Error using * Matrix dimensions must agree.

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Ahmad Agbaria
Ahmad Agbaria 2020-3-31
评论： Ahmad Agbaria 2020-3-31
when I run my code I get an error message but If i change fac with 5 for example everything runs as expected. Any idea of what can cause this?
fac is a scalar with holds the value of the element in row j column i divided by piv
Error using *
Incorrect dimensions for matrix
multiplication. Check that the
number of columns in the first
matrix matches the number of
rows in the second matrix. To
perform elementwise
multiplication, use '.*'.
Error in LU_CALC (line 15)
A(j,:)=A(j,:)-fac*A(i,:);
function [L_Matrix,U_Matrix] = LU_CALC(A)
%LU_CALC Summary of this function goes here
% Detailed explanation goes here
m = size(A,1);
L_Matrix = eye(m);
for i=1:m
for j=i+1:m
piv=A(i:i);
if piv==0
disp("Error!");
break;
end
fac=A(j:i)./piv;
A(j,:)=A(j,:)-fac*A(i,:);
end
end
U_Matrix = A;
end

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回答（1 个）

James Tursa
James Tursa 2020-3-31
编辑：James Tursa 2020-3-31
Change this
piv=A(i:i);
to this
piv=A(i,i);
And change this
fac=A(j:i)./piv;
to this
fac=A(j,i)./piv;
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James Tursa
James Tursa 2020-3-31
编辑：James Tursa 2020-3-31
A(i,j) is the (i,j)'th element of A
A(i:j) is a subset of A using linear indexing into A based on the i:j indexing. This is completely different from A(i,j)
Let's consider a 3x2 matrix just for example. So A is 3x2.
A(1,1) is the A11 element
A(1:1) is also (concidently) the A11 element because the indexing 1:1 produces the scalar index 1
A(2,1) is the A21 element
A(2:1) is an empty matrix. The indexing 2:1 means a vector starting at 2, increasing by 1, and ending at 1. Since 2 is greater than 1, the vector 2:1 is actually empty, and when using an empty vector for indexing into A you get an empty result.
E.g., A(2,3) is the A23 element, but A(2:3) is a vector containing the 2nd and 3rd elements of A which happen to be A21 and A31, not at all what you wanted.

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