How can I find one index for each row of a 2d matrix where a condition is met?

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funkadelala 2015-2-13

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评论： Geoff Hayes 2015-2-21

A is an n by n matrix. B is an n by 1 matrix.

I would like to find the last instance of an element in each row of A being greater than the element in the corresponding row of B, and return an n by 1 list of indices.

For example:

A = [1 3 2; 
     2 5 -1; 
     0 2 3]
B = [1; 
     0; 
     2]
idx = [3;
       2; 
       3]

I'm sure I could do this with a for loop, but would like to avoid it as the matrices are potentially very large.

Many thanks in advance.

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Answer 1

Geoff Hayes 2015-2-13

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Funkadelala - try using the following which assumes that A and B have been defined as above

 cell2mat(arrayfun(@(k)find(A(k,:)>B(k),1,'last'),1:size(A,1),'UniformOutput',false)')

We use arrayfun to apply the anonymous function

@(k)find(A(k,:)>B(k),1,'last')

to the kth row of A, using find to find the last index which is greater than the kth element of B. The result from arrayfun is a cell array (row), so we transpose it and convert to a matrix using cell2mat to produce the desired result of

Try the above and see what happens!

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Geoff Hayes 2015-2-21

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funkadelala's answer moved here

Thanks for the response, and sorry for the delay (I've been trying to sort out errors myself).

The above suggestion returns the following error:

Error using cat
Dimensions of matrices being concatenated are not consistent.
Error in cell2mat (line 83)
            m{n} = cat(1,c{:,n});
Error in untitled (line 60)
ans = cell2mat(arrayfun(@(k)find(A(k,:)>B(k),1,'last'),1:size(A,1),'UniformOutput',false)');

And simply using:

ans = arrayfun(@(k)find(A(k,:)>B(k,:),1,'last'),A);

Returns the error:

Attempted to access A(-0,:); index must be a positive integer or logical.
Error in @(k)find(A(k,:)>B(k,:),1,'last')
Error in untitled (line 61)
zindex = arrayfun(@(k)find(A(k,:)>B(k,:),1,'last'),A)';

Any ideas?