Introduction

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The power of a computer is its versatility: It can be programmed, i.e. its basic commands can be combined to do very different tasks.

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Typical basic commands:

data movement

``get data from memory cell 4228 into register r3''

computation

``add content of registers r1 and r2 and store the sum in r3''

program flow

``get the next command from memory cell 2001, if register r1 contains 0''

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Program: a sequence of commands that can be executed by a computer.

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The set of basic commands that a computer understands, forms a special programming language, the machine language. It is different for different kinds of computers.

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Programming in machine language requires a thorough understanding of the internal structure of the used computer (registers, built-in arithmetic and logical units etc.). It is hard to write such a program to solve a real problem.

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``Real'' problems could be

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Compute the approximate value of an integral of a special function between given bounds.

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Interpret the given human sentence to find out what kind of request is made at the hotel reservation system.

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Find all the persons in a large set of banking account data, whose account is overdrawn by a certain amount.

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To make the task of programming computers simpler, many programming languages have been invented that use higher abstractions from the particular application area.

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Some specialized programming languages:

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Mathematical abstractions are the realm of Fortran. It understands a mathematical formula like

$$x1 = -p/2 + \sqrt{{\frac{p}{2}}^2 -q}$$

in the following simple form:

x1 = -p/2 + sqrt(p**2/4 - q)

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Logical dependencies and relations are the basic commands of Prolog. It allows to formulate things like

Sentence =
(Subject phrase + Verb phrase) OR
(Subject phrase + Verb phrase + Object Phrase)

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Special database languages like SQL allow direct queries in the data, e.g.

Show all NAME fields
in data set ACCOUNTS
where (AMOUNT < -100 Euro)

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Programms written in such languages have to be translated into machine language, before they can be executed on a computer. This - rather tedious - task can itself be delegated to a computer program.

Compiler: a program that reads a program text as input and translates it into machine language.

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A class of very popular programming languages are the object oriented languages. They allow to construct programs from smaller building blocks (the objects), which contain a certain amount of information and have a given behaviour.

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An object consists of fields that contain the object's data, and of methods, which describe its possible actions.

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An operating system could organize the data on a disk using objects like

    File
      fields:  size, name, data
      methods: getSize, appendData(newData)

    Disk
      fields:  size, listOfFiles
      methods: createNewFile(name), getFreeSpace
  

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A class is the description of all objects that have the same kind of data fields and the same methods.

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The objects file1 and file2 belong to the common class File.