TURlNGAWARD LECTURE COMPUTER SCIENCE: THE EMERGENCE OF A DISCIPLINE The clontinued rapid development of computer science will require an expansion of the science base and an influx of talented new researchers. Computers have already altered the way we think and live; now they will begin to elevate our knowledge of the world. JOHN E. HOPCROFI’ It is a great honor to be a recipient of the 1986 longer adequate to meet the challenges created by Turing Award. Along with the recognition provided the expansion of knowledge. The demands of indus- by the award comes the opportunity to present this trial research laboratories and academic institutions lecture, and so to speak not only to computer scien- far surpass the current resources for producing the tists, but also to policy makers and to other members needed pool of talented researchers. To reap the of the scientific establishment. I would like to start maximum benefit from the scientific and technologi- by relating some of my experiences in the field of cal advances of computing, a national commitment computer science, and then to make some recom- must be made and sustained. mendations about the future development of the dis- Before expanding on this need, let me first relate cipline. some of the events in my career that have brought I began my professional career in computer sci- me to this position. When I received my Ph.D. in ence in 1964, when computer science was just be- electrical engineering from Stanford University, my ginning to establish itself as an academic discipline. education had included only one course in comput- Having been a pa.rt of the academic community dur- ing, which was taught by David Huffman. My last ing the formative years of computer science, I have year at Stanford coincided with the year Huffman been in a fortunate position to observe it as it spent there, and it was from him that I received a evolved and to watch it as :it matured and devel- basic introduction to switching circuits, logic design, oped. I have a great fondness for the field, and I and the theory of computation. During the spring of would like to see it continue to flourish and grow. that same year, Edward McCluskey was recruiting Even though computer science has emerged as a ma- faculty for his Digital Systems Laboratory at Prince- ture discipline, s.trong leadership and direction ton. By chance, he happened to be telephoning within the profession are still of great importance in Bernard Widrow at Stanford to discuss prospective order for it to contribute fully to science and society. Ph.D. candidates just as I was walking by Widrow’s What has impressed me most during my years in door. When Widrow saw me, he motioned me into computer science is the level of commitment of the his office and handed me the telephone, and we participants. In the early years, I saw strong individ- arranged that I would visit Princeton. The fact that I ual commitment. Later, I saw institutions joining had no formal education in computer science, ex- forces with individuals to form strong support sys- cept for Huffman’s course, did not deter McCluskey. tems for computer science. Today, technology is ad- At that time, few people had an educational back- vancing the discipline so rapidly that the combined ground in computing. After my visit, I was suffi- commitment of individuals and institutions is no ciently impressed by McCluskey’s commitment to 01987 ACM OOOl-0782,'87/0300-0198 750 computer science and with the opportunity he pre- 199 Communications of the ACM March 1987 Volume 30 Number 3 Turing Award Lecture sented me to begin a career in the developing sci- Rabin and Scott were mathematicians who devel- ence of computing that I accepted his job offer as oped a model of a computer with a finite amount of soon as it was extended. This decision significantly memory. They called this model the finite-state au- altered my career plans, for prior to McCluskey’s tomaton, and showed that the possible behaviors of telephone call, I had planned to teach electrical en- finite-state automata were precisely those behaviors gineering on the West Coast. that could be described by the regular expressions My arrival at Princeton in the fall of 1964 oc- that grew out of the work of McCulloch and Pitts. curred at a time when a dramatic change was taking This confluence of ideas from two widely different place in the computing field. Much of the course disciplines helped convince early computer scien- content in computer science had focused on the de- tists of the importance of regular expressions and sign of circuits for digital computers and minimizing finite automata. the number of transistors needed to build these cir- Chomsky, a linguist, had been studying the syntax cuits. By the mid sixties, however, technology had of natural languages. In the course of his work, he advanced to the point where transistors were about developed the concept of a context-free grammar. At to be replaced by computer chips with as many as a about the same time, two computer scientists, hundred components per chip. Thus, minimizing the Backus and Naur, were attempting to develop for- number of transistors was no longer relevant. As you malisms for describing programming languages. Be- can imagine, this had profound ramifications for fore 1960 programming languages were defined by what was then called computer science; existing lengthy and often incomplete verbal descriptions. In- courses were about to become obsolete, and new consistencies in various implementations of a lan- ones had to be developed. guage often made it difficult to change software be- Princeton asked me to develop a course in auto- tween systems. Backus and Naur developed a formal mata theory to expand the scope of the curriculum notation for describing the syntax of various pro- beyond the digital circuit design course then being gramming languages. Amazingly enough, their nota- offered. Since there were no courses or books on the tion was equivalent to the context-free grammars subject, I asked McCluskey to recommend some ma- developed by Chomsky. terials for a course on automata theory. He was not Turing had in 1936 introduced a simple model sure himself, but he gave me a list of six papers and of a computing device, which is now known as the told me that the material would probably give stu- Turing machine. This device was simple enough dents a good background in automata theory. Mc- that there could be no question that any function Cluskey’s list included works by Warren McCulloch computed by it was computable. However, Turing and Walter Pitts, John Backus and Peter Naur, Noam argued further that his model could compute every Chomsky, Michael Rabin and Dana Scott, Juris Hart- function considered computable. Simply put, any manis and Richard Stearns, and, of course, Alan computational process that could be carried out Turing. could be programmed on the Turing machine. Today At the time, I thought it strange that individuals this hypothesis is universally accepted, and the were prepared to introduce courses into the curricu- Turing machine is the foundation of modern com- lum without clearly understanding their content. In putability theory. retrospect, I realize that people who believe in the Turing’s work might have remained in the realm future of a subject and who sense its importance will of mathematics and logic were it not for a seminal invest in the subject long before they can delineate paper on the computational complexity of algo- its boundaries. rithms by mathematicians Hartmanis and Stearns. When I look back on the material in that early They measured the complexity of an algorithm by course in automata theory, I am struck by the diver- the number of steps needed for its execution and sity of these sources. In 1943 McCulloch and Pitts, used this method to develop a theory of complexity working in neurophysiology, published a paper on a classes. This paper sparked the imagination of many logical calculus for describing events in neuron nets. computer scientists and led to the establishment These events were series of electrical pulses and of complexity theory as an integral part of the could be viewed as strings of zeros and ones. The discipline. paper had a notation for describing how these strings Certain research papers are important not only for of zeros and ones combine in neurons to produce their technical contributions, but, more importantly, new strings of zeros and ones. This notation was because they provide a conceptual view or establish subsequently developed into the language of regular a paradigm for research. The work of Hartmanis and expressions for describing sets of strings. Stearns attracted researchers and focused attention March 1987 Volume 30 Number 3 Communications of the ACM 199 Turing Award Lecture on the topic of complexity. Among the more signifi- data structures. I believed that the methodology of cant advances that resulted were the classification of theoretical computer science could be used to de- the complexity of most major mathematical theories, velop a science base for algorithmic design that the reducibility of many combinatorial problems, the would be useful to the practitioner. r,oncept of NP-completeness, and a deeper under- During the 196Os, research on algorithms had been standing of concepts such (asrandomness. very unsatisfying. A researcher would publish an And so the early automata theory course served to algorithm in a journal along with execution times emphasize two important aspects of computer sci- for a small set of sample problems, and then several ence: the way it has used a multiplicity of ideas from years later, a second researcher would give an im- diverse fields to develop and expand, and the way proved algorithm along with execution times for the basic research can compound and escalate advances same set of sample problems.