How to place a number in a random position in a matrix of zeros?

I have a matrix "m" of zeros(6,7) and I would like to know how to place a variable equaling 1 into a random spot in the matrix. For example, the "1" could be randomly placed in m(1,2) or m(5,4), etc. How would I code this to pick a random row and random column?

Thank you for your help!

Answer 1

[Star Strider]

I would use linear indexing, taking advantage of how matrices are stored in MATLAB, to use one number to define both the row and column:

M = zeros(6,7);
M(randi(numel(M))) = 1

M =
     0     0     0     0     0     0     0
     0     0     0     0     0     0     0
     0     0     0     0     1     0     0
     0     0     0     0     0     0     0
     0     0     0     0     0     0     0
     0     0     0     0     0     0     0

EDIT — You can do this with more lines of code, creating the linear index in one line, and using the ind2sub function to return the subscripts.

[Andrés Leonardo Mogollón Benavides]

man thanks a lot, a hero saving lives

[Andrés Leonardo Mogollón Benavides]

and what if i want to place a vector 1*3 in a random spot in the matrix. how would be the code

[Star Strider]

This should be a new Question, however since it is at least tangential to this one, I’ll post a solution.

For row vectors less than matrix column size:

M = zeros(6,7);
v = rand(1,3);
[r,c] = ind2sub([size(M,1) size(M,2)-numel(v)+1], randi(numel(M(:,1:end-numel(v)+1))));
M(r,c:c+2) = v

This does not do any checking on vector and matrix sizes, so add those as necessary. Make appropriate changes for column vectors.

A Vote would be appreciated!

[Andrés Leonardo Mogollón Benavides]

thanks master i have already vote

[Star Strider]

Thank you!

[R. Pam]

Hey, this is very helpful. Thanks. But what if I want the vector to be randomly shown as a row or a column? I'm quite confused, is it possible?

[Star Strider]

@R. Pam — It is possible. As I mentioned, it would be necessary to make the appropriate changes in the ind2sub call, reflecting the orientation (row or column) of the vector you want to insert. It would also be necessary to add code to check the sizes of the vector and the matrix to limit the initial index appropriately.

I leave that to you.