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I have a matrix and I am actually interested in obtaining a vector that will consist of the first column in each row that has values lower certain value.

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Saint
Saint on 26 May 2017
Edited: Stephen Cobeldick on 29 May 2017
I have a matrix and I am actually interested in obtaining a vector that will consist of the first column in each row that has values lower 3. e.g if I have A=[ 5 7 8 3 2 ;6 2 1 4 4;1 2 3 4 5; 4 5 2 1 3]. I expect an Ans: [5;2;1;4]

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Accepted Answer

Saint
Saint on 29 May 2017
Edited: Saint on 29 May 2017
Thanks Star Strider and MathsReallyWork. I kind of figure it out.Ans: [5;2;1;3]
dmax=3
Nrows=4
A=[5 7 8 3 2 ;6 2 1 4 4;1 2 3 4 5; 4 5 2 1 3]
for i=1:Nrows
B=A(i,:)';
a(i)=find(B<dmax,1,'first') ;
end
b=a'

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Stephen Cobeldick
Stephen Cobeldick on 29 May 2017
It is simpler and more efficient to process the whole array at once, rather than using a loop:
>> C = A.';
>> idx = C<dmax;
>> [col,row] = find(idx & cumsum(idx,1)==1)
col =
5
2
1
3
row =
1
2
3
4

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More Answers (1)

MathReallyWorks
MathReallyWorks on 27 May 2017
Hello saint,
Your question is not clear. Please edit it.
By the description of your question I can guess that you want first column of each row provided that entry is less than 3. In that case this code works well:
A = [ 5 7 8 3 2 ;6 2 1 4 4;1 2 3 4 5; 4 5 2 1 3];
newA = A(A(:,1)<3,1)
But, then you said you are expecting [5;2;1;4] which is contradictory to your question.
[5;6;1;4] is possible as it contains all first element.

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