The very first programmers actually had to enter the binary codes themselves, this is known as machine code programming and is incredibly difficult. The next stage was to create a translator that simply converted English equivalents of the binary codes into binary so that instead of having to remember that the code 001273 05 04 meant add 5 to 4 programmers could now write ADD 5 4. This very simple improvement made life much simpler and these systems of codes were really the first programming languages, one for each type of computer. They were known as assembler languages and Assembler programming is still used for a few specialized programming tasks today.

Even this was very primitive and still told the computer what to do at the hardware level - move bytes from this memory location to that memory location, add this byte to that byte etc. It was still very difficult and took a lot of programming effort to achieve even simple tasks.

Gradually computer scientists developed higher level computer languages to make the job easier. This was just as well because at the same time users were inventing ever more complex jobs for computers to solve! This competition between the computer scientists and the users is still going on and new languages keep on appearing. This makes programming interesting but also makes it important that as a programmer you understand the concepts of programming as well as the pragmatics of doing it in one particular language.

I'll discuss some of those common concepts next, but we will keep coming back to them as we go through the course.

The common features of all programs

A long time ago a man called Edsgar Dijkstra came up with a concept called structured programming. This said that all programs could be structured in the following four ways:

•Sequences of instructions

•Loops

•Branches

•Modules

Along with these structures programs also need a few more features to make them useful:

•Data

•Operations (add, subtract, compare etc)

•Input/Output capability (e.g. to display results)

Once you understand those concepts and how a particular programming language implements them then you can write a program in that language.

Let's clear up some terminology

We already said that programming was the art of making a computer do what you want, but what is a program?

In fact there are two distinct concepts of a program. The first is the one perceived by the user - an executable file that is installed and can be run repeatedly to perform a task. For example users speak of running their Word processor program. The other concept is the program as seen by the programmer, this is the text file of instructions to the computer, written in some programming language, that can be translated into an executable file. So when you talk about a program always be clear about which concept you mean.

Basically a programmer writes a program in a high level language which is interpreted into the bytes that the computer understands. In technical speak the programmer generates source code and the interpreter generates object code. Sometimes object code has other names like: P-Code, binary code or machine code.

The interpreter has a couple of names, one being the interpreter and the other being the compiler. These terms actually refer to two different techniques of generating object code from source code. It used to be the

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