On Tue, Apr 13, 2010 at 08:11:34PM -0700, Justin Walker wrote: > Dear Forum, > > I was fiddling with lists and tried the following, which resulted in a > segmentation fault: > > gap> l:=ListWithIdenticalEntries(6,[1,2,3]);; > gap> l[1][1]:=l[3];; > gap> l; > [ [ ~[1], 2, 3 ], [ ~[2], 2, 3 ], [ ~[3], 2, 3 ], [ ~[4], 2, 3 ], [ ~[5], > 2, 3 ], > [ ~[6], 2, 3 ] ] > gap> MakeImmutable(l); > > Boom! > > I gather that the result of the second assignment is to tie the list > into knots. The following call then goes recursive and eventually blows > the stack (true?). > > I thought it worth reporting, in case it's fixable.
Dear Justin, dear Forum, Thanks for this example, I'm a bit surprised that nobody reported this before. A slightly shorter example is: gap> l := [~]; # equivalent to: l := [];; l[1] := l; [ ~ ] gap> MakeImmutable(l); Segmentation fault I tend to consider this as "feature" rather than a bug. But it should at least be documented. GAP can only make a list immutable when all its entries are immutable. MakeImmutable just recurses into the list and tries to make all entries immutable first (if they are not already immutable). In the examples above this runs into an infinite recursion. This could be fixed. For example you see above that Print or View can deal with the self-recursive structure. You can also make an immutable copy of the object: gap> l := [~];; gap> l := Immutable(l); [ ~ ] gap> IsMutable(l); false But I think that MakeImmutable is intended to be very fast on newly created objects (which may be huge and have many subobjects). If we would fix the reported problem, MakeImmutable would become much slower and we would loose its speed advantage compared to Immutable. Therefore: For self-recursive objects use don't use 'MakeImmutable', create a copy with 'Immutable' instead. Best regards, Frank BTW: Self-recursive objects are not uncommon in GAP, e.g., a group may contain a list of its conjugacy classes and each class points back to the group it belongs to. Or think of a tree structure where the nodes point back to the root. -- /// Dr. Frank Lübeck, Lehrstuhl D für Mathematik, Templergraben 64, /// \\\ 52062 Aachen, Germany \\\ /// E-mail: frank.lueb...@math.rwth-aachen.de /// \\\ WWW: http://www.math.rwth-aachen.de/~Frank.Luebeck/ \\\ _______________________________________________ Forum mailing list Forum@mail.gap-system.org http://mail.gap-system.org/mailman/listinfo/forum
