MaplePrimes - answers and comments on Question, duplicates being eliminated from a list

ListTools

jpmay — Thu, 04 Mar 2010 20:50:17 Z

The ListTools package has a routine for doing just this: ListTools:-MakeUnique

(**) L := [1,2,3,4,3,2,3,4,5,4,3,4,5,6];

               L := [1, 2, 3, 4, 3, 2, 3, 4, 5, 4, 3, 4, 5, 6]

(**) ListTools:-MakeUnique(L);

                             [1, 2, 3, 4, 5, 6]

John

Removing duplicates

Robert Israel — Thu, 04 Mar 2010 23:50:02 Z

I might try something like this. Go through your list L and check, for each element, whether that element is a duplicate of one that occurs earlier. You can do that using the three-argument form of member, since

> member(L[i], L, 'j');

will leave j as the first position where L[i] occurs in the list L. Keep track of those i for which that is true. Then use subsop to remove the duplicate entries. For efficiency, just use one subsop: if you want to remove entries number i1, i2, ..., ik, then

> subsop(i1=NULL, i2=NULL, ..., ik=NULL, L)

will do it.

list to set to list

Alex Smith — Fri, 05 Mar 2010 01:02:35 Z

What occurs to me is to convert the list L to a set, which removes duplicates, then convert back to a list.

convert(L,set);convert(%,list);

Sorry I am a little hopeless

mathperson — Fri, 05 Mar 2010 19:34:33 Z

Sorry I am a little hopeless when it comes to maple.

converting to a set makes sense but I am not sure how it would work with teh example I have

However in correspondance to the first reply from jpmay i have tried to come up with a procedure however I know it is not correct and was hoping someone would be able to help.

winnow1:= proc(L)
local i;
for i in L do
if member( L[i], L, 'j') then # 3 argument form of member
subsop(i1=NULL, i2=NULL, ..., ik=NULL, L) # removes duplicate entries
end if
end do;
return L
end proc:

Thanks again.

Alternative

Joe Riel — Fri, 05 Mar 2010 20:17:16 Z

Why not use ?ListTools:-MakeUnique, as John suggested? Of course, learning to roll your own solution is a useful exercise. Here's an interesting approach described by Roman Pearce.

list to set to list

Robert Israel — Fri, 05 Mar 2010 04:19:29 Z

That's fine if you don't care what order the elements are in.