restricting vector using if else loop. How to get rid of false values from the matrix

Hi suppose I have a 100x3 matrix representing coordinates in xyz plane. I use an if loop to select particles with a certain x value and y value and create a new matrix.

Eg:

for i=1:length(matrix)   
    if matrix(i,1)>100 & matrix(i,1)<400 & matrix(i,2)>100 & matrix(i,2)<400
       matrix_new(i,1)=matrix(i,1);
       matrix_new(i,2)=matrix(i,2);
       matrix_new(i,3)=matrix(i,3);
   end
end

The new matrix I get has zeros for the conditions that are false. How can I modify this condition so that I get a matrix without zeros.

Thanks

Answer 1

[Walter Roberson]

mask = matrix(:,1)>100 & matrix(:,1)<400 & matrix(:,2)>100 & matrix(:,2)<400;
new_matrix = matrix(mask, :);