How to find the frequency of each row with certain coordiante in a matrix?

Hello all,

I have an mx2 matrix with its rows (0,1), (-1,0), (0,1), (0,-1), (1,1), (-1,1), (1,-1),(-1,-1); I would like to find the frequency of each of above coordinates. In other words, if I have A=[1 1;0 1;-1 1;1 0;-1 1], I would like to get something like,

number of times that (1,1) has appeared=1; number of times that (0,1) has appeared=1; number of times that (-1,1) has appeared=2; number of times that (1,0) has appeared=1; number of times that (0,-1) has appeared=0; number of times that (-1,-1) has appeared=0; number of times that (-1,0) has appeared=0; number of times that (1,-1) has appeared=0;

when I use "find" command I get an error. Thank you.

Answer 1

[Azzi Abdelmalek]

A=[1 1; 0 1; 1 0; -1 1;-1 1;0 1;0 1]
[ii,jj,kk]=unique(A,'rows','stable')
f=histc(kk,1:numel(jj)); % Frequency
result=[ii f]

[Mnr]

Thanks, but even if I put "A" instead of "out", I have an error. Could you please help me?

[Azzi Abdelmalek]

Ok use this (without stable option), maybe you have an old version of Matlab

A=[1 1; 0 1; 1 0; -1 1;-1 1;0 1;0 1]
[ii,jj,kk]=unique(A,'rows')
f=histc(kk,1:numel(jj)); % Frequency
result=[ii f]

[Mnr]

Thank you so much! I really appreciate your help!

Answer 2

[Aditya Rathore]

There are two things:

1. Your row length is small (2)
2. You want to also retrieve the stored values

I suggest you make a matrix for storing frequencies

freqs = zeros(m, 2);
[rows, ~] = size(A);
for idx=1:rows
    i = A(idx, 1);
    j = A(idx, 2);
    freqs(i+2,j+2) = freqs(i+2, j+2) + 1; %Because you have -1 also
end

When retrieving a value, simply do

count = A(i+2, j+2);

This approach is useful as you can normalize the counts and get probability using

freqs = freqs / sum( freqs, 'all');

I tried it for image based task in RGB space and found this to be fastest solution.