Unable to perform assignment because the left and right sides have a different number of elements.

%%
clc
clear all
A1 = 0.015; %Excitation Amplitude
A2 = 0.035;
A3 = 0.06;
l_r = 0.617; %Wave length of the road
v = 0.04107647433; %Speed(m/s)
P = l_r/v; %Period
Om = 1/P*2*pi; %Forcing Frequency
%Om = %0.07m,2m,45m/s

k_l = 26400; %Linear stiffness
m = 483; %Mass
d = -0.473; %Stretching condition
l = 0.946; %Length of the spring
k_s = -(k_l*(l-d))/(4*d); %Spring stiffness
f_n = sqrt(k_l/m)/(2*pi); %Natural frequency

%%
fig = figure();
ax = axes();
hold(ax); 
% view([-53 33]);
grid on
A_array = linspace(0,5,100); %Excitation Amplitude
T = 150;
x0 = [0,0];
xval = zeros([],1) ; 
yval = zeros([],1) ; 
for i=1:numel(A_array)
    A = A_array(i);
    
f = @(t,x) [ x(2); ...
    -(2*k_s*(x(1)-(A1*sin(Om*t)+A2*sin(2*Om*t)+A3*sin(3*Om*t)))* ...
    (sqrt((l-d)^2 + (x(1)-(A1*sin(Om*t)+A2*sin(2*Om*t)+A3*sin(3*Om*t)))^2) - l))/ ...
    (m*(sqrt((l-d)^2 + (x(1)-(A1*sin(Om*t)+A2*sin(2*Om*t)+A3*sin(3*Om*t)))^2))) ];

    [t, x] = ode45(f,[100,T],x0);
    A = A1*sin(Om*t)+A2*sin(2*Om*t)+A3*sin(3*Om*t);
    Response_amp = (max(x(:,1)) - min(x(:,1)))/2;
%    xval(i) = Om/(2*pi) ; 
    xval(i) = A; 
    yval(i) = Response_amp ; 
    
    % plot(Om, Response_amp, '.');
end
plot(xval,yval) ; 

xlabel('A (m)')
ylabel('Response Amplitude (m)')

ax = gca;
ax.XAxis.FontSize =16;
ax.YAxis.FontSize =16;

Hi, all. I wish to plot a graph (A vs Response_amp). However, it keeps saying 'Unable to perform assignment because the left and right sides have a different number of elements.'. How am I supposed to solve this problem?

Thanks for reading.

[Geoff Hayes]

donghun - the problem is with this line

xval(i) = A; 

where you are trying to assign a 1225x1 matrix A as a scalar in xval. Is this intentional? Or are you expecting A to be a scalar?

[David Hill]

I have no idea what you are trying to graph (big picture), but you cannot assign xval(i)=A where A is (1225x1). Is your response from ode45 what you expect [t (1225x1) and x (1225x2)]?

Answer 1

[Ameer Hamza]

Just for the sake of future reference, the code related to this question can be found in this comment: https://www.mathworks.com/matlabcentral/answers/515493-how-do-i-plot-3d-graph#comment_821818