How to make an array with specific matrix values

Hello,

I have a matrix for example like this:

[1 2 3 4 5 6 7 8 9 10;
 2 2 3 3 4 4 3 3 2 2]

The first column represents time, the second ID of time. I need to make an array where there is first time value from the first group of ID of time for each groups!. So the result looks like this: [1 3 5 7 9] Unfortunately unique function is not helpful in this case.

Thank you

[Jan]

Do not post multiple threads for one problem. See https://www.mathworks.com/matlabcentral/answers/384215-how-to-make-array-with-first-values-of-matrix

Answer 1

[Jan]

I assume this works:

A = [1 2 3 4 5 6 7 8 9 10; ...
     2 2 3 3 4 4 3 3 2 2];
B = A(1, [true; diff(A(1, :)) ~= 0]);

(Just a combination of the answers of Birdman and KL - and a transposed input)

Answer 2

[Birdman]

idx=[(diff(A(:,2))==0);false]; 
A(idx,1)

[Jan]

boolean is not useful here, because diff(A(:,2))==0) is a logical already. By the way: boolean is a MuPAD command, better use logical for Matlab arrays.

[Birdman]

I edited it, thanks. :)

[Jan]

@Birdman: diff(A(:,2))==0 is a logical already, because it is the result of a comparison operator. Then there is no need to cast it to a logical.

[Birdman]

Thanks again Jan :) I am learning from you.

[Jan]

Fine :-) And perhaps some readers in the forum will learn from all these discussions also. I have learned today, where the boolean function belongs to, because I did not know it before.

Answer 3

[KL]

I assume you have a matrix of two columns,

A =  [1 2 3 4 5 6 7 8 9 10;2 2 3 3 4 4 3 3 2 2]';
A(diff(A(:,2))==0,1)
ans =
   1
   3
   5
   7
   9