# Find different combinations of 3 lists with given constraints

My inputs are 3 lists of unequal length:

A={a,b,c,d}
B={i,j,k}
C={v,w,x,y,z}


And I want to find the combine set X which looks like

X={{v,b},{w,a},{x,i},{y,j},{c,k},{d,z}}


i.e. the each sub-list should be of 2 elements and each of its elements should be from different lists A, B and C.

After this I would like to find all such possible X obtained from a given A, B and C.

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lists = {{a, b, c, d}, {i, j, k}, {v, w, x, y, z}};

pairs = Join @@ Tuples /@ Subsets[lists, {2}]
(*    {{a, i}, {a, j}, {a, k}, {b, i}, {b, j}, {b, k}, {c, i},
{c, j}, {c, k}, {d, i}, {d, j}, {d, k}, {a, v}, {a, w},
{a, x}, {a, y}, {a, z}, {b, v}, {b, w}, {b, x}, {b, y},
{b, z}, {c, v}, {c, w}, {c, x}, {c, y}, {c, z}, {d, v},
{d, w}, {d, x}, {d, y}, {d, z}, {i, v}, {i, w}, {i, x},
{i, y}, {i, z}, {j, v}, {j, w}, {j, x}, {j, y}, {j, z},
{k, v}, {k, w}, {k, x}, {k, y}, {k, z}}                    *)

XX = Select[Subsets[pairs, {Length[Flatten[lists]]/2}],
DuplicateFreeQ@*Flatten]
(*    {{{a, i}, {b, v}, {c, w}, {d, x}, {j, y}, {k, z}},
{{a, i}, {b, v}, {c, w}, {d, x}, {j, z}, {k, y}},
...
{{d, k}, {a, z}, {b, y}, {c, x}, {i, v}, {j, w}},
{{d, k}, {a, z}, {b, y}, {c, x}, {i, w}, {j, v}}}    *)


There are 1440 such lists X. If you need them in canonical order, they need to be sorted at levels 0, 1, and 2:

XXs = Map[Sort, XX, {0, 2}]
(*    {{{a, i}, {b, v}, {c, w}, {d, x}, {j, y}, {k, z}},
{{a, i}, {b, v}, {c, w}, {d, x}, {j, z}, {k, y}},
...
{{a, z}, {b, y}, {c, x}, {d, k}, {i, v}, {j, w}},
{{a, z}, {b, y}, {c, x}, {d, k}, {i, w}, {j, v}}}    *)


in which your original X appears at position 665:

X = {{v, b}, {w, a}, {x, i}, {y, j}, {c, k}, {d, z}};
Position[XXs, Map[Sort, X, {0, 1}]]
(*    {{665}}    *)

• Thank you so much!
– jar-
Jul 17 at 16:42

Try

A={a,b,c,d};
B={i,j,k};
cC={v,w,x,y,z};(*C is a predefined Mathematica name*)
Join[Tuples[{A,B}],Tuples[{A,cC}],
Tuples[{B,A}],Tuples[{B,cC}],
Tuples[{cC,A}],Tuples[{cC,B}]]