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cs7120 (Prasad)L10-COMB-FN1 Combinatorial Functions Recursion for problem solving

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cs7120 (Prasad)L10-COMB-FN2 Enumerating Initial segments (prefixes) inits [1,2,3] = [[],[1],[1,2],[1,2,3]] inits [2,3] = [ [], [2], [2,3] ] fun inits [] = [[]] | inits (x::xs) = []:: (map (fn ys => x::ys) (inits xs) ); fun inits [] = [[]] | inits xs = (inits (init xs))@[xs];

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cs7120 (Prasad)L10-COMB-FN3 Enumerating Subsequences subseq [2,3] = [[],[2],[3],[2,3]]; subseq [1,2,3] = [[], [1],[2],[3], [1,2],[1,3],[2,3],[1,2,3] ]; fun subseq [] = [[]] | subseq (x::xs) = let val ss = subseq xs in ss@(map (fn ys => x::ys) ss) end;

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cs7120 (Prasad)L10-COMB-FN4 Enumerating permutations perms [2] = [[2]] perms [1,2] = [[1,2], [2,1]] perms [0,1,2] = [[0,1,2],[1,0,2],[1,2,0], [0,2,1],[2,0,1],[2,1,0]] fun interleave x [] = [[x]] | interleave x (y::ys) = (x::y::ys) :: (map(fn xs=>y::xs)(interleave x ys)); fun perms [] = [[]] | perms (x::xs) = foldr (op @) [] (map (interleave x) (perms xs)) ;

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cs7120 (Prasad)L10-COMB-FN5 List partitions list-partitionThe list of non-empty lists [L1,L2,…,Lm] forms a list-partition of a list L iff concat [L1,L2,…,Lm] = L [] [1,2] -> [[ [1,2] ], [ [1],[2] ]] [ [0,1,2] -> [ [ [0,1,2] ], ] [ [0,1],[2] ], [ [0],[1,2] ], [ [0],[1],[2] ] ]

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cs7120 (Prasad)L10-COMB-FN6 Counting problem fun cnt_lp 0 = 0 | cnt_lp 1 = 1 | cnt_lp n = 2 * (cnt_lp (n-1)); (* cnt_lp n = 2^(n-1) for n > 0 *) Property: cnt_lp (length xs) = (length (list_partition xs))

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cs7120 (Prasad)L10-COMB-FN7 Constructing List Partitions fun lp [] = [] | lp (x::[]) = [ [[x]] ] | lp (y::x::xs) = let val aux = lp (x::xs) in () (map (fn ss => [y]::ss) aux)@ ( (map (fn ss => ( y:: ( hd ss ) ) ) :: ( tl ss )) aux) ; end;

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cs7120 (Prasad)L10-COMB-FN8 Set partition set partitionThe set of (non-empty) sets [s1,s2,…,sm] forms a set partition of s iff the sets si ’s are collectively exhaustive and pairwise-disjoint. E.g., set partitions of {1,2,3} -> { { { 1,2,3 } }, { { 1 }, { 2,3 } }, { { 1,2 }, { 3 } }, { { 2 }, { 1,3 } }, { { 1},{2},{3 } } } (Cf. list partition, number partition, etc.)

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cs7120 (Prasad)L10-COMB-FN9 (m-1) -partition of {2,3} Divide and Conquer m -partition of {1,2,3} 1 occurring with with a part in m -partition of {2,3} solitary part {1}:: 2-partitions of {1,2,3} = {1} { { {1},{2,3} } } U 11 { { {1,2},{3} }, { {2},{1,3} } }

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cs7120 (Prasad)L10-COMB-FN10 Counting problem fun cnt_m_sp 1 n = 1 | cnt_m_sp m n = if m > n then 0 else if m = n then 1 else (cnt_m_sp (m-1) (n-1)) + (m * (cnt_m_sp m (n-1))); Dependency (visualization) –Basis: Row 1 and diagonal (Half-plane inapplicable) –Recursive step: Previous row, previous column

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cs7120 (Prasad)L10-COMB-FN11 (cont’d) upto 3 6 = [3,4,5,6] fun upto m n = if (m > n) then [] else if (m = n) then [m] else m:: upto (m+1) n; fun cnt_sp n = foldr (op +) 0 (map (fn m => cnt_m_sp m n) (upto 1 n));

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cs7120 (Prasad)L10-COMB-FN12 Constructing set partitions fun set_parts s = foldr (op@) [] ( map (fn m => (m_set_parts m s)) (upto 1 (length s)) ); fun ins_all e [] = [] | ins_all e (s::ss) = (((e::s)::ss) :: ( ( map (fn ts => s :: ts) ) (ins_all e ss) ) );

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cs7120 (Prasad)L10-COMB-FN13 fun m_set_parts 1 s = [[s]] | m_set_parts m (s as hs::ts)= let val n = (length s) in if m > n then [] else if m = n then [foldr (fn (e,ss)=>[e]::ss ) [] s] else let val p1 = (m_set_parts (m-1) ts) val p2 = (m_set_parts m ts) in (map (fn ss => [hs]::ss) p1) @ (foldr (op@) [] ( map (ins_all hs) p2 ) ) end end ;

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Recursion Higher Order Functions CSCE 314 Spring 2016.

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