Edsger Dijkstra

Program testing can be used to show the presence of bugs, but never to show their absence!

Please don't fall into the trap of believing that I am terribly dogmatical about [the goto statement]. I have the uncomfortable feeling that others are making a religion out of it, as if the conceptual problems of programming could be solved by a single trick, by a simple form of coding discipline!

If we wish to count lines of code, we should not regard them as "lines produced" but as "lines spent".

How do we convince people that in programming simplicity and clarity—in short: what mathematicians call "elegance"—are not a dispensable luxury, but a crucial matter that decides between success and failure?

As long as there were no machines, programming was no problem at all; when we had a few weak computers, programming became a mild problem, and now [1972] that we have gigantic computers, programming has become a gigantic problem. […] As the power of available machines grew by a factor of more than a thousand, society's ambition to apply these new machines grew in proportion, and it was the poor programmer who found his job in this exploded field of tension between the ends and the means. The increased power of the hardware, together with the perhaps more dramatic increase in its reliability, made solutions feasible that the programmer had not dared to dream about a few years before. And now, a few years later, he had to dream about them and even worse, he had to transform such dreams into reality! It is no wonder that we found ourselves in a software crisis

Besides a mathematical inclination, an exceptionally good mastery of one's native tongue is the most vital asset of a competent programmer.

Computer Science is no more about computers than astronomy is about telescopes.

John von Neumann draws attention to what seemed to him a contrast. He remarked that for simple mechanisms, it is often easier to describe how they work than what they do, while for more complicated mechanisms, it is usually the other way around.

...Simplifications have had a much greater long-range scientific impact than individual feats of ingenuity. The opportunity for simplification is very encouraging, because in all examples that come to mind the simple and elegant systems tend to be easier and faster to design and get right, more efficient in execution, and much more reliable than the more contrived contraptions that have to be debugged into some degree of acceptability....Simplicity and elegance are unpopular because they require hard work and discipline to achieve and education to be appreciated.

The tools we use have a profound (and devious!) influence on our thinking habits, and, therefore, on our thinking abilities.

The question of whether computers can think is like the question of whether submarines can swim.

Do only what only you can do.

In their capacity as a tool, computers will be but a ripple on the surface of our culture. In their capacity as intellectual challenge, they are without precedent in the cultural history of mankind.

Always design your program as a member of a whole family of programs, including those that are likely to succeed it.

A programming language is a tool that has a profound influence on our thinking habits.

I pray daily that more of my fellow programmers may find the means of freeing themselves from the curse of compatibility.

The program and the correctness proof grow hand in hand.

The competent programmer is fully aware of the strictly limited size of his own skull; therefore he approaches the programming task in full humility, and among other things he avoids clever tricks like the plague.

Progress is possible only if we train ourselves to think about programs without thinking of them as pieces of executable code.

Software Engineering is Programming when you can’t.

So-called "natural language" is wonderful for the purposes it was created for, such as to be rude in, to tell jokes in, to cheat or to make love in (and Theorists of Literary Criticism can even be content-free in it), but it is hopelessly inadequate when we have to deal unambiguously with situations of great intricacy, situations which unavoidably arise in such activities as legislation, arbitration, mathematics or programming.

I mean, if 10 years from now, when you are doing something quick and dirty, you suddenly visualize that I am looking over your shoulders and say to yourself, "Dijkstra would not have liked this", well that would be enough immortality for me.

For me, the first challenge for computing science is to discover how to maintain order in a finite, but very large, discrete universe that is intricately intertwined. And a second, but not less important challenge is how to mould what you have achieved in solving the first problem, into a teachable discipline: it does not suffice to hone your own intellect (that will join you in your grave), you must teach others how to hone theirs. The more you concentrate on these two challenges, the clearer you will see that they are only two sides of the same coin: teaching yourself is discovering what is teachable.

Object-oriented programming is an exceptionally bad idea that could only have been invented in California.

Projects promoting programming in "natural language" are intrinsically doomed to fail.

Lisp has jokingly been called "the most intelligent way to misuse a computer". I think that description is a great compliment because it transmits the full flavor of liberation: it has assisted a number of our most gifted fellow humans in thinking previously impossible thoughts.

It is practically impossible to teach good programming style to students that have had prior exposure to BASIC: as potential programmers they are mentally mutilated beyond hope of regeneration.

Do only what only you can do.

Elegance is not a dispensable luxury but a factor that decides between success and failure.

If in physics there's something you don't understand, you can always hide behind the uncharted depths of nature. You can always blame God. You didn't make it so complex yourself. But if your program doesn't work, there is no one to hide behind. You cannot hide behind an obstinate nature. If it doesn't work, you've messed up.

Object-oriented programming is an exceptionally bad idea which could only have originated in California.

Programming is one of the most difficult branches of applied mathematics; the poorer mathematicians had better remain pure mathematicians.

Simplicity is prerequisite for reliability.

The competent programmer is fully aware of the limited size of his own skull. He therefore approaches his task with full humility, and avoids clever tricks like the plague.

The required techniques of effective reasoning are pretty formal, but as long as programming is done by people that don't master them, the software crisis will remain with us and will be considered an incurable disease. And you know what incurable diseases do: they invite the quacks and charlatans in, who in this case take the form of Software Engineering gurus.

The use of COBOL cripples the mind; its teaching should, therefore, be regarded as a criminal offence.

There are many different styles of composition. I characterize them always as Mozart versus Beethoven. When Mozart began to write at that time he had the composition ready in his mind. He wrote the manuscript and it was 'aus einem Guss' (casted as one). And it was also written very beautiful. Beethoven was an indecisive and a tinkerer and wrote down before he had the composition ready and plastered parts over to change them. There was a certain place where he plastered over nine times and one did remove that carefully to see what happened and it turned out the last version was the same as the first one.

When there were no computers programming was no problem. When we had a few weak computers, it became a mild problem. Now that we have gigantic computers, programming is a gigantic problem.

Why has elegance found so little following? Elegance has the disadvantage that hard work is needed to achieve it and a good education to appreciate it.

Write a paper promising salvation, make it a 'structured' something or a 'virtual' something, or 'abstract', 'distributed' or 'higher-order' or 'applicative' and you can almost be certain of having started a new cult.

... while we all know that unmastered complexity is at the root of the misery, we do not know what degree of simplicity can be obtained, nor to what extent the intrinsic complexity of the whole design has to show up in the interfaces. We simply do not know yet the limits of disentanglement. We do not know yet whether intrinsic intricacy can be distinguished from accidental intricacy.

If debugging is the process of removing bugs. Then programming must be the process of putting them in.

It is practically impossible to teach good programming style to students that have had prior exposure to Basic; as potential programmers they are mentally mutilated beyond hope of regeneration.

PL/I, "the fatal disease", belongs more to the problem set than to the solution set.

A program is like a poem: you cannot write a poem without writing it.

The art of programming is the art of organizing complexity, of mastering multitude and avoiding its bastard chaos as effectively as possible.

A most important, but also most elusive, aspect of any tool is its influence on the habits of those who train themselves in its use. If the tool is a programming language this influence is, whether we like it or not, an influence on our thinking habits.