Error when computing Gauss Elimination on matrix, "Undefined function 'symsum' for input arguments of type 'double'".

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Jordan Chancellor
回答： Walter Roberson 2019-2-9
Writing a function for Gauss Elimination of any sized matrix, and am trying to use a pseudocode to help me write it. In the pseudocode, a summation is used, and so when I try to include the sum in my code (line 36), I get the error "Undefined function 'symsum' for input arguments of type 'double'."
Any help would be appreciated!
function X = GaussElim(A,b)
[m,n] = size(A)
aug = [A,b]
for k = 1:n+1
m = k
for j = k+1:n
if abs(aug(m,k)) < abs(aug(j,k))
m = j
end
if aug(m,k) == 0
fprintf('No unique solution exists.')
break
else
aug([k m],:) = aug([m k],:)
if aug(n,n) == 0
fprintf('No unique solution exists.')
break
else
for j = k+1:n
M(j,k) = aug(j,k)/aug(k,k)
for p = k:n+1
aug(j,p) = aug(j,p)-(M(j,k)*aug(k,p))
end
end
end
end
end
end
for n = 1:n
x(n) = (A(n,n+1))/(A(n,n))
for i = 1:n
x(i) = (1/A(i,i))*(A(i,n+1) - symsum((A(i,j)*x(j)), j, [i+1, n]))
end
end
x(j) = x
end
A = [1 2 3; 2 3 4; 3 4 1;]
b = [4;1;2]
x = GaussElim(A,b)

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回答（1 个）

Walter Roberson
Walter Roberson 2019-2-9
You have to pass symsum a symbolic expression , followed by a symbolic variable name, followed by a lower bound, followed an upper bound . symsum takes the expression and tries to calculate from it the asymptomatic general formula for sum based on that variable . Then symsum substitutes the upper bound and the lower bound and subtracts to calculate the sum over the bounds.
Symsum is for calculating formula . Like feeding in the coefficients from a taylor series and hoping to get back the form of the function .
There is no general formula possible for indexing a matrix. It is never possible to use a symbolic variable to index a matrix in MATLAB.
You need to give up on using symsum for this . Any time you want to add a definite series of symbolic terms, calculate all of the terms into a vector and sum() the vector .
sum(A(i,i+1:n).*x(i+1:n))

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