find the last non-zero column in matrix in pre-allocated matrix

Hello, My matlab code is creating very very matrix and I didn't pre-allocate it before. Because I always need to know the size of matrix. However, due to memory problem I had to make a very large zero matrix and fill in the matrix with new value. Any my question is to how to find the last non-zero column in that matrix "in the most efficient way". for exam if I have a matrix M = zeros(10^9,6) and there is some value like the follwign matrix how to find row 6 as the answer. Thank you,

M= [3388 3279 3284 3383 3279 3278; 3388 3383 2043 2038 2038 0; 3387 3388 2038 2037 2032 2037; 3388 3389 3384 3383 2038 0; 3388 3279 3280 3389 3274 3279; 0 0 0 0 0 0; 0 0 0 0 0 0]

Answer 1

[Mohammad Abouali]

1) if you are looking for las non-zero column why the answer to your example is row 6?

2) if you are looking for last non-zero row, shouldn't the answer to your example be 5? in that case find(any(M,2),1,'last') would give 5 as the answer

Answer 2

[Geoff Hayes]

Mahsa - try using the any function to determine which rows have at least one non-zero element as

 nzRows = any(M,2);

nzRows will be a vector of ones and zeros where a one indicates that the row has at least one non-zero element, and a zero indicates that all elements in the row are all zero. The use find to find the last element in nzRows that is a one. Type doc find for details on how to use this function.

Answer 3

[Mahsa]

another way is to delete the zeros

if m = [ 1 2 3 4 5 0 0 0 0 0 0 0 0] m(m==0)=[] m= [1 2 3 4 5];

However my question is what to do if zero is one of the real values like the following example:

m = [ 1 2 0 3 4 0 5 0 0 0 0 0 0 0 0];

I'm looking for a command that gives me

m = [ 1 2 0 3 4 0 5]

Thank you

Answer 4

[Mahsa]

Oh I see it can be answered by find(any(M,1),1,'last')