Operations with arrays in Matlab

Question

I have this 2-dimensional array

x=[62,29,64;
    63,31,62;
    65,29,60;
    63,29,62;
    63,31,62;];

1st element in each column is R, 2nd is G, 3rd is B from the formula below.

I would like a function to compute the following operation:

So far, my function definition looks like this:

function[distance]=RGB_dist(x,y)

  distance=sqrt(sum(((x-y)*[3;4;2]).^2,2));

end

Tested with the matrix above, disp(RGB_dist(x,x)) outputs only zeroes. That must happen because he is calculating the distance between same vectors. How do I do to calculate the distance between any two vector(lines) from my matrix. Any help would be appreciated.

Answer 1

For two arbitrary lines (e.g. line 1 and line 2) do:

RGB_dist(x(1,:), x(2,:))

if you want all the combinations then check out pdist2. If you don't have the stats toolbox (i.e. no pdist2) then use nchoosek to create all possible pairs of rows:

I = nchoosek(1:size(x,1),2);
D = RGB_dist(x(I(:,1),:), x(I(:,2),:))

by the way, if you want to use pdist or pdist2 then you will have to alter your function to be able to find the distance between one observation and a list of observations in one shot. The easiest way is to replace your - with bsxfun like this:

sqrt(sum(((bsxfun(@minus,x,y))*[3;4;2]).^2,2));

then you can go

D = pdist(x, @RGB_dist)

both methods give you

D =

     7
     1
     1
     7
     6
     8
     0
     2
     6
     8

or

squareform(D)

ans =

     0     7     1     1     7
     7     0     6     8     0
     1     6     0     2     6
     1     8     2     0     8
     7     0     6     8     0

But this is probably wrong as this squares after multiplying by the weights. You probably don't want that so I think your final function should look like that in Divakar's answer