make a list based on two other lists

Question

I have two lists:

list1={4,6}
list2={1,2}

and I now want a function f that produces

f[list1_,list2_]:={1,1,1,1,2,2,2,2,2,2}

so 4 times the 1 and 6 times the 2. f should be generic so that

3list1={2,3,7}; 3list2={1,2,3}

yields

res={1,1,2,2,2,3,3,3,3,3,3,3}

If you know several ways to do this please provide the fastest one. thanks a lot.

Answer 1

f[list1_, list2_] := Inner[Table[#2, {#1}] &, list1, list2, Join];

Answer 2

list1 = {4, 6}
list2 = {1, 2}

ClearAll[f];
f = Join @@ MapThread[Table, {#2, List /@ #1}] &

f[list1, list2]
(* {1, 1, 1, 1, 2, 2, 2, 2, 2, 2} *)

f[{2, 3, 7}, {1, 2, 3}]
(* {1, 1, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3} *)

or

h = Join @@ (Table @@@ Transpose[{#2, List /@ #1}]) &
h[{2, 3, 7}, {1, 2, 3}]
(* {1, 1, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3} *)

or

g = Join @@ (ConstantArray @@@ Transpose[{#2, #1}])&

g2 = Inner[ConstantArray, #2, #1, Join] &

Timings:

bh = Inner[Table[#2, {#1}] &, #1, #2, Join] &; (* BobHanlon *)

{listA, listB} = RandomInteger[100, {2, 100000}];
(rs1 = bh[listA, listB]); // Timing
(* {0.265625, Null} *)
(rs2 = g[listA, listB]); // Timing
(* {0.062500, Null} *)
(rs3 = g2[listA, listB]); // Timing
(* {0.078125, Null} *)
(rs4 = h[listA, listB]); // Timing
(* {0.296875, Null} *)
(rs5 = f[listA, listB]); // Timing
(* {0.296875, Null} *)
rs1 == rs2 == rs3 == rs4 == rs5
(* True *)