Find values in array by meshgrid coordinate and assign to correct row in table without for loop
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Hi, I have the the following variables attached below.
XY is a table where each row has a X and Y coordinate of a meshgrid. I would like to take those coordinates and take the corresponding ImRef value at that coordinate and put in the XY.int column.
XY.int already contains the values I want to obtain from ImRef. I did this using a for loop but it takes extremely long and I hope there is a quicker method.
I have tried ismember but it doesn't retain the correct sequence to append it into my table.
Current (slow) approach:
for x = 1:1:height(XY)
XY.int(x) = ImRef(XY.xy(x)==xymesh);
end
EDIT: Added matlab.mat containing the other files I mentioned.
Thanks.
2 Comments
What is IMREF? Your data does not contain it:
S = load('XY.mat')
T = S.XY
Maximilian Rowe
on 6 Mar 2025
Accepted Answer
"XY is a table where each row has a X and Y coordinate of a meshgrid."
I'll take that to mean that XY.x and XY.y are drawn from two matrices X and Y that have meshgrid format.
"I would like to take those coordinates and take the corresponding ImRef value at that coordinate and put in the XY.ImRef column."
I'll take that to mean that in addition to those X and Y meshgrid matrices I speculated about, you have a third matrix ImRef, of the same size as X and Y, and what you'd like to do is to add a new variable/column called 'ImRef' to the XY table, which column contains values from matrix ImRef where points (XY.x, XY.y) are in matrices (X, Y).
If that's all true, then here's a way to do it:
% first, build X and Y matrices that might or might not be your
% meshgrid-format matrices from which XY.x and XY.y are drawn
XY = load('XY','XY').XY;
UX = unique(XY.x);
UY = unique(XY.y);
[X,Y] = meshgrid(UX,UY);
% take a look at the table (x,y) and the proposed source grid (x,y)
figure
hold on
plot(XY.x,XY.y,'.')
plot(X(:),Y(:),'.','MarkerSize',2)
% make up an ImRef matrix, same size as X and Y
ImRef = X.*Y;
% now find which table (x,y) exist in the meshgrid (x,y) and where
[ism,idx] = ismember([XY.x,XY.y],[X(:),Y(:)],'rows');
% make a new table variable 'ImRef' initially containing all NaNs
XY{:,'ImRef'} = NaN;
% put the values in place from the ImRef matrix
XY.ImRef(ism) = ImRef(idx(ism))
% in this case since I made ImRef = X.*Y, perform a sanity check that
% XY.ImRef == XY.x.*XY.*y, i.e., that the X*Y==ImRef relationship holds
% at the points in the table
isequal(XY.x.*XY.y,XY.ImRef) % good
6 Comments
Maximilian Rowe
on 6 Mar 2025
Voss
on 6 Mar 2025
"Thank you for your response."
You're welcome!
"the desired output is what is in the XY.int column."
OK, but I don't have the ImRef that XY.int comes from (if XY.int comes from ImRef).
"I don't want the product of the coordinates."
I realize that.
"I want the value of ImRef at those coordinates."
Thank you for clarifying.
I don't have your ImRef or know what it is. I made up an ImRef to use. For illustration I constructed it as the product of X and Y, but I could've used random values.
If the assumptions I made are valid, then the code will work; you need to use your X, Y, ImRef matrices.
Maximilian Rowe
on 6 Mar 2025
"ImRef array is attached, no?"
I don't see it in the mat file
S = load('XY.mat')
head(S.XY)
If you intended to upload a new mat file and replace the old one when you edited the question, it looks like that didn't work for whatever reason (when I select the mat file to attach it this comment the site says the file is from 2 hours ago - the original posting time of the question).
In any case, glad it's working now! If you have any questions, please let me know.
Checking with the updated mat file:
load('matlab.mat')
whos
[ism,idx] = ismember([XY.x,XY.y],[xmesh(:),ymesh(:)],'rows');
XY{:,'ImRef'} = NaN;
XY.ImRef(ism) = ImRef(idx(ism))
% verify XY.ImRef just populated is the same as int in the original table
XY_old = load('XY.mat','XY').XY;
isequal(XY_old.int,XY.ImRef)
Maximilian Rowe
on 6 Mar 2025
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