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list manipulation - How to use Map inside MapThread?


Given this input



lst1 = {{a, b, c}, {d, e, f}};
lst2 = {1, 2};

and the goal is to generate this output


{  {{1, a}, {1, b}, {1, c}}, 
   {{2, d}, {2, e}, {2, f}}
}

Perfect candidate for MapThread So I made this diagram first to figure what the function I want to map should be


enter image description here



So the function to use inside MapThread, needs to also use Map itself (in order to map each item into the other list). So I came up with this:


lst1 = {{a, b, c}, {d, e, f}};
lst2 = {1, 2};
foo[i_, lst_List] := List[i, #] & /@ lst
MapThread[foo[#1, #2] &, {Range[Length@lst2], lst1}]
(*  {  {{1,a}, {1,b}, {1,c}},       {{2,d}, {2,e}, {2,f}}   }  *)

Now here is the question: Is there a way to do the above without having to define an explicit function but using pure function inside MapThread?


I was getting conflict with # mapping. This is sort of the thing I was trying to do, but can't get the syntax right


(*invalid, for illustration only *)

MapThread[ 
 Function[{idx, lst},List[idx, #] & /@ lst] &  ?? ??    ,{Range[Length@lst2],lst1}]

Or if you know of a better approach to do this, that will be fine as well.



Answer



MapThread[Thread[{##}] &, {lst2, lst1}] 

Map[Thread, {lst2, lst1}\[Transpose]]

## is used so Thread gets called like Thread[{1, {a, b, c}}] As MapThread gives two arguments in this case it is equivalent to Thread[{#1, #2}]& and Composition[Thread, List]



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