Threading elements over corresponding elements in the second list

Question

I often need to merge two lists like the following:

$A=\{\{0.1\},\{0.2\},\ldots,\{1\}\}$

$B=\{\{x_1\},\{x_{21},x_{22},x_{23}\},\{x_{31},x_{32},x_{33}\},\ldots,x_N\}$

in a way to ideally obtain a new list such as:

$C = \{\{0.1,x_{1}\}, \{0.2,x_{21}\}, \{0.2,x_{22}\}, \{0.2, x_{23}\},\{0.3,x_{31}\}, \{0.3,x_{32}\}, \{0.3, x_{33}\}, \ldots, \{1, x_N\}\}$

At present I do this by inspecting the lists and manually associating them in the appropriate way. In above example, I would do

Table[{A[[k]],B[[k,1]]},{k,1,N}]

Table[{A[[k]],B[[k,2]]},{k,2,S}]

Table[{A[[k]],B[[k,3]]},{k,2,S}]

where S is the position of the last element that contains three entries in the $B$ list.

These lists are typically very long, and I wonder whether there is a way to do this efficiently.

Answer 1

I feel like it is a duplicate but I can't find it now.

MapThread[Apply[Sequence]@*Tuples@*List, {alist, blist}]

or

Flatten[Tuples /@ Transpose[{alist, blist}], 1]

,

Flatten[MapThread[Thread[{#[[1]], #2}] &, {alist, blist}], 1]

Answer 2

You might have to change this slightly for your particular problem.

    A = Table[{0.1 n}, {n, 1, 10}]
    B = Array[Subscript[x, ##] &, {10, 3}]
    Map[Flatten, Apply[Prepend, MapThread[Prepend, {A, B}], {1}]]

Answer 3

Join @@ (Thread /@ Thread[{Flatten @ a, b}])