List rearranged

Question

I have a very long list from which I will display part of it as,

a={{1, {3, 5, 8, 9}}, {2, {4, 6}}, {6, {1}}, {10, {1, 3, 5, 6, 7, 8}}};

For plotting purposes, I would like to rewrite the list in the following form

a = {{1, 3}, {1, 5}, {1, 8}, {1, 9}, {2, 4}, {2, 6}, {6, 1}, {10, 
1}, {10, 3}, {10, 5}, {10, 6}, {10, 7}, {10, 8}} ; ListPlot[a];

Since the actual list is extremely long, I appreciate it if you can help me in doing this. Please keep in mind that the first number is associated with one or more other numbers.

Thanks in advance.

Answer 1

Using Join and Thread:

Join @@ Thread /@ a
(* {{1, 3}, {1, 5}, {1, 8}, {1, 9}, {2, 4}, {2, 6}, {6, 1}, {10, 1}, {10, 3}, {10, 5}, {10, 6}, {10, 7}, {10, 8}} *)

Answer 2

I am sure there are at least 10 ways to do this in Mathematica. One that comes to mind now is

a={{1, {3, 5, 8, 9}}, {2, {4, 6}}, {6, {1}}, {10, {1, 3, 5, 6, 7, 8}}};
Flatten[Cases[a, {x_, {y___}} :> ({x, #} & /@ {y})], 1]

gives

{{1, 3}, {1, 5}, {1, 8}, {1, 9}, {2, 4}, {2, 6}, {6, 1}, {10, 1}, {10,
   3}, {10, 5}, {10, 6}, {10, 7}, {10, 8}}

Answer 3

Map[Splice@*Thread] @ a

{{1, 3}, {1, 5}, {1, 8}, {1, 9}, {2, 4}, {2, 6}, {6, 1}, {10, 1}, 
 {10, 3}, {10, 5}, {10, 6}, {10, 7}, {10, 8}}

Also

Splice @* Thread /@ a  (* thanks: LukasLang *)

{{1, 3}, {1, 5}, {1, 8}, {1, 9}, {2, 4}, {2, 6}, {6, 1}, {10, 1}, 
 {10, 3}, {10, 5}, {10, 6}, {10, 7}, {10, 8}}

For versions older than 12.0 replace Splice with Apply[Sequence].

Answer 4

a//Table[Splice@Distribute[#[[i]],List], {i,1,Length@#}]& 

(* {{1, 3}, {1, 5}, {1, 8}, {1, 9}, {2, 4}, {2, 6}, {6, 1}, {10, 1}, {10,   3}, {10, 5}, {10, 6}, {10, 7}, {10, 8}}  *)