# Matrixes not multiplying correctly using for loops

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Mason Reilly am 19 Mär. 2024
Beantwortet: Catalytic am 20 Mär. 2024
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)
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Torsten am 20 Mär. 2024
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 am 20 Mär. 2024
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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### Antworten (2)

Matt J am 20 Mär. 2024
Bearbeitet: Matt J am 20 Mär. 2024
[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
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Catalytic am 20 Mär. 2024
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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