burned with disappointment. I really felt a hot flash, I was so upset. It had to look right, and it didn’t, at that time. I’m happy to say that I open my books now and I like what I see. Even though they don’t match the 1968 book exactly, the way they differ are pleasing to me.

What it was like writing teX.

Structured programming gave me a different feeling from programming the old way—a feeling of confidence that I didn’t have to debug something immediately as I wrote it. Even more important, I didn’t have to mock-up the unwritten parts of the program. I didn’t have to do fast prototyping or something like that, because when you use structured programming methodology you have more confidence that it’s going to be right, that you don’t have to try it out first. In fact, I wrote all of the code for TeX over a period of seven months, before I even typed it into a computer. It wasn’t until March 1978 that I spent three weeks debugging everything I had written up to that time.

I found that writing software was much more difficult than anything else I had done in my life. I had to keep so many things in my head at once. I couldn’t just put it down and start something else. It really took over my life during this period. I used to think there were different kinds of tasks: writing a paper, writing a book, teaching a class, things like that. I could juggle all of those simultaneously. But software was an order of magnitude harder. I couldn’t do that and still teach a good Stanford class. The other parts of my life were largely on hold, including The Art of Computer Programming. My life was pretty much typography.

 

teX leads to a new way of programming. Literate programming, in my mind, was the greatest spin-off of the TeX project. I learned a new way to program. I love programming, but I really love literate programming. The idea of literate programming is that I’m writing a program for a human being to read rather than a computer to read. It’s still a program and it’s still doing the stuff, but I’m a teacher to a person. I’m addressing my program to a thinking being, but I’m also being exact enough

so that a computer can understand it as well. Now I can’t imagine trying to write a program any other way.

As I’m writing The Art of Computer Programming, I realized the key to good exposition is to say everything twice: informally and formally. The reader gets to lodge it in his brain in two different ways, and they reinforce each other. In writing a computer program, it’s also natural to say everything in the program twice. You say it in English, what the goals of this part of the program are, but then you say it in your computer language. You alternate between the informal and the formal. Literate programming enforces this idea.

In the comments you also explain what doesn’t work, or any subtleties. You can say, “Now note the following. Here is the tricky part in line 5, and it works because of this.” You can explain all of the things that a maintainer needs to know. All this goes in as part of the literate program, and makes the program easier to debug, easier to maintain, and better in quality.

 

after teX, Don gets to go back to mathematics. We finished the TeX project; the climax was in 1986. After a sabbatical in Boston I came back to Stanford and plunged into what I consider my main life’s work: analysis of algorithms. That’s a very mathematical thing, and so instead of having font design visitors to my project, I had great algorithmic analysts visiting my project. I started working on some powerful mathematical approaches to analysis of algorithms that were unheard of in the 1960s when I started the field. Here

“at age 55 i
became ‘Professor
emeritus of the
art of computer
Programming,’
with a capital ‘t.’
i love that title.”

I am in math mode, and thriving on the beauties of this subject.

One of the problems out there that was fascinating is the study of random graphs. Graphs are one of the main focuses of Volume 4, all the combinatorial algorithms, because they’re ubiquitous in applications.

frustrated with the rate of
progress, he “retires” to devote
himself to “the art.”

I wasn’t really as happy as I let on. I mean, I was certainly enjoying the research I was doing, but I wasn’t making any progress at all on Volume 4. I’m doing this work on random graphs, and I’m learning all of these things. But at the end of the year, how much more had been done? I’ve still got 11 feet of preprints stacked up in my closet that I haven’t touched, because I had to put that all on hold for the TeX project. I figured the thing that I’m going to be able to do best for the world is finishing The Art of Computer Programming.

The only way to do it was to stop being a professor full time. I really had to be a writer full time. So, at age 55 I became “Professor Emeritus of The Art of Computer Programming,” with a capital “T.” I love that title.

Don is a master at straddling
the path between engineering
and science.

I always thought that the best way to sum up my professional work is that it has been an almost equal mix of theory and practice. The theory I do gives me the vocabulary and the ways to do practical things that can make giant steps instead of small steps when I’m doing a practical problem. The practice I do makes me able to consider better and more robust theories, theories that are richer than if they’re just purely inspired by other theories. There’s this symbiotic relationship between those things. At least four times in my life when I was asked to give a kind of philosophical talk about the way I look at my professional work, the title was “Theory and Practice.” My main message to the theorists is, “Your life is only half there unless you also get nurtured by practical work.”

Software is hard. My experience with TeX taught me to have much more admiration for colleagues that are devot-