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Data Science Wolfram Language Symbolic Computations

Use a key collision function to combine results of JoinAcross[ ]?

Caitlin Ramsey

Posted 9 years ago

In the Wolfram Language Documentation for JoinAcross, there is an example that uses a KeyCollisionFunction to change the keys (labels) that result from a join across operation: in[]: JoinAcross[{<\|a -> 1, b -> X\|>}, {<\|a -> 1, b -> Y\|>}, Key[a], "Inner", KeyCollisionFunction -> Function[x, {left[x], right[x]}]] This produces: out[]: {<\|a -> 1, left[b] -> X, right[b] -> Y\|>} I have a similar problem, but I would like to Join two arrays (or associations) on Key A, and combine the values associated with Key B. That is... in[]: JoinAcross[{<\|a -> 1, b -> X\|>}, {<\|a -> 1, b -> Y\|>}, Key[a], "Inner", KeyCollisionFunction -> SOME FUNCTION ] To produce: {<\|a -> 1, b -> {X, Y}\|>} Note, instead of modifying the keys left[b] and right[b], we keep the original key b and combined the values from left and right, into a list associated with the original key b ->{X, Y}. It seems like there should be a straightforward way to do this, but I am not very handy with the syntax for pure functions. Can anyone explain how to do this?

POSTED BY: Caitlin Ramsey

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Kuba Podkalicki

Posted 9 years ago

Yes, I agree it is a simple code, we need a better example to test. From what OP has already said I don't know if we have 2 lists of associations (as `JoinAccros` requires) or it is just a single list that is supposed to be merged in a fancy way. In the latter case we can adjust my simple approach: list = {<\|a -> 1, b -> 1\|>, <\|a -> 1, b -> 2\|>, <\|a -> 2, b -> 3\|>, <\| a -> 2, b -> 4\|>, <\|a -> 3, b -> 4\|> (unmatched addition)} Values @ GroupBy[list, Key[a], MapAt[First, Key[a]] @* Merge[Identity]] {<\|a -> 1, b -> {1, 2}\|>, <\|a -> 2, b -> {3, 4}\|>, <\|a -> 3, b -> {4}\|>} For `a->3` there isn't any mach, should `b's` value be wrapped with `{}`?, probably depends of OP's needs too. Alternatively, the same code but with more readable operators order: GroupBy[list, Key[a], Merge[Identity] /* MapAt[First, Key[a]] ] // Values

POSTED BY: Kuba Podkalicki

Sander Huisman

Sander Huisman, University of Twente

Posted 9 years ago

Nice solution for the simple case, however this does not work when there are two different 'a' matches: MapAt[First, Key[a]]@ Merge[Identity]@{<\|a -> 1, b -> 1\|>, <\|a -> 1, b -> 2\|>, <\|a -> 2, b -> 3\|>, <\|a -> 2, b -> 4\|>} Furthermore you don't have a set of 'left' and 'right' associations. I was playing with Merge and GroupBy, but these don't perform a 'join', which is what she wants (I think)... Some more elaborate example would be necessary to see what is wanted...

POSTED BY: Sander Huisman

Kuba Podkalicki

Posted 9 years ago

I suppose a more general example is needed (this one doesn't need `JoinAcross` specific features) but here is something what may be good enough: MapAt[First, Key[a]] @ Merge[Identity] @ {<\|a -> 1, b -> X\|>, <\|a -> 1, b -> Y\|>} <\|a -> 1, b -> {X, Y}\|>

POSTED BY: Kuba Podkalicki

Sander Huisman

Sander Huisman, University of Twente

Posted 9 years ago

Hi Caitlin, I'm pretty sure JoinAcross can not do what you want 'out of the box'. However we can make a function that collects the left and right terms and forms a single key: ClearAll[MergeLeftRight] MergeLeftRight[assoc_Association]:=Module[{keys,new,newassoc}, keys=Cases[Keys[assoc],left[_]\|right[_]]; keys=GatherBy[keys,First]; new=(First[First[#]]->(assoc/@#))&/@keys; newassoc=assoc; KeyDropFrom[newassoc,Join@@keys]; newassoc~Join~Association[new] ] out=JoinAcross[{<\|a->1,b->X,c->3\|>,<\|a->2,b->3\|>},{<\|a->1,b->Y,c->2\|>,<\|a->1,b->z,c->4\|>,<\|a->2,b->cc,c->1\|>},Key[a],"Inner",KeyCollisionFunction->Function[x,{left[x],right[x]}]] out//Column out = MergeLeftRight/@out; out // Column Let me know if this is what you want...

Hi Caitlin,

I'm pretty sure JoinAcross can not do what you want 'out of the box'. However we can make a function that collects the left and right terms and forms a single key:

ClearAll[MergeLeftRight]
MergeLeftRight[assoc_Association]:=Module[{keys,new,newassoc},
    keys=Cases[Keys[assoc],left[_]|right[_]];
    keys=GatherBy[keys,First];
    new=(First[First[#]]->(assoc/@#))&/@keys;
    newassoc=assoc;
    KeyDropFrom[newassoc,Join@@keys];
    newassoc~Join~Association[new]
]

out=JoinAcross[{<|a->1,b->X,c->3|>,<|a->2,b->3|>},{<|a->1,b->Y,c->2|>,<|a->1,b->z,c->4|>,<|a->2,b->cc,c->1|>},Key[a],"Inner",KeyCollisionFunction->Function[x,{left[x],right[x]}]]
out//Column

out = MergeLeftRight/@out;
out // Column

Let me know if this is what you want...

POSTED BY: Sander Huisman

Caitlin Ramsey

Posted 9 years ago

I do not always have exactly two, and they are part of many associations, with one, two, or more associations that match on Key a. I want to track more than b, but b happens to be the only one that has the form List. (The values associated with other keys would usually be individual strings.)