Matrixes not multiplying correctly using for loops

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Mason Reilly
Mason Reilly 2024-3-19
回答： Catalytic 2024-3-20
I have two matrixes (3x4 and 4x3) that I'm trying to multiply with the use of if and for loops, but it appears there is an issue with the indexing of the matrixes. The problem contains code meant to check the dimensions of each matrix and ensure that the column count of the first matrix is equal to the row count of the second matrix. However, when I run the code, it gives me an error of "Index in position 2 exceeds array bounds. Index must not exceed 3". Is there an issue in the for loop that is perhaps messing with the multiplication?
af = [ af '.txt' ] ; a = load(af) ; [n,m] = size(a) ;
bf = [ bf '.txt' ] ; b = load(bf) ; n1 = size(b,1) ; m1 = size(b,2) ;
if n ~= m1
error('incorrect dimensions (rows)') ;
end
if m ~= n1
error('incorrect dimensions (columns)') ;
end
c = zeros(n,m) ;
for i = 1 : n
for j = 1 : m
c(i,j) = a(i,j) * b(i,j) ;
end
end
disp(c)
  4 个评论
显示 2更早的评论
Torsten
Torsten 2024-3-20
The element (i,j) of C = A*B is given by
sum_k (a_ik * b_kj)
Do you think that
for i = 1 : n
for j = 1 : m
c(i,j) = a(i,j) * b(i,j) ;
end
end
will give a result for c(i,j) that is equivalent to that sum ?
Chuguang Pan
Chuguang Pan 2024-3-20
The matrix multiplication is , suppose that the size of a is [n m], the size of b is [m p], the size of c is [n p].

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回答（2 个）

Matt J
Matt J 2024-3-20
编辑：Matt J 2024-3-20
[n,w]=size(a);
[h,m]=size(b);
assert(w==h,'Inner dimensions incompatible')
c=zeros(n,m);
for i = 1 : n
for j = 1 : m
c(i,j) = a(i,:) * b(:,j) ;
end
end
Catalytic
Catalytic 2024-3-20
One loop -
[n,w]=size(a);
[h,m]=size(b);
if w~=h
error('Inner dimensions do not agree.')
end
c=0;
for k = 1 : w
c = c + a(:,k).*b(k,:);
end

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