XML CO538 Anonymous Questions and Answers Keyword Index

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Keyword reference for demo.occ

Question 9 (2003):

In `demo.occ', why does the Fibonacci list produce negatives ? Is it to do with the maximum size a number can be ?

Answer 9:

The numbers in the Fibonacci sequence increase very rapidly, and soon exceed the capacity of a 32-bit INT, so integer overflow occurs. The results are then effectively meaningless, and are as likely to be negative as positive.

Unlike programming languages such as Java, occam normally checks for such errors - prefering to crash rather than give wrong answers. However, the component processes in demo.occ use explicitly unchecked arithmetic operators (PLUS, MINUS, TIMES), and not the usual checked ones (+, -, *). If they did not, the run of demo would be rather short!

Keywords: demo.occ , overflow