How to interleave data in a certain way

Question

Here's the deal: I have a table that has the form

 list = {{A,{a,b,c}},{B,{d,e,f}},{C,{g,h,i,j}}}

and so on, where the innermost lists can have an arbitary number of elements that differs from list to list, and I'd like it to look like

{{A,a},{A,b},{A,c},{B,d},{B,e},{B,f},{C,g},{C,h},{C,i},{C,j}}

and so on. How do I do this?

My first answer was the following monstrosity: Flatten[Inner[Outer[##] /. x_[y_] :> {x, y} &, Sequence @@ Transpose[l], List], 1] — Patrick Stevens, Commented Nov 24, 2015 at 15:27
@belisariushassettled I am definitely of the "add more stuff until it works" school of programming. — Patrick Stevens, Commented Nov 24, 2015 at 15:31
@PatrickStevens The Mathematica environment somehow encourages that. It is unbelievable how much one can entangle a simple problem by following a line of thought instead of giving it a break and think again! — Dr. belisarius, Commented Nov 24, 2015 at 15:34

Pillsy · Accepted Answer · 2015-11-24 15:40:34Z

6

Here's a simple one-liner:

Apply[Join]@*Map[Thread]@list

(* {{A, a}, {A, b}, {A, c}, {B, d}, {B, e}, {B, f}, {C, g}, {C, h}, {C, i}, {C, j}} *)

answered Nov 24, 2015 at 15:40

Pillsy

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1

$\begingroup$ Love the mix of operator forms and composition. $\endgroup$
– march
Commented Nov 24, 2015 at 16:52

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ybeltukov · Accepted Answer · 2015-11-24 16:04:17Z

9

Since V10.0 you can use the following clear expression

Catenate[Thread /@ list]
(* {{A, a}, {A, b}, {A, c}, {B, d}, {B, e}, {B, f}, {C, g}, {C, h}, {C, 
  i}, {C, j}} *)

answered Nov 24, 2015 at 16:04

ybeltukov

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$\begingroup$ I checked that timings of Catenate[...] and Faltten[...,1] are almost the same. $\endgroup$
– ybeltukov
Commented Nov 24, 2015 at 16:17
$\begingroup$ +1 I always forget about Catenate! $\endgroup$
– Pillsy
Commented Nov 24, 2015 at 17:15

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2 Answers 2