How to store a variable no. of data points in a matrix?

Hello everyone. The question is..suppose i am using a for loop. At the end of each loop it is giving me a data point only if a certain condition is satisfied otherwise not. I want to store these data points in a k*2 matrix where value of k depends on the condition being satisfied or not.So the no. of data points generated is not previously known. Is is possible to store the data points in a matrix if we dont know the exact dimension of the matrix at the first place?? Please help! Thanx!!

Answer 1

[Jan]

Create the output with a maximum size at first to avoid the extremely time-consuming iterative growing of an array:

data  = zeros(N, 2);
index = 0;
for k = 1:N
  if FLAG  % Apply your condition here
    index = index + 1;
    data(index, 1:2) = rand(1,2);  % Your values to be stored
  end
end
data = data(1:index, :);  % Crop unused elements

This is much faster, when the size of the output is over a certain limit. Notice that for the iterative growing of a 1000x2 array, you have to obtain sum(1:1000)*2*8 bytes from the operating system and copy the same amount of data: more than 800kB althought the result has 16kB only.

[roro6992]

Yea it works nice. Thanx alot!!

Answer 2

[Mischa Kim]

Sure. Simply append the data point to the existing matrix of data points:

for ii = 1:N
  ...
  if FLAG     % if condition is true, append data point to matrix
     data = [data; data_point];
     ...
  end
end

Make sure that data_point is a 1x2 (row) vector.

[roro6992]

But i want the data matrix to be of the same dimension of the no. of selected data points. I mean if k points are selected the matrix wud be kx2.