Repeat elements in list, but the number of times each element is repeated is provided by a separate list

Join @@ MapThread[Table, {A,B}]

{1, 1, 1, 2, 3, 3, 3, 3, 4, 4}

Join @@ Table @@@ Transpose @ {A,B}

{1, 1, 1, 2, 3, 3, 3, 3, 4, 4}

Join @@ MapThread[ConstantArray, {A, B}]

{1, 1, 1, 2, 3, 3, 3, 3, 4, 4}

Also

Internal`RepetitionFromMultiplicity @ Transpose[{A, B}]

{1, 1, 1, 2, 3, 3, 3, 3, 4, 4}

kglr's proposals are nice, and the last (undocumented) one is very nice. As a variation, here is a solution using Inner[] + Flatten[]:

Flatten[Inner[ConstantArray, {1, 2, 3, 4}, {3, 1, 4, 2}, List]]
   {1, 1, 1, 2, 3, 3, 3, 3, 4, 4}

As kglr notes, a shorter version is

Inner[ConstantArray, {1, 2, 3, 4}, {3, 1, 4, 2}, Join]

or in version 10 and later,

Inner[Table, {1, 2, 3, 4}, {3, 1, 4, 2}, Join]

a = {1, 2, 3, 4};

b = {3, 1, 4, 2};

Using ConstantArray

c = Flatten[ConstantArray[#[[1]], #[[2]]] & /@
   Transpose[{a, b}]]

(* {1, 1, 1, 2, 3, 3, 3, 3, 4, 4} *)

or using Table

c = Flatten[Table[#[[1]], {#[[2]]}] & /@
   Transpose[{a, b}]]

(* {1, 1, 1, 2, 3, 3, 3, 3, 4, 4} *)

Related