# DOUBLE cannot convert the input expression into a double array.

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Akshay Pratap Singh on 25 Oct 2019
Answered: Stephan on 25 Oct 2019
Hi, I wrote the following code to generate the smmoth curves. When executed, the program shows the following errors:
The following error occurred converting from sym to double:
Error using symengine (line 58)
DOUBLE cannot convert the input expression into a double array.
If the input expression contains a symbolic variable, use VPA.
clear all
clc
% syms x
format longEng
syms y1 y2
h=4;
h1=2.88;
h2=0.26325;
alfa=0.5;
beta=0.5;
R1=-1;
R3=-1;
q=0;
kh=0.2;
kv=0.0;
psi=atan(kh/(1-kv));
gma=14;
delta=20;
phi=30;
a=0.5;
b=1;
L=h+h1+h2;
psi=atan(kh/(1-kv));
da1=delta*(pi/180); pha1=phi*(pi/180)
l=pha1+da1;
m=pha1-psi;
n=psi+da1;
B=(2*q*b*(b/(2*a+b)))/(gma*(h+h1)*(h+h1))
alphacm=atan((sin(l)*sin(m)+(((sin(l)^2)*(sin(m)^2))-B*cos(n)*sin(m)*cos(l)+cos(m)*sin(l)*sin(m)*cos(l))^0.5)/((cos(m)*sin(l))-B*cos(n)))
kg1=((tan(alphacm-phi)+tan(psi))*cos(alphacm-phi))/(tan(alphacm)*(cos(alphacm-phi-delta)))
r1=(b/(h+h1))*(b/(2*a+b))*(tan(alphacm))
kq1=r1*kg1
% pgx=gma*(1-kv)*kg1*x
pqx=(1-kv)*q*kq1
%
delm3=-0.5*(1-R3)*delta;
k3=(2*cos(phi-psi)^2)/(cos(phi-psi)^2*(1+R3)+cos(psi)*cos(delm3+psi)*(1-R3)*(1+sqrt((sin(phi+delm3)*sin(phi-psi))/cos(delm3+psi)))^2);
i=0;
for x=0:1:L
i=i+1;
%
%
delm23=0.5*(3-1)*delta;
k23=1+0.5*(3-1).*((cos(phi-psi).^2./(cos(psi).*(cos(delm23+psi).*...
(-sqrt((sin(phi+delm23).*sin(phi-psi))./(cos(delm23+psi)))+1).^2)))-1);
R2=3*(beta*(1-y1))^0.5;
delm213=0.5*(R2-1)*delta;
k213=1+0.5*(R2-1).*((cos(phi-psi).^2./(cos(psi).*(cos(delm213+psi).*...
(-sqrt((sin(phi+delm213).*sin(phi-psi))./(cos(delm213+psi)))+1).^2)))-1);
delm201=0.5*(1-R2).*delta;
k201=(2*cos(phi-psi)^2)/(cos(phi-psi)^2*(1+R2)+cos(psi)*cos(delm201+psi)...
*(1-R2)*(1+sqrt((sin(phi+delm201)*sin(phi-psi))/cos(delm201+psi)))^2);
delm43=0.5*(3-1)*delta;
k43=1+0.5*(3-1)*((cos(phi-psi)^2/(cos(psi)*(cos(delm43+psi)*(-sqrt((sin(phi+delm43)...
*sin(phi-psi))/(cos(delm43+psi)))+1)^2)))-1)
R4=3*(alfa*y2)^0.5;
delm413=0.5*(R4-1)*delta;
k413=1+0.5*(R4-1)*((cos(phi-psi)^2/(cos(psi)*(cos(delm413+psi)*(-sqrt((sin(phi+delm413)...
*sin(phi-psi))/(cos(delm413+psi)))+1)^2)))-1)
delm401=0.5*(1-R4).*delta;
k401=(2*cos(phi-psi)^2)/(cos(phi-psi)^2*(1+R4)+cos(psi)*cos(delm401+psi)...
*(1-R4)*(1+sqrt((sin(phi+delm401)*sin(phi-psi))/cos(delm401+psi)))^2);
%
H23=(k23*y1*cos(delm23));
h23=gma*(x-h)^2*vpa(subs(H23,0,(1-(1/beta))))
H213=(k213*cos(delm213)*y1);
h213=gma*(x-h)^2*vpa(subs(H213,(1-(1/beta)),(1-(1/(9*beta)))))
H201=(k201*y1*cos(delm201));
h201=gma*(x-h)^2*vpa(subs(H201,(1-(1/(9*beta))),1));
U=h23+h213+h201;
%
H123=(k23*y1*cos(delm23));
h123=gma*h1^2*vpa(subs(H23,0,(1-(1/beta))));
H1213=(k213*cos(delm213)*y1);
h1213=gma*h1^2*vpa(subs(H213,(1-(1/beta)),(1-(1/(9*beta)))));
H1201=(k201*y1*cos(delm201));
h1201=gma*h1^2*vpa(subs(H201,(1-(1/(9*beta))),1));
U1=h123+h1213+h1201;
%
H401=(k401*y2*cos(delm401));
h401=gma*(x-h-h1)^2*vpa(subs(H401,0,(1/(9*alfa))));
H413=(k413*y2*cos(delm413));
h413=gma*(x-h-h1)^2*vpa(subs(H413,(1/(9*alfa)),(1/alfa)));
H43=(k43*y2*cos(delm43));
h43=gma*(x-h-h1)^2*vpa(subs(H43,(1/alfa),1));
%
Hb401=(k401*cos(delm401));
hb401=(gma*(x+h)+q)*(x-h-h1)*vpa(subs(H401,0,(1/(9*alfa))));
Hb413=(k413*cos(delm413));
hb413=(gma*(x+h)+q)*(x-h-h1)*vpa(subs(H413,(1/(9*alfa)),(1/alfa)));
Hb43=k43*cos(delm43);
hb43=(gma*(x+h)+q)*(x-h-h1)*vpa(subs(H43,(1/alfa),1));
%
h4=h401+h413+h43+hb401+hb413+hb43;
%
h3=0.5*gma*k3*(x-h-h1)^2*cos(delta)+gma*x*k3*(x-h-h1)*cos(delta);
%
M23=(k23*y1*(1-y1)*cos(delta));
m23=gma*(x-h)^3*vpa(subs(M23,0,(1-(1/beta))));
M213=(k213*cos(delm213)*y1*(1-y1));
m213=gma*(x-h)^3*vpa(subs(M213,(1-(1/beta)),(1-(1/(9*beta)))));
M201=(k201*y1*cos(delm201)*(1-y1));
m201=gma*(x-h)^3*vpa(subs(H201,(1-(1/(9*beta))),1));
m2=m23+m213+m201;
%
M123=(k23*y1*(1-y1)*cos(delta));
m123=gma*(x-h)^3*vpa(subs(M23,0,(1-(1/beta))));
M1213=(k213*cos(delm213)*y1*(1-y1));
m1213=gma*(x-h)^3*vpa(subs(M213,(1-(1/beta)),(1-(1/(9*beta)))));
M1201=(k201*y1*cos(delm201)*(1-y1));
m1201=gma*(x-h)^3*vpa(subs(H201,(1-(1/(9*beta))),1));
m12=m123+m1213+m1201;
%
M401=(k401*cos(delm401)*y2^2);
m401=gma*(x-h-h1)^3*vpa(subs(M401,0,(1/(9*alfa))));
M413=(k413*cos(delm413)*y2^2);
m413=gma*(x-h-h1)^3*vpa(subs(M413,(1/(9*alfa)),(1/alfa)));
M43=(k43*cos(delta)*y2^2);
m43=gma*(x-h-h1)^3*vpa(subs(M43,(1/alfa),1));
%
Mb401=(k401*cos(delm401)*y2);
mb401=(gma*(x+h)+q)*(x-h-h1)^2*vpa(subs(M401,0,(1/(9*alfa))));
Mb413=(k413*cos(delm413)*y2);
mb413=(gma*(x+h)+q)*(x-h-h1)^2*vpa(subs(M413,(1/(9*alfa)),(1/alfa)));
Mb43=(k43*cos(delta)*y2);
mb43=(gma*(x+h)+q)*(x-h-h1)^2*vpa(subs(M43,(1/alfa),1));
%
m4=m401+m413+m43+mb401+mb413+mb43;
%
m3=0.5*gma*k3*(x-h-h1)^3*cos(delta)*(2/3)+gma*x*k3*(x-h-h1)^2*cos(delta)*(1/2);
%
if(x<h)
SF(i)=-0.5*(1-kv)*kg1*gma*x^2*cos(delta)-(1-kv)*q*kq1*x*cos(delta);
SF1(i)=0;
BM(i)=(1/6)*(1-kv)*kg1*gma*x^3*cos(delta)+(1-kv)*q*kq1*x*(x/2)*cos(delta);
BM1(i)=0;
elseif(x>=h && x<(h+h1))
SF(i)=-0.5*kg1*(1-kv)*gma*x^2*cos(delta)-(1-kv)*q*kq1*x*cos(delta)+U;
SF1(i)=0;
BM(i)=(1/6)*kg1*(1-kv)*gma*x^3*cos(delta)+(1-kv)*q*kq1*x*(x/2)*cos(delta)-m2;
BM1(i)=0;
else
SF(i)=-0.5*kg1*(1-kv)*gma*(h+h1)^2*cos(da1) + U1 -pqx*(h+h1)*cos(delta)-h4+h3;
SF1(i)=0;
BM(i)=0.5*kg1*(1-kv)*gma*(h+h1)^2*(((h+h1)/3)+(x-h-h1))*cos(da1)-m12+pqx*(h+h1)*(((h+h1)/2)+(x-h-h1))*cos(delta)-m4+m3;
BM1(i)=0;
end
end
x=0:1:L;
f=max(BM)
subplot(2,1,1)
plot(x,SF,x,SF1)
grid on
xlabel('Length of the beam in m')
ylabel('Shear force in KN/m')
title('Shear force diagram')
subplot(2,1,2)
plot(x,BM,x,BM1)
grid on
xlabel('Length of the beam in m')
ylabel('Bending Moment in KN-m/m')
title('Bending Moment diagram')

Stephan on 25 Oct 2019
During the first run of your loop U=0.0, which can be converted to double in line 144 and stored in SF(1). But in the second run with i=2 U is becomming a nonlinear equation depending on the symbolic variables y1 and y2. This expression can not be converted to double, because y1 and y2 dont have values:
U = % Result of i = 1
0.0
U = % Result of i = 2
6.0365253422831727711781109416652*y1 + (0.050605831776812582566138498663122*y1*cos(30.0*(0.5 - 0.5*y1)^(1/2) - 10.0))/(0.0054220534046584909892291248567631*(0.5 - 0.5*y1)^(1/2) - 0.98058067569092015962081232865823*cos(30.0*(0.5 - 0.5*y1)^(1/2) - 10.197395559849880775082908712648)*(((0.99909591574838996219654063679627*sin(30.0*(0.5 - 0.5*y1)^(1/2) - 40.0))/cos(30.0*(0.5 - 0.5*y1)^(1/2) - 10.197395559849880775082908712648))^(1/2) + 1.0)^2*(3.0*(0.5 - 0.5*y1)^(1/2) - 1.0) + 0.0018073511348861636630763749522544) + 14*y1*cos(30.0*(0.38888888888888888888888888888889*y1 + 0.5)^(1/2) - 10.0)*(((0.0018431437409397227774065611925384*cos(30.0*(0.38888888888888888888888888888889*y1 + 0.5)^(1/2) - 9.8026044401501192249170912873524)^(7/9))/((-0.99909591574838996219654063679627*cos(30.0*(0.38888888888888888888888888888889*y1 + 0.5)^(1/2) - 9.8026044401501192249170912873524)^(7/9)*sin(30.0*(0.38888888888888888888888888888889*y1 + 0.5)^(1/2) + 20.0))^(1/2) + 0.77777777777777777777777777777778)^2 + 0.77777777777777777777777777777778)*(1.5*(0.38888888888888888888888888888889*y1 + 0.5)^(1/2) + 0.38888888888888888888888888888889) + 1.0)
The following error occurred converting from sym to double:
Unable to convert expression into double array.
Error in signal_check (line 145)
SF(i)=-0.5*kg1*(1-kv)*gma*x^2*cos(delta)-(1-kv)*q*kq1*x*cos(delta)+U;
This is the point where things go wrong - you should think about how to fix this. I guess in line 75 where you calculate U will be a starting point for adapting your code:
U=h23+h213+h201;