# How to make an array with specific matrix values

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Filip Konecny on 22 Feb 2018
Commented: Jan on 22 Feb 2018
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

#### 1 Comment

Jan on 22 Feb 2018

Jan on 22 Feb 2018
Edited: Jan on 22 Feb 2018
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)

Birdman on 22 Feb 2018
Edited: Birdman on 22 Feb 2018
idx=[(diff(A(:,2))==0);false];
A(idx,1)

Jan on 22 Feb 2018
@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 on 22 Feb 2018
Thanks again Jan :) I am learning from you.
Jan on 22 Feb 2018
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.

KL on 22 Feb 2018
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