问题描述
让我们说这3个向量定义了以下稀疏矩阵:
Let's say we have the following sparse matrix defined by this 3 vectors:
[lines, columns, values] = find(A)
lines =
1
2
3
5
1
3
3
5
4
5
columns =
1
2
2
2
3
3
4
4
5
5
values =
3
4
7
3
1
5
9
6
2
5
我要实现的目标是访问(2，2)位置的元素
What I am trying to achieve is to access the element at the position (2, 2)
我知道您可以执行values(lines == 2)(values(columns == 2))，它将返回第二行(列)中的所有值.
I know you can do values(lines == 2) (values(columns == 2)) which will return all the values from second row (column).
我的问题是，如何执行values(lines == 2 && columns == 2)之类的操作来获取A(2,2)处的值?
My question is how can you do something like values(lines == 2 && columns == 2) to get the value at A(2,2)?
推荐答案
如果您要访问稀疏矩阵或与此相关的任何其他矩阵中的元素.这没有任何开销.如果有人能证明我做错了，我非常希望看到它的基准，因为那样的话，我就错过了非常重要的事情！
If you want to access elements in a sparse matrix, or any other matrix for that matter. This has no overhead whatsoever. If anyone can prove me wrong on this one, I would very much like to see a benchmark of it, because then I have missed something very important!
a = Q(2,2);
如果您要添加元素到稀疏矩阵中，这也非常简单.我也不认为有任何更快方法来做到这一点.同样，如果有人可以证明我错了，请分享您的基准测试结果！
If you want to add elements to a sparse matrix, this is also very simple. I don't think there are any faster ways to do this either. Again, if anyone can prove me wrong, please share your benchmark results!
如果您有:
lines = [ 1
2
3
5
1];
columns = [1
2
2
2
3];
values = [3
4
7
3
1];
Q = sparse(lines, columns, values)
(1, 1) -> 3
(2, 2) -> 4
(3, 2) -> 7
(5, 2) -> 3
(1, 3) -> 1
[linesF, columnsF, valuesF] = find(Q)
%% Now add the value 12 to position (3,1)
linesF = [linesF; 3];
columnsF = [columnsF; 1];
valuesF = [valuesF; 12];
Q = sparse(linesF, columnsF, valuesF)
(1, 1) -> 3
(3, 1) -> 12
(2, 2) -> 4
(3, 2) -> 7
(5, 2) -> 3
(1, 3) -> 1
之所以行之有效，是因为没有人说必须对行向量和列向量进行任何排序.
This works because there is nothing saying the row and column vectors must be sorted in any way.
S = sprand(10000,10000,0.0005);
tic
for ii = 1:1000
var = S(r,c);
end
toc
Elapsed time is 0.010705 seconds.
[i,j,s] = find(S);
tic
for ii = 1:1000
var = s((i == r & j == c); % (r,c) is a random non-zero element in s
end
toc
Elapsed time is 0.296547 seconds.
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