Error using evalin Undefined function or variable 'x'. Error in sym/eval (line 11) s = evalin('ca​ller',vect​orize(map2​mat(char(x​))));

i was trying to solve a simple system of equations, when the program gives me this error:

Error using evalin
Undefined function or variable 'x'.

Error in sym/eval (line 11)
s = evalin('caller',vectorize(map2mat(char(x))));

Error in es_6_analisi_cinematica (line 27)
xBposizioni=eval(solB.xBsol);

in fact when I go to evaluate the solutions of the system inside solB, it solves the system in x and y, rather than xBsol and yBsol. sorry that the script may seem strange but I'm Italian

i get the same problem using vpa instead of eval, the system of equations is solved with other variables, x and y

This is the script:

clear
AC=0.15;
BC=0.2;
AD=0.35;
xA=0;
yA=0;
rA=[xA yA 0];
xC=AC;
yC=0;
rC=[xC yC 0];

ii=0;
for phi=10^(-17):pi/36:2*pi
    
xD=AD*cos(phi);
yD=AD*sin(phi);
rD=[xD yD 0];
xBsol=sym('xBsol');
yBsol=sym('yBsol');
eqB1='xBsol*sin(phi)-yBsol*cos(phi)=0';
eqB2='(xC-xBsol)^2+yBsol^2-BC^2=0';
solB=solve(eqB1,eqB2,'xBsol','yBsol');
xBposizioni=eval(solB.xBsol);
yBposizioni=eval(solB.yBsol);
xB1=xBposizioni(1)
yB1=yBposizioni(1)
xB2=xBposizioni(2)
yB2=yBposizioni(2)

if (phi>=0 && phi<=pi)
if yB1>=0 xB=xB1;yB=yB1;
else
    xB=xB2;yB=yB2;
end
end

if (phi>pi && phi< 2*pi)
    if yB1<0 
        xB=xB1; yB=yB1;
    else
        xB=xB2; yB=yB2;
    end
end


rB=[xB yB 0];

figure(1)
plot([xA,xB],[yA,yB],'k-',[xB, xC],[yB,yC],'r-',[xA,xC],[yA,yC],'b-',[xB,xD],[yB,yD],'g-')
text(xA,yA,'A')
text(xB,yB,'B')
text(xC,yC,'C')
text(xD,yD,'D')
hold on
xlim([-0.5 0.5])
ylim([-0.5 0.5])

end

[Stephen23]

https://www.mathworks.com/matlabcentral/answers/56166-symbolic-computaion-of-complex-number-and-using-eval-function-doesnot#answer_68084

https://www.mathworks.com/matlabcentral/answers/6437-matlab-gives-me-a-formula-when-i-want-a-value#answer_8911

https://www.mathworks.com/matlabcentral/answers/376032-using-eval-undefined-function-or-variable#answer_299055

https://www.mathworks.com/matlabcentral/answers/368517-how-do-you-use-the-function-eval#answer_292491

https://www.mathworks.com/matlabcentral/answers/518936-trying-to-get-a-symbolic-derivative-from-a-string-that-the-user-inputs

etc.etc.

Answer 1

[Stephan]

clear
AC=0.15;
BC=0.2;
AD=0.35;
xA=0;
yA=0;
rA=[xA yA 0];
xC=AC;
yC=0;
rC=[xC yC 0];
syms xBsol yBsol

ii=0;
for phi=10^(-17):pi/36:2*pi
    
xD=AD*cos(phi);
yD=AD*sin(phi);
rD=[xD yD 0];

eqB1=xBsol*sin(phi)-yBsol*cos(phi)==0;
eqB2=(xC-xBsol)^2+yBsol^2-BC^2==0;
solB=solve(eqB1,eqB2,xBsol,yBsol);
xBposizioni=eval(solB.xBsol);
yBposizioni=eval(solB.yBsol);
xB1=xBposizioni(1);
yB1=yBposizioni(1);
xB2=xBposizioni(2);
yB2=yBposizioni(2);

if (phi>=0 && phi<=pi)
if yB1>=0
    xB=xB1;
    yB=yB1;
else
    xB=xB2;yB=yB2;
end
end

if (phi>pi && phi< 2*pi)
    if yB1<0 
        xB=xB1; yB=yB1;
    else
        xB=xB2; yB=yB2;
    end
end


rB=[xB yB 0];

figure(1)
plot([xA,xB],[yA,yB],'k-',[xB, xC],[yB,yC],'r-',[xA,xC],[yA,yC],'b-',[xB,xD],[yB,yD],'g-')
text(xA,yA,'A')
text(xB,yB,'B')
text(xC,yC,'C')
text(xD,yD,'D')
hold on
xlim([-0.5 0.5])
ylim([-0.5 0.5])

end