Storing value in a matrix from a for loop.

Hi all, I'm quite new to matlab and coding in general, and I'm having problems getting my code to do what I want it to do. I'm trying to look at 1 lon and lat then for all the next lon and lats check to see if they're a small enough distance apart. I want it then to put all the answers that are within that distance into a row of a matrix then move to the next row once the inner for loop has ended. The problem I'm having is that the output of closeIx is just the entries "i" on a diagonal and zero's everywhere else (an identity matrix). I'm not sure how to make it so it actually put the incecies of the entries that are within that delta tolerance in the row. I'm sorry if its hard to run through without the loaded file, I'm not sure if I can put that in here someway, but let me know if there's any further things I can provided to better understand the question I'm asking. Thanks so much, heres my code:

load trackmat.mat
%File with latitudes and longitudes that are used in the study its 7949 x 1
%double 
size = length(Lat);
closeIx = zeros(size,size);
delta = 0.05;
thisIx = 1;
for i = 1:size-1
     %current latitude and longitude for particle we're looking at

    currlat = Lat(i,1);
    currlon = Lon(i,1);
    
    for j = 1+i       
   
    %latitude and longitude of the other points the particle goes to other than the current one
        nextlat = Lat(j);
        nextlon = Lon(j);

        difflat = abs((nextlat-currlat));
        difflon = abs((nextlon-currlon));
    
        if difflat<delta && difflon<delta
            closeIx(i,thisIx) = j;
        end
        
        thisIx = thisIx + 1;
    end 
end

Answer 1

[Torsten]

Don't use "size" as a variable name - it's a MATLAB - reserved function:

https://uk.mathworks.com/help/matlab/ref/double.size.html

Maybe like this ?

Or better a distance matrix d with

d(i,j) = max(abs(Lat(i)-Lat(j)),abs(Lon(i)-Lon(j))) 

?

load trackmat.mat
%File with latitudes and longitudes that are used in the study its 7949 x 1
%double 
s = numel(Lat);
closeIx = zeros(s);
delta = 0.05;
for i = 1:s-1
     %current latitude and longitude for particle we're looking at
    currlat = Lat(i);
    currlon = Lon(i);
    thisIx = 0;
    
    for j = i+1:s      
   
    %latitude and longitude of the other points the particle goes to other than the current one
        nextlat = Lat(j);
        nextlon = Lon(j);
        difflat = abs((nextlat-currlat));
        difflon = abs((nextlon-currlon));
    
        if difflat<delta && difflon<delta
            thisIx = thisIx + 1;
            closeIx(i,thisIx) = j;
        end
        
    end 
end

[Sophie]

Hi, thanks so much for the answer. That worked in fixing my problem, thank you!