Sir Tony Hoare

Sir Charles Antony Richard Hoare, known to colleagues and generations of students simply as Tony Hoare and to readers of the research literature as C. A. R. Hoare, was one of the founding figures of computer science. He invented Quicksort, one of the most widely used sorting algorithms ever devised. He created Hoare logic, the axiomatic method for proving that programs are correct. He designed Communicating Sequential Processes (CSP), the mathematical theory of concurrency that shaped languages such as occam and inspired Go. He won the ACM Turing Award in 1980 and was knighted in 2000. He died peacefully at home in Cambridge, England, surrounded by his family, on 5 March 2026, aged 92.

Hoare was born in Colombo, in what was then British Ceylon (now Sri Lanka), on 11 January 1934. His father, Henry, was a colonial civil servant and his mother, Marjorie (née Villiers), was the daughter of a tea planter. He was educated in England at the Dragon School in Oxford and the King's School, Canterbury, and then read Classics and Philosophy ("Greats") at Merton College, Oxford, graduating in 1956. His philosophy studies, including tutorials with John Lucas, gave him a lifelong interest in logic and in the foundations of reasoning.

After graduation he did national service in the Royal Navy, where he learned Russian. In 1958 he returned to Oxford for a postgraduate certificate in statistics, and there he first programmed a computer, learning Autocode on the Ferranti Mercury from Leslie Fox. He then went to Moscow State University as a British Council exchange student, working on machine translation under the mathematician Andrey Kolmogorov. While trying to sort words for a Russian to English dictionary in 1959 he conceived Quicksort. He published it in 1961, and it remains a standard algorithm taught to every computer science student.

In 1960 he joined Elliott Brothers, a small London computer manufacturer, where he led the team that implemented a compiler for ALGOL 60 and later rose to chief scientist of the research division. There he met Jill Pym, a member of his team, whom he married in 1962. In 1965, while designing the type system for ALGOL W, he introduced the null reference, a decision he would later call his "billion-dollar mistake" because of the countless crashes and security flaws it caused in software ever since. The difficulties of a large operating system project at Elliott also taught him hard lessons about complexity that he carried for the rest of his career.

In 1968 Hoare became Professor of Computing Science at the Queen's University of Belfast. In 1969 he published "An Axiomatic Basis for Computer Programming", introducing what is now called Hoare logic and the Hoare triple, the foundation of modern program verification. In Belfast he also developed the monitor concept for structuring operating systems and, with Edsger Dijkstra, the dining philosophers problem, and he co-wrote the influential book Structured Programming (1972) with Dijkstra and Ole-Johan Dahl.

In 1977 he returned to Oxford to lead the Programming Research Group after the death of Christopher Strachey. He built Oxford into an international centre for formal methods and programming language theory. His 1978 paper on Communicating Sequential Processes, expanded into a 1985 book, gave the world a precise way to reason about programs that run in parallel; it underpinned the occam language for the Inmos Transputer and later informed industrial verification tools. Collaborations with industry, including work with IBM on the CICS system and with Inmos, earned Queen's Awards for Technological Achievement. In 1988 he became the first holder of the Christopher Strachey Professorship of Computing. With He Jifeng he later developed the Unifying Theories of Programming.

His 1980 Turing Award lecture, "The Emperor's Old Clothes", is still widely quoted for its plea for simplicity in design. He received many further honours, including Fellowship of the Royal Society (1982), the IEE Faraday Medal, the Kyoto Prize (2000), the IEEE John von Neumann Medal (2011) and the Royal Society's Royal Medal (2023).

After retiring from Oxford in 1999 he joined Microsoft Research in Cambridge, where he continued to champion verified software and remained active well into his eighties. Colleagues remember him as modest, generous with credit and devoted to his students, many of whom became leading computer scientists. Of his leadership he said, "I just found some really good people, then I let them get on with it."

Tony and Jill Hoare had three children. Their son Matthew died of leukaemia in 1981. Tony Hoare is survived by Jill, his wife of 63 years, by their son Tom and daughter Jo, and by his granddaughters.