How can I properly loop through a simple 2D matrix? - matlab

Question

I don't understand why I can't find any clear answer on this! Such a basic thing...
I have a normal 2D matrix, such as this one:
matrix = zeros(w, h);
Where w, h ∈ Z+, which means w and h are positive integers greater than zero.
I need to loop through the matrix and do something with it's element. For this task, we can assume h=3. Therefore, I tried this:
test = zeros(w, 3);
for i = 1:test
point = test(:,i);
[point(0), point(1), point(2)] = MY_CONVERSION(point(0), point(1),point(2));
test(:,i) = point;
end
The MY_CONVERSION is varargin function and for 3 parameters defined as:
function [rho, alpha, z] = MY_CONVERSION(r, alpha, epsilon)
Of course, in real code the test contains data, not just zeros. But this code alone could, for example, fill matrix with random numbers. But it doesn't work.
I also tried using three variables to store the column values:
for i = 1:Xi
[a,b,c] = Xi(:,i);
[a,b,c] = mysph2cyl(a, b, c);
rho(:,i) = a,b,c;
end
Matlab marks it as incorrect syntax:
An assignment statement makes an assignment to multiple values, but
the expression on the right side of the assignment does not appear to
produce multiple values.

Answer 1

The first piece of code is what you need to get it working. However, your for loop is incorrect. You probably want to iterative over all of the columns, so do for i = 1 : size(test,2), not test. size(test,2) determines how many columns your matrix has.
Therefore:
test = zeros(w, 3);
for i = 1:size(test,2) %// Change here
point = test(:,i);
[point(0), point(1), point(2)] = MY_CONVERSION(point(0), point(1),point(2));
test(:,i) = point;
end
Also, your second piece of code, you need to fix that for loop problem like I mentioned above. The first statement inside the for loop is superfluous. You don't need it. The third line of code needs to have [] surrounding a,b,c.

Answer 2

First, matrix = zeros(w, h); creates confusion: you are probably thinking of w as width and h as height. But the first argument of zeros is height: for example, zeros(2,3) is
0 0 0
0 0 0
Generally: row index first, column index second.
Then you have for i = 1:test but test is a matrix. You need a number here.
Here is a working example similar to your first fragment:
w = 2; h =3;
test = zeros(w, h);
for i = 1:h
point = test(:,i);
point = cos(point)*i; % sample conversion
test(:,i) = point;
end
Output:
1 2 3
1 2 3