Graham Smith <myotistwo <at> gmail.com> writes: > > I assume this has a "proper" name, but I don't know what it is and wondered > if anyone knew of a package that might do the following, or something > similar. > > As an example, assume I have borrowed and read 10 books on R , and I have > subjectively given each of them a "value" score in terms of how useful I > think they are. I also know how much each costs in terms of money. > > What I would like to do is to calculate the costs of every possible > combination of the 10 books, and plot the total monetary value for each of > these possible combination with their associated subjective value totals, > to help decide which combination of books represents the best value for > money. > > I know that some specialist decision analysis software does this sort of > thing, but was hoping R might have an appropriate package.
Perhaps you can specify your question more precisely, or differently. The way I interpret it, if there are no interactions in price (e.g. you get a discount for buying more than one book at a time) or in value (e.g. you learn more from one book having read another), then you get the best value/price ratio by taking only the book with the highest value/price. (If you take no books at all, your value/price ratio is undefined.) The algebra below shows that combining a lower value/price book with a higher one always lowers your overall value/price ratio. If you redefine your problem, you might find the combn() or expand.grid() functions, along with various versions of apply(), to be useful. If you have too large a search space you might take a look at the simulated annealing (SANN) option of optim(). =================== if a1/b1 > a2/b2 (1) and a1, b1, a2, b2 > 0 show a1/b1 > (a1+a2)/(b1+b2) (2) i.e. a1/b1 - (a1+a2)/(b1+b2) > 0 or (a1(b1+b2)-(a1+a2)b1)/(b1+b2) = (a1*b2-a2*b1)/(b1+b2) > 0 the numerator is (a1*b2-a2*b1): (1) implies that a1*b2>a2*b1 so the numerator is positive qed ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
