Translating vector into locations in a matrix

I'm attempting to set up an n row by 4 column matrix using data from column vectors Z1, Z2, & Z3. The problem is I need to place the random values from these column vectors into random locations in the n row by 4 column matrix. How can I specifically put certain values from the column vectors into certain locations in the K matrix? I know the K matrix will be always 4 columns wide, but could be any length. The rows of the K matrix will repeat every 4 utilizing a different value from one of the Z1, Z2, & Z3 column vectors. (I'm trying to come up with a local stiffness matrix that I can later transform into a global stiffness matrix). In the following code I'm attempting to set up a structure where the every 4th row in column 1 of Matrix K is row 1 - Z2(1,1), row 5 - Z2(5,1), row 9 - Z2(9,1), and so on. Any help would be appreciated.

 K = zeros(4*member_count,4);
    for r = 1:4:4*member_count;
        for i = 1:Z2_count;
            s(i,r) = Z2(i,1);
            for j = 1:r
                K(j,1) = s(i,r)                
            end            
        end
    end

Answer 1

[Cedric]

If you need to generate random indices that can appear multiple times, you'll be interested in RANDI. Ex.:

 >> randi(10, 1, 10)
 ans =
     1     9    10     7     8     8     4     7     2     8

you see here that 8 appears three times, and 5 doesn't appear.

If you need to shuffle indices randomly so you essentially have a random filling of K but indices must be unique, you'll be interested in RANDPERM. Ex.:

 >> randperm(10)
 ans =
     2     7     3    10     4     1     9     5     6     8

you see here that all integers between 1 and 10 are present, but they were permuted randomly.

I don't fully understand what you want to do, but here one way to perform the last bit of your statement without a loop:

 >> Z2 = (10:10:100).'              % Simple example.
 Z2 =
    10
    20
    30
    40
    50
    60
    70
    80
    90
   100

 >> idx = 1:4:numel(Z2)
 idx =
     1     5     9

 >> K = zeros(numel(Z2), 4)
 K =
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0

 >> K(idx,1) = Z2(idx)
 K =
    10     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
    50     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
    90     0     0     0
     0     0     0     0

[Mark]

Thank you Cedric. This absolutely answered my question. I ended up using it multiple times in my situation.

K = zeros(4*member_count,4);
idx1 = 1:4:4*member_count;
K(idx1,1) = Z2;
K(idx1,2) = Z3;
K(idx1,3) = -Z2;
K(idx1,4) = -Z3;

[Cedric]

Great, I'm glad it helped!