How can I create a sub-matrix if the number of rows change according to main-matrix?

% I have 9x1 sized A matrix as below:
A=[2.36; 2.36; 2.36; 1.93; 1.93; 1.41; 1.41; 1.41; 0.39]
% from 3rd row to 4th row
% from 5th row to 6th row
% from 8th row to 9th row, values of rows change
% I wanna have B=[4;6;9]. it is just an example! I wanna have a general code including the value changes based on from ith row to jth row...In the end B matrix could be like B= [j;...;....] (maybe 15x1 sized matrix)
P.S : The number of columns is always "1"

 % How can I solve that problem? Many Thanks!

Answer 1

[Andrei Bobrov]

A=[2.36; 2.36; 2.36; 1.93; 1.93; 1.41; 1.41; 1.41; 0.39];

[a,b,c] = unique(A,'first');
[ignored,ii] = sort(b);
b1 = b(ii);
a1 = a(ii);

B = b1(2:end);
out = diff(a1);

or

D = diff(A(:));
B = find([false;D~=0]);
out = D(B - 1);

[Purushottama Rao]

 @Andrei: Sir,I tried your code and got the following error meesage

A=[2.36; 2.36; 2.36; 1.93; 1.93; 1.41; 1.41; 1.41; 0.39]; [a,b,c] = unique(A,'first'); [~,ii] = sort(b); b1 = b(ii); a1 = a(ii); B = b1(2:end); out = diff(a1); ??? [~,ii] = sort(b); | Error: Expression or statement is incorrect--possibly unbalanced (, {, or [.

what was that

[Andrei Bobrov]

Corrected. You are using an old version of MATLAB.

Answer 2

[Purushottama Rao]

clc
k=0;
for i=2:(length(A))
if (A(i)~=A(i-1))
k=k+1
b(k)=i
end
end

Try out this..