Specifying an array with a specific number of copies of two elements

Question

To specify an array of length L in Mathematica, where each entry is someElement, I'm accustomed to writing:

exArray=Array[someElement &, L];

However, let's say that I want to (quickly) generate an array of length L where there are n copies of elementOne and L- n copies of elementTwo. Maybe elementOne is the string character A and elementTwo is the string character B. I'd like to be able to position the two elements in two different ways: (1) where the first n elements are all elementOne and the remaining elements are all elementTwo, and (2) where we have a uniform random sample from the set of all possible arrays where there are n copies of elementOne and L - n copies of elementTwo.

Are there simple "one-liners" to do (1) + (2)?

Answer 1

For requirement (1):

Join @@ ConstantArray @@@ {{"A", 5}, {"B", 3}}

{"A", "A", "A", "A", "A", "B", "B", "B"}

Then for requirement (2):

RandomSample[%]

{"A", "B", "A", "B", "A", "A", "B", "A"}

Or as a "one-liner:"

Join @@ ConstantArray @@@ {{"A", 5}, {"B", 3}} // RandomSample

Or as a function:

f[a_, b_, L_Integer, n_Integer] /; L >= n :=
  Join @@ ConstantArray @@@ {{a, n}, {b, L - n}}

Now:

f["A", "B", 7, 2] // RandomSample

{"B", "A", "B", "B", "A", "B", "B"}

Answer 2

Extended comment about ConstantArray vs Array:

a = ConstantArray[0, 10000000]; // AbsoluteTiming
(* {0.066234, Null} *)

MaxMemoryUsed[]
MemoryInUse[]
(* 102399424 *)
(* 102175120 *)

a =.;
a = Array[0 &, 10000000]; // AbsoluteTiming
(* {0.989285, Null} *)

MaxMemoryUsed[]
MemoryInUse[]
(* 342186712 *)
(* 102188984 *)

ConstantArray is faster and more memory efficient for long numerical arrays.

Answer 3

MapApply and Splice came with V 13.1 and

RandomSample @ MapApply[Splice @* Table] @ {{"A", 5}, {"B", 3}}

{"A", "A", "B", "A", "B", "B", "A", "A"}