MATLAB: Error using symengine Unable to convert expression into double array.

error in sym/doublesymbolicSymbolic Math Toolbox

What could be the problem with the code below? I can't get numerical answer for the error_dvd
clear all
syms a n lamb th h0 l hn deltal
eqd=n*lamb/(sin(a-th)-sin(a));
eqa=1/2*atan(h0/l);
eqth=2*a-atan(hn/l);
d_a=diff(eqd,a);
a_l=diff(eqa,l);
d_th=diff(eqd,th);
th_l=diff(eqth,l);
th_a=diff(eqth,a);
a_h0=diff(eqa,h0);
th_hn=diff(eqth,hn);
deltal=0.01;
deltah0=0.006;
deltahn=0.006;
a=1/2*atan(h0/l);
th=atan(h0/l)-atan(hn/l)
n=-1;
h0=0.26;
hn=0.079;
lamb=405*10^(-9);
l=0.15;
ltermi=abs((d_a*a_l+d_th*th_l+d_th*th_a*a_l))*abs(deltal);
h0termi=abs(d_a*a_h0+d_th*th_a*a_h0)*abs(deltah0);
hntermi=abs(d_th*th_hn)*abs(deltahn);
error_dvd(1)=double(subs(ltermi+h0termi+hntermi))
d_dvd(1)=n*lamb/(sin(atan(hn/l)-1/2*atan(h0/l))-sin(1/2*atan(h0/l)))

Best Answer

Running:
ReasonsDoubleIsNotPossible = symvar(subs(ltermi+h0termi+hntermi))
just before ‘error_dvd’ produces: 
ReasonsDoubleIsNotPossible =
[ h0, hn, l]
The presence of symbolic variables in that expression prevents the double evaluation. Those variables nust have numeric values.
The problem is with respect to the order of the statements.
This does what you want: 
syms a n lamb th h0 l hn deltal
eqd=n*lamb/(sin(a-th)-sin(a));
eqa=1/2*atan(h0/l);
eqth=2*a-atan(hn/l);
d_a=diff(eqd,a);
a_l=diff(eqa,l);
d_th=diff(eqd,th);
th_l=diff(eqth,l);
th_a=diff(eqth,a);
a_h0=diff(eqa,h0);
th_hn=diff(eqth,hn);
deltal=0.01;
deltah0=0.006;
deltahn=0.006;
n=sym(-1);
h0=sym(0.26);
hn=sym(0.079);
lamb=sym(405*1E-9);
l=sym(0.15);
a=1/2*atan(h0/l);
th=atan(h0/l)-atan(hn/l)
ltermi=abs((d_a*a_l+d_th*th_l+d_th*th_a*a_l))*abs(deltal);
h0termi=abs(d_a*a_h0+d_th*th_a*a_h0)*abs(deltah0);
hntermi=abs(d_th*th_hn)*abs(deltahn);
error_dvd(1)=double(subs(ltermi+h0termi+hntermi))
d_dvd(1)=n*lamb/(sin(atan(hn/l)-1/2*atan(h0/l))-sin(1/2*atan(h0/l)))
and with the addition of the vpa call: 
d_dvd(1)=vpa(n*lamb/(sin(atan(hn/l)-1/2*atan(h0/l))-sin(1/2*atan(h0/l))))
produces: 
error_dvd =
    57.3505e-009
d_dvd =
    0.00000075120302179556347730706133365097
.
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