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.

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Adam Thompson 2020-3-11

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https://ww2.mathworks.cn/matlabcentral/answers/510367-unable-to-perform-assignment-because-the-size-of-the-left-side-is-1-by-1-and-the-size-of-the-right-s

评论： Star Strider 2020-3-12

% 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;

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Guillaume 2020-3-11

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

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采纳的回答

Star Strider 2020-3-11

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https://ww2.mathworks.cn/matlabcentral/answers/510367-unable-to-perform-assignment-because-the-size-of-the-left-side-is-1-by-1-and-the-size-of-the-right-s#answer_419679

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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.

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Adam Thompson 2020-3-12

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

Star Strider 2020-3-12

My pleasure!

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

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