How to store two variables (x.y) from a loop?

Question

I am generating two different coordinates (x, y) within a loop. In my code, I have just realised that it is saving the last variable from the loop. I am, however, trying to save all the iterations from the setsize variable. I already tried to save using something like:

circleposition = [0:length(setsize) x(i),y(i)];

But, it seems that I am not doing it correctly, getting the following error:

Subscript indices must either be real positive integers or logicals.- 
Error using vertcat
Dimensions of matrices being concatenated are not consistent.

Here is my original code:

setsize = 9; 
r = 340;
cx = 500;
cy = 500;
anglesegment = 2 * pi/setsize;
circleposition = []; 

for i = drange (0:setsize)
    x = r * cos(i*anglesegment) + cx;
    y = r * sin(i*anglesegment) + cy;
    circleposition = [x,y];
end

Output:

circleposition =
             0    1.0000
      840.0000  500.0000

It runs only with the first/last iteration. I need to get 9 x's and 9 y's (depending the setsize, variable).

Eliahu Aaron · Answer 1 · 2019-05-15T10:54:21.157

Hear is code that works:

setsize = 9; 
r = 340;
cx = 500;
cy = 500;
anglesegment = 2 * pi/setsize;
circleposition = zeros(setsize + 1, 2); % Changed from circleposition = []

for i = drange (0:setsize)
    x = r * cos(i*anglesegment) + cx;
    y = r * sin(i*anglesegment) + cy;
    circleposition((i+1),:) = [x,y]; % Changed from circleposition = [x,y];
end

Explanation:

The fix was Changing circleposition = [x,y]; to circleposition((i+1),:) = [x,y]. Without ((i+1),:), you are changing the data of circleposition, not adding to it.

Changing circleposition = []; to circleposition = zeros(setsize + 1, 2); was not required, its just recommended to allocate memory for speed, not an issue for small number of elements.

It'd be good to also explain why the OP code didn't work (what was wrong with it). — am304, May 15 '19 at 10:48

score 1 · Accepted Answer · answered May 15 '19 at 10:50

It's kind of hard to follow, which error message comes from which attempt, but let's have a look.

I don't have access to the Parallel Computing Toolbox, which seems necessary to use the for-loop over distributed range drange, but I assume, this loop can be replaced by for i = 0:setsize for testing.

Now, when starting at i = 0, you would try to access x(0) and y(0), which is not allowed (Subscript indices must either be real positive integers or logicals). Also, you would get 10 values instead of 9, as you stated in your question. So, let's start at i = 1.

To store all 9 pairs of x and y, your circleposition should be an 9 x 2 array So, initialize that by, for example, circleposition = zeros(setsize, 2).

Last, you need to use proper indexing to store [x, y] at the i-th row of circleposition, i.e. circleposition(i, :).

So, the corrected code (attention on the replaced drange part) could look like this:

setsize = 9; 
r = 340;
cx = 500;
cy = 500;
anglesegment = 2 * pi/setsize;
circleposition = zeros(setsize, 2);     % Initialize circleposition appropriately

for i = 1:setsize                       % Start at i = 1
    x = r * cos(i*anglesegment) + cx;
    y = r * sin(i*anglesegment) + cy;
    circleposition(i, :) = [x, y];      % Correct indexing of the row
end 

circleposition                          % Output

The output would then be:

circleposition =
   760.46   718.55
   559.04   834.83
   330.00   794.45
   180.50   616.29
   180.50   383.71
   330.00   205.55
   559.04   165.17
   760.46   281.45
   840.00   500.00

On the second error (Error using vertcat. Dimensions of matrices being concatenated are not consistent.): I don't see, where you used vertical concatenation at all!?

How to store two variables (x.y) from a loop?

2 Answers2