[英]Subtracting rows and columns from a vector at non-zero indices MATLAB
Suppose I have the following matrix in MATLAB: 假设我在MATLAB中具有以下矩阵:
A =[0 0 4 0;
0 5 0 3;
1 2 0 0];
given the following vectors: 给定以下向量:
b1 = [1 2 3];
b2 = [2 3 4 5];
the output should look like this: 输出应如下所示:
C1 =[0 0 3 0;
0 3 0 1;
-2 -1 0 0];
C2 =[0 0 0 0;
0 2 0 -2;
-2 -1 0 0];
C1 and C2 are column-wise and row-wise subtraction of the original matrix A from vectors happening at non-zero elements . C1和C2是原始矩阵A在非零元素处发生的矢量的按列和按行减法。 Note A in reality is sparse matrix . 注意,实际上A是稀疏矩阵 。 Obviously answers without using loop is appreciated! 显然不使用循环的答案表示赞赏! Thank you 谢谢
This one might be a little more memory efficient: 这可能会提高内存效率:
A =[0 0 4 0;
0 5 0 3;
1 2 0 0];
b1 = [1 2 3].'; % transpose so it's a column vector
b2 = [2 3 4 5].';
[Arows Acols Avals] = find(A);
C1 = sparse([Arows;Arows], [Acols;Acols], [Avals;-b1(Arows)]);
C2 = sparse([Arows;Arows], [Acols;Acols], [Avals;-b2(Acols)]);
Results: 结果:
>> full(C1)
ans =
0 0 3 0
0 3 0 1
-2 -1 0 0
>> full(C2)
ans =
0 0 0 0
0 2 0 -2
-1 -1 0 0
This takes advantage of the fact that sparse
adds the values given for duplicate subscripts. 这利用了sparse
将重复下标给出的值相加这一事实。 A
can be sparse or full. A
可以是稀疏的或完整的。
No need to use a loop. 无需使用循环。 First perform the subtractions and then replace the elements that should remain 0
. 首先执行减法,然后替换应保持为0
的元素。
C1 = A - repmat(b1.',1,size(A,2));
C2 = A - repmat(b2,size(A,1),1);
C1(A==0)=0;
C2(A==0)=0;
C1 =
0 0 3 0
0 3 0 1
-2 -1 0 0
C2 =
0 0 0 0
0 2 0 -2
-1 -1 0 0
Test on Sparse Matrix 测试稀疏矩阵
You can also confirm that this will work on Sparse Matirces 您还可以确认这将适用于稀疏母ir
A = sparse(10,10);
A(5:6,5:6)=rand(2);
b1 = rand(10,1);
b2 = rand(1,10);
B1 = A - repmat(b1,1,size(A,2));
B2 = A - repmat(b2,size(A,1),1);
B1(A==0)=0;
B2(A==0)=0;
C1 = A ~= 0; // save none zero elements of A
b1 = b1.'; // transpose b1
b1 = [b1, b1, b1, b1]; // create matrix of same size as A
C1 = C1.*b1;
C1 = A-C1;
C1: C1:
0 0 3 0
0 3 0 1
-2 -1 0 0
Next is C2 接下来是C2
C2 = A ~= 0;
k = [b2; b2; b2];
C2 = C2.*k;
C2 = A-C2;
C2: C2:
0 0 0 0
0 2 0 -2
-1 -1 0 0
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