operations on specific row

Hi,

I've a for example a 5x3 matrix

2015 52 7

2015 53 12

2015 54 20

3096 77 7

3096 83 11

and I want apply the function p = polyfit(x,y,1) to third (independent variable) and second (dependent variable) column. Moreover I want apply the regressions to data which have the same value in the first column. In this case a first regression on the first 3 data and a second regression on the last 2 data. How can I define this criteria?

Thanks

Gianluca

Answer 1

[Mitch Martelli]

Hi,

for your first question you can use :

p = polyfit(A(:,3),A(:,2),1);

Pay attention to use one like degree .

For the second question the following code should work , but i dont know if there is a way to dont use for loop:

b = unique(A(:,1));
degree=1;
p=zeros(size(b,1),degree+1);
for i=1:size(b,1)
    ind = find(A(:,1)==b(i));
    p(i,:) = polyfit(A(ind,3),A(ind,2),1);
end

[gianluca]

Hi Mitch,

your code works perfectly.

Thank you for your help and quickness!

Best regards

Gianluca