Dear GAP Forum,

There is no problem (although some Computer Scientists would say that it is a
bad idea) with a GAP function modifying its arguments. As a an example here
is a function which swaps the first elements of its two list arguments and
returns true if they are equal, false if not, and fail if one of the lists had
no first entry.

foo := function(l1,l2)
    local x;
    if not IsBound(l1[1]) or not IsBound(l2[1]) then
	return fail;
    fi;
    if l1[1] = l2[1] then 
           return true;
    fi;
    x := l1[1]; 
    l1[1] := l2[1];
    l2[1] := x;
    return false;
end;


I can then do:
gap> l := [1,2,3,4]; m:= [4,3,2,1];
[ 1, 2, 3, 4 ]
[ 4, 3, 2, 1 ]
gap> foo(l,m);
false
gap> l;
[ 4, 2, 3, 4 ]
gap> m;
[ 1, 3, 2, 1 ]
gap> 

I hope this helps,

Steve Linton

-- 
Steve Linton	School of Computer Science  &
      Centre for Interdisciplinary Research in Computational Algebra
	     University of St Andrews 	 Tel   +44 (1334) 463269
http://www-theory.dcs.st-and.ac.uk/~sal	 Fax   +44 (1334) 463278   

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