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I learned how to query Dataset for rows that contain a certain value, but now I need to query for a long series of And in chain on the value of several columns. So in short I am trying to identify the row where x={x1, ..., xn} takes a certain value.

For a Dataset like this

  ds = Dataset[{Association[{"x1" -> myX, "x2" -> myX, "x3" -> myX, 
  "x4" -> myX, "y" -> myY}], 
Association[{"x1" -> myotherX, "x2" -> myotherX, "x3" -> myX, 
  "x4" -> myX, "y" -> myotherY}]}];

I can do 

   ds[Select[(#x1 == myotherX && #x2 == myotherX && #x3 == myX && #x4 == 
  myX) &], "y"]

but the expression is long to type and less and less convenient as from 2 argument I go to a larger numbers.

Ideally I wanted to able to call something like

  ReadRow[ds,<|"x1"->myX,"x2"->myX,"x3"->myX,"x4"->myX|>,"y"]

Any idea how to do this? something like this should be a standard way to deal with Dataset.

My best result so far is to "translate" the Association into a check for the value I am looking for and put all the entered keys of the association

  RuleToEquality[a_, b_] := Slot[a] == b
  MgPyResults[
        Select[(KeyValueMap[RuleToEquality, <|x1 -> myX|>][[1]]) &], "y"]

This, however, does not match the entries, despite the 

InputForm[
     Tr[KeyValueMap[RuleToEquality, <|x1 -> myX, x2 -> myX|>], And]]

gives the correct

 Slot[x1] == myX && Slot[x2] == myX
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  • $\begingroup$ Your verbal description and you code examples don't seem consistent to me. Further, a data set with only one row doesn't make for a good example. I am voting to close this question as unclear. $\endgroup$ – m_goldberg Aug 29 '15 at 13:11
  • $\begingroup$ One row is more than enough to retrieve one row. Why should I put on the code more entries than needed? anyways, I am editing the question, maybe is only my point of view. $\endgroup$ – Rho Phi Aug 29 '15 at 13:19
  • $\begingroup$ @RobertoFranceschini, "Why should I put on the code more entries than needed?" non-matching entries are typically needed to test potential false positives. $\endgroup$ – alancalvitti Sep 3 '15 at 21:57
  • $\begingroup$ In your ReadRow[ds,<|x1->myX,x2->myX,x3->myX,x4->myX|>,"y"] the Keys in your example are strings so x1... should be wrapped in quotes. $\endgroup$ – alancalvitti Sep 3 '15 at 22:00
  • $\begingroup$ correct(ed), thanks for "compiling" my question :) $\endgroup$ – Rho Phi Sep 14 '15 at 13:48
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If you have your $\{column, value\}$ pairs in a list

findList = {{"x1", myX}, {"x2", myX}, {"x3", myX}, {"x4", myX}};

then you can build the Select criteria function from it.

selectCriteria[find_List] :=
 And @@ Function[{item}, #[item[[1]]] == item[[2]]] /@ find &

selectCriteria builds up the Equal pairs and chains them with And. It returns a function that can be used with Select.

ds[Select[selectCriteria[findList]], "y"]
(* {myY} *)

If your pairs are in an Association then convert it to a List before calling selectCriteria.

findAssc = <|"x1" -> myX, "x2" -> myX, "x3" -> myX, "x4" -> myX|>;

ds[Select[selectCriteria[List @@@ Normal@findAssc]], "y"]
(* {myY} *)

Hope this helps.

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Possibly this will be of use to you:

ds = Dataset[{
     <|"x1" -> myX, "x2" -> myX, "x3" -> myX, "x4" -> myX, "y" -> myY|>,
     <| "x1" -> myotherX, "x2" -> myotherX, "x3" -> myX, "x4" -> myX, "y" -> myotherY|>
   }];

sel = <|"x1" -> myX, "x2" -> myX, "x3" -> myX, "x4" -> myX|>;

sel2 = <|"x1" -> myotherX, "x2" -> myotherX, "x3" -> myX, "x4" -> myX|>;

pull[sel_] := Select[sel == KeyTake[#, Keys@sel] &];

ds[pull @ sel, "y"]
ds[pull @ sel2, "y"]

{myY}

{myotherY}
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