Dear Forum,
is there a reason why the versions Sort and SortParallel exist with
an optional Argument func which permits to define an own comparison
function while for Sortex and SortingPerm such a version does not
exist?
There is no fundamental reason for this (and in the next release such
functionality will be available), the most likely reason is that it
can be easily emulated, see below.
And if so, is there a simple way to obtain what
SortingPerm(ConjugacyClasses, cmp)
would do (for a given function cmp) if there were such a version?
Yes. You can use `SortParallel' to build a permutation (in fact this
is what `Sortex' does internally):
gap> l:=[6,2,9,4,5];
gap> pos:=[1..Length(l)]; # list of positions
gap> SortParallel(l,pos,function(a,b) return a>b;end); # reverse
sorting function
gap> l;
[ 9, 6, 5, 4, 2 ]
Now `PermList(pos)' is the inverse of permutation that will do the
same sort:
gap> l:=[6,2,9,4,5]; # old list again
gap> Permuted(l,PermList(pos)^-1);
[ 9, 6, 5, 4, 2 ]
All the best,
Alexander Hulpke
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