How to take the co-ordinate values in MatLab

A = [1 0  3  3;
     0 0  4  5;
     5 0 98 56; 
     0 2  4  6]

In the above 4*4 matrix, each cell has got x and y coordinates. I need to extract the x and y co-ordinate values for the cells excepting 0.

For example, the coordinates are placed in the below manner having the zero cell omitted

x 1 1 1,....
y 1 3 4,....

Any leads?

[Adam Danz]

Where are the X and Y values within the A matrix? Another way of asking that question, which values of A correspond with X and which with Y?

[vigneshwaran Loganathan]

PFA, if you plot the matrix, the output will have x,y and the index values. I have to arrange the co-ordinates in below manner leaving the index values which is zero

x 1 1 1,.... y 1 3 4,....

[Adam Danz]

I see, so the values in 'A' are all y values and the row indices are the x values. I'll post an answer in a few minutes.

[Image Analyst]

Vigneshwaran, what a bad way to name variables! Why deliberately choose variable names that fly in the face of hundreds of years of convention? You're choosing x to be the vertical/row coordinate instead of the conventional y. And you're choosing the horizontal/column coordinate of the matrix to be y, instead of the conventional x. Why? Why do it the opposite of everyone else in the world for no reason???

By the way, if you had given the full x and y instead of just the first three numbers and ..., you could have prevented a lot of confusion.

Answer 1

[Nanditha Nirmal]

Hi,

The code below will give you the coordinates you want. This is what you would want to do:

A = [1 0 3 3;0 0 4 5;5 0 98 56; 0 2 4 6];
x=[];
y=[];
for i=1:4
  for j=1:4
      if A(i,j) ~= 0
          x=[x ;i];
          y=[y ;j];
      end
  end
end

[vigneshwaran Loganathan]

Thank alot, it helped

[Adam Danz]

You can avoid both for-loops and speed up the code with

%Your data (y values)
A = [1 0 3 3;0 0 4 5;5 0 98 56; 0 2 4 6];

% Transpose A (to match your example)
aT = A'; 
% Your x values
x = repmat((1:size(aT,1))', 1, size(aT,2));
y = repmat(1:size(aT,1), size(aT,2), 1);
% Identify where A==0
zeroIdx = aT==0; 
% Extract coordinates where A~=0
xCoord = x(~zeroIdx); 
yCoord = y(~zeroIdx);

Answer 2

[Adam Danz]

If I'm understanding correctly, your matrix A are all y values and the row indices are the X values (based on your comments above).

%Your data (y values)
A = [1 0 3 3;0 0 4 5;5 0 98 56; 0 2 4 6];

% Your x values
x = repmat((1:size(A,1))', 1, size(A,2)); 

% Identify where A==0
zeroIdx = A==0; 

% Extract coordinates where A~=0
xCoord = x(~zeroIdx); 
yCoord = A(~zeroIdx); 

% Test: draw your data, circle the chosen coordinates
figure; 
plot(A); 
hold on
plot(xCoord, yCoord, 'ko')

[vigneshwaran Loganathan]

Thanks for your time, the above answer by Nandita solved my case Your code gives me the index value

[Adam Danz]

Nanditha's code gives you the index values. The for-loops in her code record the values of i and j which are index values of A. My code provides the index values for x and the 'A' values for y which is what I thought you were describing by, " if you plot the matrix, the output will have x,y and the index values".

See the comment I left under Nanditha's solution for a simplification.

Answer 3

[Image Analyst]

Personally I'd use meshgrid() - a useful function that you might want to learn about:

[rows, columns] = size(A)
[x, y] = meshgrid(1:columns, 1:rows)
mask = A~=0
x = x(mask)
y = y(mask)

[Adam Danz]

Vigneshwaran, this is the fastest, simplest solution. However, if you want it to conform to your example, you'll need to transpose A and switch the values of x and y.

aT = A'; 
[rows, columns] = size(aT)
[y, x] = meshgrid(1:columns, 1:rows)
mask = aT~=0
x = x(mask)
y = y(mask)

But reconsider the names of your x and y variables as suggested by Image Analyst above.

[vigneshwaran Loganathan]

Thanks both, it helped and i will consider the suggestion too