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I would like to generate a matrix whose dimension is L^2 and whose entries can be only in a finite set but in random positions in the matrix.

such as

 M[b_]:=RandomReals[{0,b},{L,L}]

where b is an integer. However the latter gives random values in the range 0 to b, while I want only entries that can be only 0 or b....namely several b distributed randomly in the matrix and all the other elements 0.

Is this possible?

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You need RandomChoice,

RandomChoice[{0, b}, {4, 3}]
(* {{0, 0, 0}, {0, b, 0}, {b, b, 0}, {0, 0, b}} *)
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For the case with two elements 0 and b you can also use:

b RandomInteger[1, {4, 4}]

{{0, 0, b, 0}, {0, 0, b, b}, {b, b, 0, 0}, {0, b, 0, 0}}

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  • $\begingroup$ Thank you so much! $\endgroup$ – Galuoises Feb 5 '18 at 19:26
  • $\begingroup$ @Galuoises, my pleasure. $\endgroup$ – kglr Feb 5 '18 at 19:32

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