What wrong with the indexing code?

2018 2 8
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更新时间：2018 2 8
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If I want to fill in a matrix, of which I have its indices without a for loop, how could I do it? Suppose that the (row(i),col(i)) elements to be filled are given.
value = 189;
row = [ 1 3 6 7 8 ];
col = [ 1 1 4 4 5];
B(row,col) = value

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Walter Roberson
Walter Roberson 2018-2-8

0 个投票

B = full( sparse(row, col, value) );
or
B = accumarray([row(:), col(:)], value);

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GEORGIOS BEKAS
GEORGIOS BEKAS 2018-2-8
how about using arrayfun/bsxfun or something?
Walter Roberson
Walter Roberson 2018-2-8
Not at all easily with arrayfun or bsxfun as assignment through function calls is difficult and never less than ugly.

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Birdman
Birdman 2018-2-8

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B(sub2ind([max(row) max(col)],row,col)) = value

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GEORGIOS BEKAS
GEORGIOS BEKAS 2018-2-8
as usual; can you explain that?
Birdman
Birdman 2018-2-8
编辑：Birdman 2018-2-8
sub2ind function converts the equivalent indexes of given rows and columns and it should also know the size of your matrix. The inputs are accordingly given. For instance, the linear index representation for a matrix having 2 rows and 5 columns, the index of the element at
row=2;
col=3;
is
6
GEORGIOS BEKAS
GEORGIOS BEKAS 2018-2-8
It does not solve the problem. I want B(row(i), col(i)) = value, without using a for loop.
Birdman
Birdman 2018-2-8
I did not use for loop here,as you can see.
GEORGIOS BEKAS
GEORGIOS BEKAS 2018-2-8
sz = [max(row), max(col)]
B(sub2ind(sz, row, col)) = value
B = reshape(B, sz)
Walter Roberson
Walter Roberson 2018-2-8
The reshape() is not needed.
Birdman
Birdman 2018-2-8
Doesn't my answer also work Walter?

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