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How to extract specific values from a table

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karishma koshy
karishma koshy on 15 Jul 2019
Commented: Adam Danz on 17 Jul 2019
Dear All I'm aiming to measure the distance between all points in one frame and all points in the next frame. So I want to Create a table (let’s call this “table i”)which only contains points in frame i (where “i" is the index of that frame) Create another table (“table i+1”), which only contains points in frame i+1 Create a 2D distance matrix (width equal to the number of points in frame i and height equal to the number of points in frame i+1) where each element is the Pythagoras distance between two points. How can I implement this as a code ? Thank you

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Adam Danz
Adam Danz on 17 Jul 2019
There are 30 coordinates in each frame. Let's call them c1,c2, c3....c30. Do you want the distance between c1 in frame 1 and [c1:c30] in frame 2? And do you want those distances for all points and between all 1000 frames?
I ask these questions because I'm unsure how you want to organize the data.
For example, do you want a [30 x 30 x 1000] array named A where A(i,j,k) is the distance between xy(i) and xy(j) from frame (k-1) to frame (k)?
This should be well defined before moving forward.
Also, you'll use pdist() or pdist2().
karishma koshy
karishma koshy on 17 Jul 2019
Yes I want the distance between c1 in frame 1 to c1:c30 in frame 2. Then I want that differences that to be assigned in the form of a 2D distance matrix (width equal to the number of points in frame i and height equal to the number of points in frame i+1) where each element is the Pythagoras distance between two points. I want to how I can use nested for loop for this.
Right now I only need frame 1 and frame 2 alone.
Thank you so much in advance for your help.
Adam Danz
Adam Danz on 17 Jul 2019
Following my example in my answer, here's how to calculate the distance between point 'p' in frame 'f' to all points in frame 'f+1'.
The example shows distances between the 3rd point of frame 4 and all points in frame 5.
f = 4; %frame number 1:1000
p = 3; %point number (within frame) 1:30
% Find distance between point p of frame f and all points in frame f+1
fIdx = T.Frame == f; %index for frame f
f2Idx = T.Frame == f+1; %index for frame f+1
pIdx = max(find(fIdx,p)); %index of point p in frame f
d = pdist2([T.centreX(pIdx),T.centreY(pIdx)],[T.centreX(f2Idx),T.centreY(f2Idx)])';
Note that d(p) should be 0 since the (x,y) coordinates do not change between frames.

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Accepted Answer

Adam Danz
Adam Danz on 15 Jul 2019
Edited: Adam Danz on 16 Jul 2019
If I understand your question correctly, you want to measure the distance each (x,y) coordinate travels between frames. Is that correct? It looks like the (x,y) centers of your circles do not change between frames. For example, the first (x,y) coordinate at frame 1 is (144.09, 131,56) and this coordinate is the same for frame 2, 3, etc... When I run the code below, the distance is always 0. Maybe I'm not understanding your problem.
%% Import data and store in table "T"
opts = spreadsheetImportOptions("NumVariables", 4);
opts.Sheet = "in";
opts.DataRange = "A2:D30001";
opts.VariableNames = ["centreX", "centreY", "Radius", "Frame"];
opts.SelectedVariableNames = ["centreX", "centreY", "Radius", "Frame"];
opts.VariableTypes = ["double", "double", "double", "double"];
T = readtable("C:\Users\adanz\Documents\MATLAB\savehere\matlabCentralDocs_trash\DataTable_SER.xlsx", opts, "UseExcel", false);
%% Calculate distance between frames
nFrames = max(T.Frame);
distFcn = @(x1,x2,y1,y2)sqrt((x2-x1).^2 + (y2-y1).^2); %distance function
dist = nan(sum(T.Frame == 1), nFrames-1); %assumes there are an equal number of frames for all frames
for i = 2:nFrames
% Index for frame i
idx2 = T.Frame == i;
% Index for frame i-1
idx1 = T.Frame == i-1;
% Calculate distance
dist(:,i-1) = distFcn(T.centreX(idx2),T.centreX(idx1),T.centreY(idx2),T.centreY(idx1));
end

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