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Convert a list of tuples into a crosstab of key with list of values with that key
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// take an order list of tuple key value and convert to a crosstab of key & values let crosstab set = Seq.fold (fun a (x,y) -> if (fst (List.head a)) = x then [(x,[y] @ (snd (List.head a)))] @ a.Tail else [(x,[y])] @ a) [(fst (Seq.head set),[])] set (* takes advantage of tail of a single linked list being a list, to immutably accumulate without copying data. example with type anotation crosstab [(1,'a'); (1,'b');(1,'c');(2,'A');(2,'B');(3,'x')] = [(3, ['x']); (2, ['B'; 'A']); (1, ['c'; 'b'; 'a'])] let crosstab (set : ('a * 'b) seq) : ('a * 'b list) list = let keyofhead (a : ('a * 'b list) list) : 'a = fst (List.head a) let crosshead (a : ('a * 'b list) list) : 'b list = snd (List.head a) let statewithfirstkey : ('a * 'b list) list = [(fst (Seq.head set),[])] Seq.fold (fun a (x,y) -> if (keyofhead a) = x then [(x,[y] @ (crosshead a))] @ a.Tail else [(x,[y])] @ a) statewithfirstkey set *)
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seq
crosstab
pivot
seq
crosstab
pivot
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