Solve in a loop - Unable to perform assignment because the indices on the left side are not compatible with the size of the right side

Hello,

I wrote this code in order to estimate the value of x :

for i=2:length(TimeReel)
syms x  
eqn = x == (Qin*(Cin-GpureFluo(i))-GpureFluo(i)*(pi()*Qcr.*sin(acos((Qin+x-Qp)./Qcr))-(Qin+x-Qp)./2/pi().*(2*acos((Qin+x-Qp)./Qcr)))-hw*pi()*rw*rw*(GpureFluo(i)-GpureFluo(i-1))/dt(i))/GpureFluo(i);
S(i) = solve(eqn);
end

I have to use solve because the value of x is found in the acos. All others values are known

If I run this, the following message appears :

"Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.

Error in sym/privsubsasgn (line 1133)

L_tilde2 = builtin('subsasgn',L_tilde,struct('type','()','subs',{varargin}),R_tilde);

Error in sym/subsasgn (line 970)

C = privsubsasgn(L,R,inds{:});"

Could you help ?

Thanks :)

N.S.

[Walter Roberson]

Side note:

You are going to run into precision problems due to floating point round-off. I suggest something closer to

Q = @(v) sym(v);
Qin_s = Q(Qin);
Cin_s = Q(Cin);
GpureFluo_s = Q(GpureFluo);
Pi = Q(pi);
Qcr_s = Q(Qcr);
Qin_s = Q(Qin);
Qp_s = Q(Qp);
hw_s = Q(hw);
rw_s = Q(rw);
dt_s = Q(dt);

syms x

S = cell(length(TimeReel),1);
for i=2:length(TimeReel)
  eqn = x == (Qin_s*(Cin_s-GpureFluo_s(i))-GpureFluo_s(i)*(Pi*Qcr_s.*sin(acos((Qin_s+x-Qp_s)./Qcr_s))-(Qin_s+x-Qp_s)./2/Pi.*(2*acos((Qin_s+x-Qp_s)./Qcr_s)))-hw_s*Pi*rw_s^2*(GpureFluo_s(i)-GpureFluo_s(i-1))/dt_s(i))/GpureFluo_s(i);
  S{i} = solve(eqn);
end

Answer 1

[Dyuman Joshi]

It is most likely that there are multiple solutions or no solutions for equations, which can't be stored in a single placeholder.

Remember that trignometric functions are periodic in nature and multiple solutions for an equation is a possibility.

You can initialize the array S as a cell array to store the solutions and use indexing to access them -

n = length(TimeReel);
S = cell(n-1,1);

for i=2:n
syms x  
eqn = x == (Qin*(Cin-GpureFluo(i))-GpureFluo(i)*(pi()*Qcr.*sin(acos((Qin+x-Qp)./Qcr))-(Qin+x-Qp)./2/pi().*(2*acos((Qin+x-Qp)./Qcr)))-hw*pi()*rw*rw*(GpureFluo(i)-GpureFluo(i-1))/dt(i))/GpureFluo(i);
S{i,1} = solve(eqn);
end

Note the answers stored are symbolic numbers. Use double() to convert the solutions to double precision numbers.

[NASI]

Thanks!

Answer 2

[Walter Roberson]

You would get that error if solve does not return exactly one solution, such as if it returns no solutions or returns two solutions pi apart. Store the output in a cell array