Error using evalin

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('calle...

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

Maria Giulia

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

Accepted Answer

Stephan

Edited: Stephan

on 23 Nov 2020

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


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Регистрация: 27.11.2016

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1

Найти экстремаль функционала

27.11.2016, 23:31. Показов 3177. Ответов 1

Прошу помощи. Необходимо найти экстремаль функционала.
В моем решении появляется три неизвестных при решении уравнения Эйлера?
Ошибка в моем понимании функции или же нужно решить систему из двух уравнений с тремя неизвестными, чтобы все работало?

Matlab M 
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clear all  % очищаем память
format long  % формат отображения чисел с 14 знаками
%disp('Решаем пример 1a')  % выводим заголовок задачи
syms x t Dx D2x   % описали символические переменные  
F=Dx.^2+x.^2+(2*x)*exp(t);  % вводим подынтегральную функцию
x0=0;  % вводим граничные условия
x1=0;
t0=0;
t1=1;
fprintf('Подынтегральная функция: F=%sn',char(F))
fprintf('Граничные условия: x(%d)=%d;   x(%d)=%dn',t0,x0,t1,x1)  
dFdx=diff(F,x)  % вычисляем Fy
dFdx1=diff(F,Dx)  % вычисляем Fy' 
d_dFdx1_dt = diff(dFdx1,t)  % (Fy')/x
d_dFdx1_dx=diff(dFdx1,x)  % (Fy')/y
d_dFdx1_dx1 = diff(dFdx1,Dx)  % (Fy')/y'=Fy'y' - условие Лежандра
dFx1dt = d_dFdx1_dt + d_dFdx1_dx*Dx + d_dFdx1_dx1*D2x  % d(Fy')/dx 
Euler = (dFdx-dFx1dt) 
%dFdx1 = simple(diff(F,Dx))  % Fy'
%dFdx1_dt=diff(dFdx1,t);
%deqEuler = [char(dFdx-dFx1dt) '=C']  % составили уравнение
 
 
%Euler = simple(dFdx-dFx1dt)   % левая часть уравнения Эйлера
deqEuler = [char(Euler ) '=C']  % составили уравнение
Sol = dsolve(deqEuler,'t')  % решаем уравнение Эйлера
if length(Sol)~=1  % решений нет или более одного
   error('Нет решений или более одного решения!');
end   
SolLeft = subs(Sol,t,sym(t0))  % подставляем x1
SolRight = subs(Sol,t,sym(t1))  % подставляем x2 
EqLeft = [char(SolLeft) '=' char(sym(x0))]  % приравняли y1
EqRight = [char(SolRight) '=' char(sym(x1))]  % приравняли y2
Con = solve(EqLeft,EqRight);  % решаем систему уравнений
C=Con.C    % присвоили полученные решения
C1=Con.C1  % символическим переменным C и C1
%C2=Con.C2  % символическим переменным C и C1
Sol1b = vpa(eval(Sol),14);  % подставляем C1, C в решение с точностью 14 знаков
fprintf('Уравнение экстремали:n%sn',char(Sol1b)) 
tpl = linspace(t0,t1);  % массив абсцисс
x0b = subs(Sol1b,t,tpl);  % вычисляем ординаты
plot ( tpl, x0b, '-r' )  % рисуем график
title ( 'bfF=Dx.^2+x.^2+(2*x)*exp(t)' )  % заголовок
xlabel('t')  % метка оси OX
ylabel('x(t)')  % метка оси OY
Leg = diff(dFdx1,Dx)  % Fy'y' - достаточное условие Лежандра
Matlab M 
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Подынтегральная функция: F=Dx^2 + 2*x*exp(t) + x^2
Граничные условия: x(0)=0;   x(1)=0
 
dFdx =
 
2*x + 2*exp(t)
 
 
dFdx1 =
 
2*Dx
 
 
d_dFdx1_dt =
 
0
 
 
d_dFdx1_dx =
 
0
 
 
d_dFdx1_dx1 =
 
2
 
 
dFx1dt =
 
2*D2x
 
 
Euler =
 
2*x - 2*D2x + 2*exp(t)
 
 
deqEuler =
 
2*x - 2*D2x + 2*exp(t)=C
 
 
Sol =
 
C/4 - exp(t)/4 + exp(t)*(t/2 + (C*exp(-t))/4) + C2*exp(t) + C1*exp(-t)
 
 
SolLeft =
 
C/2 + C1 + C2 - 1/4
 
 
SolRight =
 
C/4 - exp(1)/4 + C1*exp(-1) + C2*exp(1) + exp(1)*((C*exp(-1))/4 + 1/2)
 
 
EqLeft =
 
C/2 + C1 + C2 - 1/4=0
 
 
EqRight =
 
C/4 - exp(1)/4 + C1*exp(-1) + C2*exp(1) + exp(1)*((C*exp(-1))/4 + 1/2)=0
 
Warning: Support of strings that are not valid variable names or define a number will be
removed in a future release. To create symbolic expressions, first create symbolic variables
and then use operations on them. 
> In sym>convertExpression (line 1536)
  In sym>convertChar (line 1441)
  In sym>tomupad (line 1198)
  In sym (line 177)
  In solve>getEqns (line 405)
  In solve (line 225)
  In test2 (line 34) 
Warning: Support of strings that are not valid variable names or define a number will be
removed in a future release. To create symbolic expressions, first create symbolic variables
and then use operations on them. 
> In sym>convertExpression (line 1536)
  In sym>convertChar (line 1441)
  In sym>tomupad (line 1198)
  In sym (line 177)
  In solve>getEqns (line 405)
  In solve (line 225)
  In test2 (line 34) 
Warning: Do not specify equations and variables as character strings. Instead, create symbolic
variables with syms. 
> In solve>getEqns (line 445)
  In solve (line 225)
  In test2 (line 34) 
 
C =
 
-(exp(2) - 4*C2 + 4*C2*exp(2) + 1)/(2*(exp(1) - 1))
 
 
C1 =
 
(exp(1)*(exp(1) - 4*C2 + 4*C2*exp(1) + 1))/(4*(exp(1) - 1))
 
Error using evalin
Undefined function or variable 'C2'.
 
Error in sym/eval (line 11)
s = evalin('caller',vectorize(map2mat(char(x))));
 
Error in test2 (line 38)
Sol1b = vpa(eval(Sol),14);  % подставляем C1, C в решение с точностью 14
знаков

Скажите как удалить тему с форума?

Миниатюры

Найти экстремаль функционала
 

__________________
Помощь в написании контрольных, курсовых и дипломных работ, диссертаций здесь



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