letter B to be selected by one program, and all those living in Chicago by another.

1GL/2GL/3GL/4GL/5GL
             (first, second, third, fourth, and fifth generation programming languages)

In the computer industry, these abbreviations are widely used to represent major steps or "generations" in the evolution of programming languages.

First-generation languages (1GL) were machine code (strings of 0s and 1s).

Second-generation language (2GL) is assembly language. A typical 2GL  instruction may be to add the contents of two addresses, looking something  like this:

                 ADD    FF, F8

An assembler then converts the statements into machine language.

Third-generation language (3GL) is a "high-level" programming languages, such as Pascal, C, or Java. Java
language statements look like this do while statement:

import javabook.chapter4.dice;
public class GameStarter
{
        public static void main (String args [ ] )
        {
                Dice a_dice = new Dice();
                int dice_value = 0;
                do
                {
                        a_dice.throwDice();
                        dice_value = a_dice.getDiceValue();

A compiler converts the statements of a specific high-level programming language into machine language.  (In the case of Java, the output is called bytecode, which is converted into appropriate machine language  by a Java virtual machine that runs as part of an operating system platform.) A 3GL language requires a considerable amount of programming knowledge.

Fourth-generation language (4GL) is designed to be closer to natural language than a 3GL language.  The programmer can specify a problem in general terms and leave the computer to provide a solution.  MS Access application software database allows a non expert to query a database.  The programme develops the required query code.

4GL In computing, is a type of programming language designed for the rapid programming of applications but  often lacking the ability to control the individual parts of the computer. Such a language typically provides easy ways of designing screens and reports, and of using databases. Other ‘generations’ (the term implies a class of language rather than a chronological sequence) are machine code (first generation); assembly languages, or low-level languages (second); and conventional high-level languages such as BASIC and PASCAL (third).

Fifth-generation language (5GL) is programming that uses a visual or graphical development interface to create source language that is usually compiled with a 3GL or 4GL language compiler. Microsoft, Borland, IBM, and other companies make 5GL visual programming products for developing applications in Java, for example. Visual programming allows you to easily envision object-oriented class hierarchies and drag icons to assemble program components. Microbrew AppWare and IBM's VisualAge for Java are examples of 5GL "languages."

Writing instructions in a programming language for the control of a computer. Applications programming is for end-user programs, such as accounts programs or word processing packages. Systems programming is for operating systems and the like, which are concerned more with the internal workings of the computer.

There are several programming styles:
procedural programming, in which programs are written as lists of instructions for the computer to obey in sequence, is by far the most popular. It is the ‘natural’ style, closely matching the computer's own sequential operation; declarative programming, as used in the programming language PROLOG, does not describe how to solve a problem, but rather describes the logical structure of the problem. Running such a program is more like proving an assertion than following a procedure; functional programming is a style based largely on the definition of functions. There are very few functional programming languages, HOPE and ML being the most widely used, though many more conventional languages (for example C) make extensive use of functions; object-oriented programming, the most recently developed style, involves viewing a program as a collection of objects that behave in certain ways when they are passed certain ‘messages’. For example, an object might be defined to represent a table of figures, which will be displayed on screen when a ‘display’ message is received