EWD249 - 37

before printing starts, and under this assumption we have exploited that the computation of the primes can be conceived independently of the printing. Of our original problem statement we have not taken into account very much more than that at least a thousand different prime numbers do exist (we had to assume this for the problem statement to make sense). At this stage it is still fairly immaterial what the concept "prime number" really means. Also: we have not committed ourselves in the least as regards the specific layout requirements of the print-out to be produced. Apparently it is the strength of our approach that the consequences of these two rather independent aspects of our original problem statement seem to have been allocated in the respective refinements of our two constituent actions. It suggests that we have been more or less successful in our effort to apply the golden principle "divide and rule".

Resuming our discussion, however, we have to ask ourselves, to what extent the two subcomputations can now be conceived independently of each other. To be more precise "Have we now reached the stage that the design of the two subalgorithms (that have to evoke the two subcomputations) can be conceived by two programmers, working independently of each other?".

When the two actions can no longer be regarded as invoked by instructions from the well-understood repertoire, neither can the variable "table p" any longer be regarded as an implicitly available resource. And in a way similar to the one in which we in which we have to decompose the actions into subcomputations, we have to choose how the variable "table p" will be composed, viz. what data structure we select to represent the information to be handed over via "table p" from the first action to the second. At some point this has to be decided and the questions are "when?" and "how?".

In principle, there seem to be two ways out of this. The first one is to try to postpone the decision on how to structure "table p" into (more neutral, less problem-bound) components. If we postpone the decision on how to structure "table p", the next thing to do is to refine one of the actions or both. We can do so, assuming a proper set of operations on the still mysterious object "table p"; finally we collect these operations and in view of their demands we design the most attractive structure of "table p".