Vectorization for Two Nested for-loops

6 次查看（过去 30 天）
Mohammed
Mohammed 2024-7-1
编辑： Stephen23 2024-7-1
Hello all,
I'm trying to vectorize some operation between two 2D matrices, A and B. Basically, the process is multiplying every row verctor from matrix B by every vector from matrix A and store the value in matrix C. The correct non-vectorized code looks as follows:
A = [2 3 4; 5 2 1; 1 4 3];
B = [3 4 4; 4 2 1; 4 4 1];
C = zeros(3,3); % Initializing C
for ii=1:size(A,1)
for jj=1:size(B,1)
AA = A(ii,:);
BB = transpose(B(jj,:));
C(jj,ii) = sum(BB*AA, 'all')
end
end
C
The resultant C should looks as:
C =
99 88 88
63 56 56
81 72 72
Can I get help here?
Thanks!

请先登录，再回答此问题。

回答（2 个）

Tomoaki Takagi
Tomoaki Takagi 2024-7-1
Try:
A = [2 3 4; 5 2 1; 1 4 3];
B = [3 4 4; 4 2 1; 4 4 1];
C = repmat(A,3).*repelem(B,3,3);
C = reshape(sum(C,2),3,3)'
Stephen23
Stephen23 2024-7-1
编辑：Stephen23 2024-7-1
Ran in:
Without creating multiple copies of data in memory:
A = [2 3 4; 5 2 1; 1 4 3];
B = [3 4 4; 4 2 1; 4 4 1];
C = sum(permute(A,[3,1,2,4]).*permute(B,[1,3,4,2]),3:4)
C = 3x3
99 88 88 63 56 56 81 72 72
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>
The same basic approach, more verbose but possibly more efficient:
As = size(A);
Bs = size(B);
C = sum(reshape(A,[1,As]).*reshape(B,[Bs(1),1,1,Bs(2)]),3:4)
C = 3x3
99 88 88 63 56 56 81 72 72
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>

类别

在 Help Center 和 File Exchange 中查找有关 Creating and Concatenating Matrices 的更多信息

标签

产品

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by