software is available for operational use by a certain date, then all designs that are so complex that they could not meet the deadline must be rejected. Similarly, designs that will cost in excess of the user’s budget to implement must go.
Although the application of constraints can severely limit the number of choices available to the designer, the size of selection will still seem unmanageable. After all a very large number, less a large number, is still a very large number. So a designer still needs some help in tackling the selection problem. This help comes in the form of what are termed design heuristics or, if you prefer, rules-of-thumb.
1.2.3 Design heuristics
Let us turn again to problem of adding fifty numbers and its 6.85x1081 adding trees. Just suppose that it is possible to identify five of the numbers as being related in some way so that they could be close together in the adding tree, e.g., they are all octal, perhaps, whereas the others are not. We know that the prospect of adding these five numbers will give rise to 236 possible adding trees. The evaluation of the 236 alternatives is perfectly feasible and the optimal adding sub-tree for these five numbers could be selected. We could then be able to replace the five numbers and with a single node representing the optimal adding sub-tree. So that we have now achieved a reduction in the numbers to be added from 50 to 46, i.e.,45 numbers and a sub-total from the sub-tree.
The process could be repeated taking five numbers at a time and replacing them, similarly, with a single satisfactory sub-tree or design.
The process would involve thirteen cycles, each one involving the evaluation of 236 designs, except the last. The last cycle would involve only two numbers and this would permit only once design. Consequently, the total process would involve the evaluation of 12x236, i.e., 2832 separate designs (Emery, 1969).
Even with this reduction in the number of separate designs, there is still a lot to do. If it takes a quarter of an hour to examine and assess each alternative, how long would it take to design the optimal adding tree using the above procedure? Design would now take 708 hours. Or , assuming that one designer is working on the problem, just under 18 working man-weeks. So that design is no longer impossible but merely tedious and formidable.
The above is an example of a design heuristic, a rule-of thumb used by the designer to reduce drastically the size of selection until it becomes a manageable problem. The source of such design heuristics may experience or the native cunning of the designer, or it may be part of the folklore of the profession.
The similarity with folklore does not stop there. You are, no doubt, familiar with a number of folkloric expressions of the ‘red sky at night, shepherds’ delight’ type. On hearing this from the lips of some weather-beaten sage, one can almost hear the quotation marks. And so it is with designers. The expert at designing adding trees for fifty numbers, when holding forth in the local drinking establishment, might well say:
Take five numbers that for some reason should be close together on the tree, replace them with a single node representing the optimal sub-tree, and repeat until one tree remains.
<< 上一页 [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] 下一页