mjhjmtu

I want to combine two list in the following way without using Table or any loop structure.

genList[n_] := Table[RandomInteger[1, 10, 4], n] (* list generating function*)

The two lists that needs to be combined are:-

list1 = genList[3]
list2 = genList[4]

What I want to achieve is as follows:-

Partition[
 Flatten[
 Table[list1[[i]], list2[[j]], i, 1 
 Length[list1],j,1 Length[list2]]
 ]
 ,8]

So I simply need to enumerate each of the elements of list 1 combined with all of the elements of list 2.

What would be an efficient way of doing this with large lists?

Let's say with Length[list1] = 100 and Length[list2] = 200.

Also Length[list2] > Length[list1]

Join @@@ Tuples[list1, list2] (* or *)
Flatten /@ Tuples[list1, list2] (* or *)
Join @@ Outer[Join, list1, list2, 1]

5, 3, 5, 4, 9, 2, 10, 6, 5, 3, 5, 4, 10, 6, 4, 7,

5, 3, 5, 4, 2,
1, 7, 1, 5, 3, 5, 4, 6, 6, 1, 9,

2, 6, 9, 5, 9, 2, 10, 6, 2,
6, 9, 5, 10, 6, 4, 7,

2, 6, 9, 5, 2, 1, 7, 1, 2, 6, 9, 5, 6, 6,
1, 9, 10, 8, 9, 6, 9, 2, 10, 6,

10, 8, 9, 6, 10, 6, 4, 7, 10,
8, 9, 6, 2, 1, 7, 1, 10, 8, 9, 6, 6, 6, 1, 9

This is a variant of kglr's approach that employs ArrayFlatten instead of mapping Flatten. Notice also that I changed genList so that it produces packed arrays; this is crucial for performance.

genList[n_] := RandomInteger[1, 10, n, 4];
m = 100;
n = 100;
list1 = genList[m];
list2 = genList[n];

a = Partition[Flatten[Table[list1[[i]], list2[[j]], i, 1 Length[list1], j, 1 Length[list2]]], 8]; // RepeatedTiming // First
b = Flatten /@ Tuples[list1, list2]; // RepeatedTiming // First
c = ArrayReshape[
 Tuples[list1, list2],
 Length[list1] Length[list2], Dimensions[list1][[2]] + Dimensions[list2][[2]]
 ]; // RepeatedTiming // First
a == b == c

0.026

0.0012

0.000095

True