Unable to perform assignment because the size of the left side is 1-by-1 and the size of the right side is 0-by-1.

% Y asymptote of curve
Rd = 2;
% Release point x
XL = -1;
% Starting point x
X0 = 2.5;

% Find A for launch angles 0-45deg

A_values = ones(1,45);
B_values = ones(1,45);
LaunchAngle = ones(1, 45);
for qL = 1:45
   LaunchAngle(1,qL) = qL;
   qLrad = deg2rad(qL);
   Rd = 2;   % Y asymptote of curve
   XL = -1;  % Release point x
   X0 = 2.5; % Starting point x
   syms A 
   syms B
   F = [(((-2*A*Rd*exp(A*(XL-B)))/((exp(A*(XL-B))+1)^2))-tan(qLrad)== 0), ((-Rd + ((2*Rd) /(1 + exp(A*(X0 - B))))) == 0)];
   AB = vpasolve(F, [A, B])
   A_values(1,qL) = AB.A;
   B_values(1,qL) = AB.B;
end

I understand i'm currently trying to store the incorrect number of elements but I'm new to MATLAB and don't see how I'm doing this.

Offending line is:

A_values(1,qL) = AB.A;

[Guillaume]

A 0x1 array is an empty array. Sounds like vpasolve failed to find a solution.

Answer 1

[Star Strider]

The problem is that vpasolve could not find a solution, so it returned an empty vector. The Symbolic Math Toolbox is good for many things although not for iterative calculations.

Try this instead:

for qL = 1:45
   LaunchAngle(1,qL) = qL;
   qLrad = deg2rad(qL);
   Rd = 2;   % Y asymptote of curve
   XL = -1;  % Release point x
   X0 = 2.5; % Starting point x
%    syms A 
%    syms B
   F = @(A,B) [(((-2*A*Rd*exp(A*(XL-B)))/((exp(A*(XL-B))+1)^2))-tan(qLrad)), ((-Rd + ((2*Rd) /(1 + exp(A*(X0 - B))))))];
   AB0 = [1; 1];
   AB = fsolve(@(b)F(b(1),b(2)), AB0)
   A_values(:,qL) = AB(1);
   B_values(:,qL) = AB(2);
end

Experiment with the initial parameter estimates (‘AB0’) if necessary to get different results.

[Adam Thompson]

thanks, would using fsolve not require me to convert A and B from vp to double?

[Star Strider]

What does ‘vp’ mean?

The results are double, however they are results! Note that there are no real roots, so it is best to use:

AB0 = [1; 1]*1j;

to find the complex roots.

[Adam Thompson]

When using your code I get the error:

"Error using fsolve

FSOLVE requires all values returned by functions to be of data type double.

Error in

AB = fsolve(@(b)F(b(1),b(2)), AB0)

"

[Star Strider]

The code I poasted runs without error.

The complete code:

% Y asymptote of curve
Rd = 2;
% Release point x
XL = -1;
% Starting point x
X0 = 2.5;
% Find A for launch angles 0-45deg
A_values = ones(1,45);
B_values = ones(1,45);
LaunchAngle = ones(1, 45);
for qL = 1:45
   LaunchAngle(1,qL) = qL;
   qLrad = deg2rad(qL);
   Rd = 2;   % Y asymptote of curve
   XL = -1;  % Release point x
   X0 = 2.5; % Starting point x

   F = @(A,B) [(((-2*A*Rd*exp(A*(XL-B)))/((exp(A*(XL-B))+1)^2))-tan(qLrad)), ((-Rd + ((2*Rd) /(1 + exp(A*(X0 - B))))))];
   AB0 = [1; 1]*1j;
   AB = fsolve(@(b)F(b(1),b(2)), AB0);
   A_values(:,qL) = AB(1);
   B_values(:,qL) = AB(2);
end

To plot them:

figure
plot(LaunchAngle, real(A_values), '-b',    LaunchAngle,imag(A_values), '--b')
hold on
plot(LaunchAngle, real(B_values), '-g',    LaunchAngle,imag(B_values), '--g')
hold off
grid
legend('\Re(A)', '\Im(A)', '\Re(B)', '\Im(B)', 'Location','E')
xlabel('Launch Angle')
ylabel('Value')

producing:

Proving that it works!

[Adam Thompson]

apologies, typo my end. Thanks for your help star strider

[Star Strider]

My pleasure!

If my Answer helped you solve your problem, please Accept it!