How to find a number location in a multiple sub-matrices?

I have a combined matric "z" which is included 3 sub-matrices "a", "b" and "c". RNG is a random number from matrix z. So, I want to know that this RNG is belongs to which sub-matrices?

a = [2;3;4;5];
  b = [6;7;9;10;23];
  c = [56;32];

z = [a;b;c];
rand = randperm(size(z,1));
RNG = z(rand(1),:);

RNG = % belong to which sub-matrices?

Answer 1

[Walter Roberson]

   [tf, idx] = ismember(RNG, z);
   [count, binnum] = histc(idx, cumsum([1, length(a), length(b), length(c)]);
   varnames = {'a', 'b', 'c'};
   fprintf('value %d came from matrix %s\n', RNG, varnames{binnum});

Answer 2

[Geoff Hayes]

Mohammad - rand is the name of a built-in MATLAB function so you shouldn't be using it as the name of a local variable. As for which sub-matrix does the RNG value belong to, you can just use the index into z to determine that. For example, if

 z = [a;b;c];
 randValues = randperm(size(z,1));
 RNG = z(randValues(1),:);

then the index into z is

 randIndex = randValues(1);

Now if we consider the number of elements of each sub-matrix, then we can determine the upper bound on each sub-matrix with respect to the index into z as follows

 subMatricesIdcsUpperBounds = cumsum([length(a) length(b) length(c)]);

which is

 subMatricesIdcsUpperBounds =
     4     9    11

We can interpret the above as a corresponding to indices 1 through 4 into z, b corresponding to indices 5 through 9 of z, and c corresponding to indices 10 through 11 of z.

Then we just do

 find(randIndex <= subMatricesIdcsUpperBounds,1)

which returns an index k which tells us that RNG can be found in the kth sub-matrix.