# Generating permutations of at most 'n' elements, and where a specific subset of elements always appears

I often run into a situation where I'd like to generate a set of permutations with at most $n$ elements (here $n = 5$):

Permutations[{"t1","t2","t3","t4","t5","t6","t7","t8","t9","t10","t11","t12","t13"}, 5]


I'd like to also specify that a specific subset of elements always appears in each subset (without generating the entire list of permutations and scanning through it, or scanning through the permutations in lexicographic order: Generating a permutation of elements in chunks). For example, could one generate a list of all length $n = 5$ permutations for the above example where the subset of elements {"t2","t5","t7"} always appears (in any order)?

Is there a (fast) way to ask Mathematica to do this? One solution would be to ask for all length $q = 2$ subsets of {"t1","t2","t3","t4","t5","t6","t7","t8","t9","t10","t11","t12","t13"}, concatenate these subsets with the list {"t2","t5","t7"}, generate the permutations for each subset, then concatenate each list of permutations. However, is there maybe a nicer solution?

• I think the only way to get the result is to pick the lists you want from the all Permutations[list,5]. It will be so easy this way. Something like this: Cases[allPermutations, x_ /; SubsetQ[x, {"t2","t5","t7"}]] Commented Dec 27, 2014 at 2:59
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l = {"t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9", "t10", "t11", "t12", "t13"};
lAlways = {"t2", "t5", "t7"};

f[n_Integer, l_List, always_List] :=
Module[{lVar = Complement[l, always], toSel = n - Length@always},
Flatten[Permutations /@ (Join[always, #] & /@ Subsets[lVar, {toSel}]), 1]]
f[5, l, lAlways] // Length

(*5400*)

f[5, l, lAlways][[;; 5]] // Column


Similar to Algohi's proposal but without SubsetQ so that it runs also in version 8.

list = {"t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9", "t10",
"t11", "t12", "t13"};

list1 = {"t2", "t5", "t7"};

t = Permutations[list, {5}];
Length[t]
(* 154440  *)

t1= Select[t, Intersection[#, list1] == list1 &];

Length[t1 ]
(* 5400 *)

t1//Short


$\{\{\text{t1},\text{t2},\text{t3},\text{t5},\text{t7}\},\{\text{t1},\text{t2},\text{t3},\text{t7},\text{t5}\},\{\text{t1},\text{t2},\text{t4},\text{t5},\text{t7}\},\{\text{t1},\text{t2},\text{t4},\text{t7},\text{t5}\},\{\text{t1},\text{t2},\text{t5},\text{t3},\text{t7}\},\langle\langle 5391\rangle\rangle ,\{\text{t13},\text{t12},\text{t5},\text{t2},\text{t7}\},\{\text{t13},\text{t12},\text{t5},\text{t7},\text{t2}\},\{\text{t13},\text{t12},\text{t7},\text{t2},\text{t5}\},\{\text{t13},\text{t12},\text{t7},\text{t5},\text{t2}\}\}$

Best regards,
Wolfgang

• The OP specifically stated "(without generating the entire list of permutations and scanning through it)". I believe this solution is doing exactly that :) Commented Dec 30, 2014 at 14:23
• @belisarius: you are right, I neglected the condition "without generating the entire list ...", but I simply wanted to make Algohi's proposal running in V8. BTW you didn't object to his comment. BTW2: your solution shows that Algohi was wrong in stating that his is the "only way". Commented Dec 30, 2014 at 22:23
• I've never seen a problem with only one possible solution in Mathematica :) Commented Dec 30, 2014 at 22:42
• @belisarius: that's what I was saying. Commented Jan 1, 2015 at 12:16