how can I select matrix values for those cols and rows for which an f(col,row)==true?

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Jasper van Casteren
评论： Guillaume 2017-9-28
I have a huge matrix A (10k*10k) and I have a function that returns false/true for each combination of a row and a column number : f(row, col) = boolean. I need to do A(f(row,col)) = X;
How to do that quickly (I need to do this very often)?
I saw arrayfun and spfun but the element function does not allow for row and column arguments.

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回答（1 个）

Guillaume
Guillaume 2017-9-28
编辑：Guillaume 2017-9-28
[rows, cols] = ndgrid(1:size(A, 1), 1:size(A, 2));
Then, if f can operate directly with pairs of matrices, simply:
A(f(rows, cols)) = X;
otherwise if f only works with scalar values:
A(arrayfun(@f, rows, cols)) = X;
  2 个评论
Guillaume
Guillaume 2017-9-28
Comment by Jasper van Casteren moved here:
thank you for this, but
[rows, cols] = ndgrid(1:size(A, 1), 1:size(A, 2));
takes more than a second, and allocates two huge matrices. Is there no more efficient way?
Guillaume
Guillaume 2017-9-28
Depending on how f is implemented you may get away with:
A(f((1:size(A, 1))', 1:size(A, 2))
or
A(bsxfun(@f, (1:size(A, 1))', 1:size(A, 2))
but ultimately, it's a trade-off of memory vs speed. If a loop works better for you then use that.
It's also possible that modifying the way f works may provide some / lots of / no potential for improvement.

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