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

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)

Answer 1

[Walter Roberson]

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))