Question

我在这样的囚室中有一些线性方程式(每小时等数有所不同):

equs = {  2*X1+X2+6 ,  3*X2-X1  }

How can I solve these equation with Matlab? I can get my answer simply by this function :

ans = solve(equs(1), equs(2));

But, as the number of equations differ each time, I want this to be done automatically.

Answer 1

I am assuming that you want the equations to be equal to 0, and that no equals sign appears in the equations.

Parse the expressions to find the coefficients - put into a matrix (A). I am using here a near trick that assumes that the variables are always x1, x2, etc. Also you must write the * sign for multiplications. The FindCoeffs function finds the coefficients by assigning ones and zeros to the variables. Then, you can solve the equations using linsolve.

 function FindEquations() 

     a = { x1+x2 - 6  , x1 - x2 - 2 };
     A = [];
     B = [];
     for i=1:numel(a)
        [b,l] = FindCoeefs(a{i}, numel(a));
        A(end+1,:) = l;
        B(end+1) = -b;
    end
    linsolve(A,transpose(B))
end

function [b,p] = FindCoeefs(expr, n)
    for j=1:n
        eval(sprintf( x%d=0; ,j));
    end
    b = eval([expr  ; ]);

    p = zeros(1,n);
    for i=1:n
        for j=1:n
            eval(sprintf( x%d=0; ,j));
        end
        eval(sprintf( x%d=1; ,i));

        p(i) = eval([expr  ; ]) - b;    
    end

end

Answer 2

可以通过使用:

solve(equs{:})

Works for the unknown, too (in case they are not properly determined by symvar):

uvar={x,y}
solve(equs{:},uvar{:})

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