9
$\begingroup$

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.

$\endgroup$
4
  • $\begingroup$ A related thread. You'll be particularly interested in the use of Flatten[]. $\endgroup$ Jun 4, 2015 at 20:29
  • 2
    $\begingroup$ Flatten[Tuples /@ (Transpose@{a, b}), 1]? $\endgroup$
    – N.J.Evans
    Jun 4, 2015 at 20:36
  • $\begingroup$ closely related $\endgroup$
    – Kuba
    Jun 4, 2015 at 20:40
  • 2
    $\begingroup$ Also Function[Null, {##}, Listable][Flatten@alist, blist]~Flatten~1 $\endgroup$
    – garej
    Feb 24, 2016 at 13:00

3 Answers 3

6
$\begingroup$

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

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

enter image description here

or

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

,

Flatten[MapThread[Thread[{#[[1]], #2}] &, {alist, blist}], 1]
$\endgroup$
0
4
$\begingroup$

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}]]
$\endgroup$
1
$\begingroup$
Join @@ (Thread /@ Thread[{Flatten @ a, b}])      

enter image description here

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.