Indexing matrix using logicals

I'm trying to index a large matrix, with the goal of finding/indexing the first value to meet a threshold. Right now I'm doing it in a loop, but it's rather slow.

For example: a=[10 13 14 15 16;... 11 12 15 16 17;... 3 5 8 9 12]; threshold=11.5;

I want it to return: ans=[2;2;5];

I've tried screwing with find, but all I can seem to get it to return is: ans=[4;5;7;8;10;11;13;14;15];

Thanks, - Matt

Answer 1

[Matt Fig]

% Given:
a=[10 13 14 15 16;11 12 15 16 17; 3 5 8 9 12];  % Array
T = 11.5;  % Threshold.
% The approach:
L = mod(findstr(reshape(a.',1,numel(a))<T,[1,0]),size(a,2))+1

[Azzi Abdelmalek]

What if

 a=[10 13 14 15 16;11 12 15 16 17; 3 5 8 9 11];  % Array

[Matt Fig]

Interesting, what would you expect in that case, Azzi?

Perhaps [2 2 nan] would be most useful?

[Azzi Abdelmalek]

I prefer [2 2 5],

[Matt Fig]

But from the OP: "with the goal of finding/indexing the first value to meet a threshold"

a(3,5) does not meet the threshold.

[Azzi Abdelmalek]

Yes, but it's the nearest

[Matt Fig]

To Matt H,

If you want to fill the value with nan (or zero, just replace the nan with 0 in below code) when no member of the row matches, you could use this:

% This array has no element meet the 
% threshold in the third row.
a = [10 13 14 15 16;11 12 15 16 17; 3 5 8 9 11];
% So we use NaN as the filler.
[I,J] = find(a>11.5);
A = accumarray(I,J,[size(a,1),1], @min, NaN)

Answer 2

[Sean de Wolski]

If you can guarantee that each row in A has at least one value greater than the threshold, you can use the index output from max():

a=[10 13 14 15 16; 11 12 15 16 17; 3 5 8 9 12]; 
thresh = 11.5;
[~,index] = max(a>thresh,[],2);

Answer 3

[Azzi Abdelmalek]

a=[10 13 14 15 16;11 12 15 16 17; 3 5 8 9 12];  
[n,m]=size(a)
b=a<11.5
out=arrayfun(@(x) max(find(b(x,:)==1)),1:n)+1
out(out>m)=m

%or

out(out>m)=nan

[Matt Kindig]

Another way, just for fun (assuming that each row is sorted, which is assumed if we are interested in the "first" match):

n = size(a,2);  
b  = a > thresh;
first = n-sum(b,2)+1;
%if first > n, then not found-- set to NaN;
first( first > n)= NaN;