Mathematica Asked by Money Sets You Free on December 3, 2020
I want to create a list of 3-element subsets of ${1,2,cdots,12}$ where no two elements in each subset can have difference of 1 or 11. I am trying to solve the following:
Find the number of all possible triangles that can be created by choosing 3 points of a 12-sided polygon, but no sides of the triangles are also the sides of the polygon.
The following attempt fails because it returns just a list of all subsets without restriction.
Select[Subsets[Range[12], {3}]
, (Abs[#[[1]] - #[[2]]] != 1 || Abs[#[[1]] - #[[2]]] != 11) &&
(Abs[#[[1]] - #[[3]]] != 1 || Abs[#[[1]] - #[[3]]] != 11) &&
(Abs[#[[3]] - #[[2]]] != 1 || Abs[#[[3]] - #[[2]]] != 11) &]
I just got the solution as follows, but can it be simplified?
Select[Subsets[Range[12], {3}]
, ! MemberQ[{1, 11}, Abs[#[[1]] - #[[2]]]] &&
! MemberQ[{1, 11}, Abs[#[[1]] - #[[3]]]] &&
! MemberQ[{1, 11}, Abs[#[[3]] - #[[2]]]] &] // Length
test[sublist_] := ContainsNone[Abs[Subtract @@@ Subsets[sublist,{2}]], {1,11}]
Select[Subsets[Range[12], {3}], test]
For your problem in the comments, the number of triangles in a regular polygon which do not share any sides with that polygon is $n (n - 4) (n - 5)/6$ provided $nge6$. It would be much more efficient to use this result directly than to list them and count them.
Correct answer by flinty on December 3, 2020
You may use SubsetCount
. This is an experimental function in version 12.1.1 so behaviour may change.
Select[
SubsetCount[#, {j_, k_} /; Or @@ Thread[j - k == {1, 11}]] == 0 &
]@Subsets[Range[12], {3}]
Hope this helps.
Answered by Edmund on December 3, 2020
This should be quite a bit faster than your original solution:
Select[Subsets[Range[12], {3}], ! MemberQ[Abs[ListCorrelate[{-1, 1}, #, 1]], 1 | 11] &]
Answered by J. M.'s discontentment on December 3, 2020
Not an answer,only a review.
the question is equivalence to $$1leq a < b <c leq 12,b-ageq 2,c-bgeq 2$$ and when $a=1$, $cnot=12$ or when $c=12$,$anot=1$
If we mapping ${a,b,c}$ to ${a,b-1,c-2}={i,j,k}$
the question is equivalence to $$2leq i < j <k leq 9$$ or $$1=i,2leq j<kleq 9$$ or $$2leq i<jleq 9,k=10$$
so the number of subsets is ${8choose 3}+2{8 choose 2}=112$
Similarly the general result is ${n-4choose 3}+2{n-4choose 2}$ where the $n$ is the length of subsets ${1,2,cdots,n}$ ( here $n=12$)
Answered by cvgmt on December 3, 2020
Several additional alternatives:
res0 = DeleteCases[{1, _, 12} | ({a_, b_, _} /; b == a + 1) |
({_, a_, b_} /; b == a + 1)] @ Subsets[Range[12], {3}]; // RepeatedTiming // First
0.00042
res1 = Select[DeleteCases[{1, _, 12}] @ Subsets[Range[12], {3}], FreeQ[1] @* Differences];
// RepeatedTiming // First
0.00047
res2 = Select[Union @ Join[Subsets[Range[2, 12], {3}], Subsets[Range[11], {3}]],
FreeQ[1] @* Differences]; // RepeatedTiming // First
0.00051
Comparison with methods from flinty's (res3
), J.M.'s (res4
) and Edmund's (res5
) answers:
res3 = Select[Subsets[Range[12], {3}], test]; // RepeatedTiming // First
0.0034
res4 = Select[Subsets[Range[12], {3}],
!MemberQ[Abs[ListCorrelate[{-1, 1}, #, 1]], 1 | 11] &]; // RepeatedTiming // First
0.0016
res5 = Select[SubsetCount[#, {j_, k_} /; Or @@ Thread[j - k == {1, 11}]] == 0 &]@
Subsets[Range[12], {3}]; // RepeatedTiming // First
0.260
res0 == res1 == res2 == res3 == res4 == res5
True
Answered by kglr on December 3, 2020
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