It is with great sadness that we report the passing of Christopher Wallace, arguably the greatest and most influential thinker in Computer Science in Australia.

The text below presents the highlights of Chris' distinguished academic career, as he described them prior to his death. This text illustrates Chris' modesty and self-effacing nature. However, what it fails to convey is Chris' brilliance and his impact on the people who knew him.

Chris was an inspiration to those who had the privilege of knowing him and working with him. At the professional level, he had a towering intellect, encyclopaedic knowledge and uncompromising commitment to research. At the personal level, Chris' ethics and his belief in altruism and the goodness of people permeated his interactions with his colleagues and work associates. As a teacher, he mentored many staff and students, most of which are nowadays at the head of academia and industry both in Australia and overseas.

Chris was a remarkable man. His knowledge and insight were impressive, and despite his modest claims to the contrary, spanned many fields of endeavour. We could ask Chris questions about anything, and he invariably would give an authoritative answer. Despite this, Chris thought that there was nothing special about him, and was often rather surprised that the rest of us could not figure out the things he could.

Chris inspired loyalty and affection from all who knew him. Just seeing him come to work, even after he retired, gave us the feeling that all is well with the world. His passing leaves a large void in the field of Computer Science both in Australia and abroad, but most of all, for those of us who had the privilege of knowing him.

Career details

One of the second generation of computer scientists, Wallace was a pioneer in both hardware design and the use of computers for data analysis, as well as being an exceptionally dedicated teacher and postgraduate supervisor.

He entered computer science from nuclear physics having developed, for his postgraduate degree, a complex computer program for analysis of cosmic ray events on SILLIAC, Sydney University’s first computer, one of the first three computers to be built in Australia.

Appointed lecturer in Physics in Sydney in 1960, he was sent almost immediately to the University of Illinois to copy the original design of ILLIAC II, a duplicate of which was to be built in Sydney. ILLIAC II was not in fact completed at that stage and, after an initial less than warm welcome by a department who seemed unsure exactly what this Australian was doing in their midst, or even where Australia was, his talents were recognized and he was invited to join their staff, under very generous conditions, to assist in ILLIAC II design. He remained there for two years, helping in particular to design the input output channels and aspects of the advanced control unit. In the event, Sydney decided it would be too expensive to build a copy of ILLIAC II, although a successful copy (the Golem) was built in Israel using circuit designs that he developed in partnership with another scientist. In spite of considerable financial and academic inducements to remain in America, Wallace’s heart remained firmly in Australia and he returned to the School of Physics at Sydney University as lecturer in the Basser Computing Laboratory under Professor Bennett. Here he assisted with the installation and amendment of the KDF9 Computer, purchased, instead of ILLIAC II, to replace SILLIAC. With Brian Rousewell he redesigned and rebuilt the Direct Memory Access subsection of the KDF9, increasing its peak channel performance while halving the hardware, and they also designed and constructed a high speed data link between the KDF9 and a Control Data Computer. As a teacher, he developed the hardware component of the undergraduate course in Computing and, in 1967, of the first Honours level Computing Course in Australia.

It was also during this period that he published the suggested design for a fast multiplier/divide unit, now known as the Wallace Tree. This design eventually formed the basis of multiply units in most modern computers.

In 1968, with David Boulton, he developed the innovative theory and software for which, among academic circles, he is most widely known: the ‘SNOB’ program for unsupervised classification. This was the first application of Minimum Message Length (MML) Inference, loosely based on a theory first put forward by a mediaeval monk, William of Occam, commonly known as Occam’s Razor: ‘Entia non sunt multiplicanda praeter necessitatem’.

That same year, at the age of 35, Wallace was appointed to the Foundation Chair of Computer Science, initially called Information Science, at Monash University. The following years were not easy. Computer Science in the early years was (and some would say still is) widely misunderstood in the Science Faculties as well as in the Humanities. The struggle to establish it as an academic discipline rather than simply a ‘trade skill’ consumed much of his energy for many years in this post, and depleted his health. Nevertheless he was fortunate in attracting a small but outstanding staff and his department for some years was producing about half the PhD computing graduates in Australia, a significant number of whom he actively supervised himself. As well as much important research using MML, important advances in the development of secure operating systems were made under his supervision.

Although the fundamental theory of MML was first published in 1975 in the computing literature, exposure in the statistical literature was delayed by referees who, to quote one, found the idea ‘repugnant’, presumably because it was such a radical departure from standard statistical method. Finally, with the help of a friendly English academic statistician, Peter Freeman, a paper in ‘proper’ statistical jargon was published in 1987. Subsequent progress on MML led to many successful applications in machine learning, model selection and estimation, and this progress continues. Although the software is now widely used, Wallace was always concerned that the theory was not generally understood. Fortunately, within days of his death he was able to put the finishing touches to his book on the theory and application of MML, which will be published posthumously in America.

Like many scientists Wallace was a man of few words outside his scientific interests, but he will be remembered with affection by all who knew him well for his generosity of spirit and total dedication to his work, his students and colleagues, and his country.