Finding minimum value between certain rows

Hi,

I have a 9x3 matrix.

A= [0.08 34.1 2; 0.03 34.2 2; 0.04 34.3 2; 0.05 34.4 1; 0.07 34.5 1; 0.04 34.6 1; 0.02 34.7 1; 0.03 34.8 2; 0.08 34.9 2]

I want to find the minimum value in the first column but not for all the rows together. As you can see the first three rows have a value of 2 in the third column so I want to find the minimum value in the first column for the first three rows. Then rows 4 to 7 all have a 1 in the last column so I want to find the minimum value in column 1 from rows 4-7. Then rows 8 to 9 have a 2 in the third column so I want to find the minimum value in the first column in rows 8-9. In the end I want this...

B= [0.03 34.2 2; 0.02 34.7 1; 0.03 34.8 2]

The second thing I want to do is similar to the first thing but instead of finding the minimum value I want to find the average of the column 1 from Matrix A between certain rows (same row selection as when finding the minimum). In the end I want this...

C= [0.05 2; 0.045 1; 0.055 2]

I hope that made sense. Thanks!

Answer 1

[Azzi Abdelmalek]

idx= [diff(A(:,3)')==0 0];
ii2=find(idx==0);
ii1=[1 ii2(1:end-1)+1];
B=zeros(numel(ii1),size(A,2));
C=zeros(numel(ii1),2);
for k=1:numel(ii1)
  jj=ii1(k):ii2(k);
  [minv,jdx]=min(A(jj,1));
  B(k,:)=A(jj(jdx),:);
  C(k,:)=[mean(A(jj,1)) A(jj(1),3)];
end
B,C

Answer 2

[Andrei Bobrov]

i0 = [true;diff(A(:,3))~=0];
ii = cumsum(i0);
am = accumarray(ii,(1:numel(ii))',[],@(x){A(x(A(x,1)==min(A(x,1))),:)});
B = cat(1,am{:});
C = [accumarray(ii,A(:,1),[],@mean),A(i0,3)];

[Azzi Abdelmalek]

I will correct it like this

am= accumarray(ii,(1:numel(ii))',[],@(x) {A(find(A(x,1)==min(A(x,1)))+min(x)-1,:)})

Or

am= accumarray(ii,(1:numel(ii))',[],@(x) {A([ logical(zeros(min(x)-1,1));A(x,1)==min(A(x,1))],:)})

[Andrei Bobrov]

Thank you Azzi. I am corrected.

am = accumarray(ii,(1:numel(ii))',[],@(x){A(x(A(x,1)==min(A(x,1))),:)});