Question

I want to write such a code for two matrixes:

x=[1 1 1 2 2 3 3 3 3]';
y=[11 21 31 24 32 33 13 37 3]';

How can I find out the mean value of each group of numbers in y, that have the same index in x?

My algorithm can look like:

If     x(1)=x(2) counter1=1 sum1=y(1)+y(2) 
       x(2)=x(3) counter1=2 sum1=sum+y(3)  Define  newx1=x(1) newy1=sum1/counter1
       x(3)<>x(4) 
       x(4)=x(5) counter2=1 sum2=y(4)+y(3)  Define  newx2=x(4) newy2=sum2/counter2
       x(5)<>x(6) 
       x(6)=x(7) counter3=1 sum3=y(6)+y(7)  
       x(7)=x(8) counter3=2 sum3=sum+y(8)  
       x(8)=x(9) counter3=3 sum3=sum+y(9)  Define  newx1=x(6) newy3=sum3/counter3

My problem here is using more then two counters in a loop. Probably, I should write something like:

s=1:8
for k=1:9
if x(k)=x(k+s);
else
s=s+1;
end
end

which didn’t work:(
I’ll be greatfull for any help or advise!

Answer 1

If I understood you correctly, you want the mean of each group of numbers that have the same index in “x”. In that case…

function FindAverages
x=[1 1 1 2 2 3 3 3 3]';
y=[11 21 31 24 32 33 13 37 3]';

means = [];
indexes = unique(x);
for i=1:numel(indexes)
    index = indexes(i);
    indexInY = (x==index);
    means(end+1) = mean(y(indexInY));
end
disp(means);