Dear Forum, Siddiqua Mazhar wrote:
> If we have a list like [[2,1],[5,2,1],[6,5,2,1],[9,10]] and I want to find > the result in the form of [6,5,2,1].[9,10] and want to delete all those list > that are contatined in the other. How may I progrme this > > I do like let FV:= [[2,1],[5,2,1],[6,5,2,1],[9,10]]; > > Intersection:=function(FV) > local A,B,a; > for A in FV do > for B in FV do > if Intersection(A,B)=B then > a:=Position(FV,B); > Unbind(FV[a]); > fi; > od; > od; > > return FV; > end; > > However, this program gives me empty list. Could you please guide me in this > program. The first problem is that the function should not be called 'Intersection'; note that this would overwrite the GAP function with this name, and then one would not be able to call this function inside. In fact, trying to read the above code into GAP would cause an error message because GAP's 'Intersection' is write protected. The second problem is that GAP's 'Intersection', when called with two lists, returns the *sorted* list of all those elements that appear in these two lists. Therefore, the condition 'Intersection( A, B ) = B' is *not* satisfied if 'B' is not sorted, even if all entries of 'B' appear in 'A'. (In particular, it is not true that the above function will return the empty list when it is called with the above list 'FV'.) The third problem is that if the condition would be replaced for example by 'Intersection( A, B ) = Set( B )' then the return value would really be an empty list, because the two for loops compare among others the intersection of 'A' and 'A' with 'Set( A )', and then 'A' gets kicked out from the list. (Perhaps the fourth problem is that the argument FV gets changed in place, which is probably not intended. If this behaviour is intended then there is no need to return a result.) All the best, Thomas _______________________________________________ Forum mailing list Forum@mail.gap-system.org http://mail.gap-system.org/mailman/listinfo/forum
