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

The code is for basic Euler to solve Second order ODEs, I'm really stuck for how to resolve this error message:

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

Error in basicEuler (line 23)

v1(i + 1) = v1(i) + h*(eq1(x1(i), x2(i), v1(i), v2(i))); '

Any help/advice is welcome

Thank you

function [] = basicEuler(h,N)
h = 0.5; N = 500;
t = 1:h:N;

x1 = zeros(size(t)); x1(1) = -7;
x2 = zeros(size(t)); x2(1) =  7;
x3 = zeros(size(t)); x3(1) = -3;
v1 = zeros(size(t)); v1(1) = -3;
v2 = zeros(size(t)); v2(1) = -2;    
v3 = zeros(size(t)); v3(1) = -1;
F  = zeros(size(t)); F(1) = myForce(0);

for i = 1:(length(t)-1)

    F(i + 1) = myForce(i);
    
    x1(i + 1) = x1(i) + h*v1(i);   
    x2(i + 1) = x2(i) + h*v2(i);
    x3(i + 1) = x3(i) + h*v3(i);
    
    v1(i + 1) = v1(i) + h*(eq1(x1(i), x2(i), v1(i), v2(i)));
    v2(i + 1) = v2(i) + h*(eq2(x1(i), x2(i), x3(i), v1(i), v2(i), v3(i)));
    v3(i + 1) = v3(i) + h*(eq3(x2(i), x3(i), v2(i), v3(i), F(i)));
   
end

figure(1);
plot(t,x1)
title('basicEuler x1(t)')
xlabel('Time')
ylabel('Displacement')

figure(2);
plot(t,x2)
title('basicEuler x2(t)')
xlabel('Time')
ylabel('Displacement')

figure(3);
plot(t,x3)
title('basicEuler x3(t)')
xlabel('Time')
ylabel('Displacement')

end

[Torsten]

Where did you define the functions for the ODEs (eq1,eq2 and eq3) ?

[Walter Roberson]

eq1 is returning a vector result.

[Cody]

As seperate scripts

function [equation1] = eq1(x1,x2,v1,v2)

global c1 c2 c3 k1 k2 k3 m1 m2 m3

equation1 = (-( v1*( c1 + c2 ) + ( v2*c2 ) - ( x1*( k1 + k2 ) ) + ( x2*k2 ) ) ) /m1 ;

end

function [equation2] = eq2(x1,x2,x3,v1,v2,v3)

global c1 c2 c3 k1 k2 k3 m1 m2 m3

equation2 = ( ( v1*c2 ) - ( v2*( c2 + c3 ) ) + ( v3*c3 ) + ( x1*k2 ) - ( x2*( k2 + k3 ) ) + ( x3*k3 ) )/m2 ;

end

function [equation3] = eq3(x2,x3,v2,v3,F)

global c1 c2 c3 k1 k2 k3 m1 m2 m3

equation3 = ( ( v2*c3 ) - ( v3*c3 ) + ( x2*k3 ) - ( x3*k3 ) + F)/m3 ;

end

with c1,c2,c3,k1,k2,k3 being separate variables defined through respective functions

[Torsten]

Your global variables seem to be arrays, not scalars as required.

Answer 1

[SAI SRUJAN]

Hi cody,

I understand that you are working solve second-order ODEs using the Euler method. You're encountering an error that says, "Unable to perform assignment because the left and right sides have a different number of elements," which occurs at line 23 in your code.

v1(i + 1) = v1(i) + h*(eq1(x1(i), x2(i), v1(i), v2(i)));

This error typically indicates that when you're trying to assign a value to 'v1(i + 1)', MATLAB expects a single value on the right side of the assignment, but it's receiving an array or a different number of elements instead.

It is critical to verify that the global variables 'c1',' c2', 'c3', 'k1', 'k2', 'k3', 'm1', 'm2', and 'm3', which are utilized within your 'eq1', 'eq2', and 'eq3' functions, are correctly initialized and assigned scalar values prior to invoking the 'basicEuler' function.

Please ensure that the equation, yields a single scalar value as a result.

equation1 = (-( v1*( c1 + c2 ) + ( v2*c2 ) - ( x1*( k1 + k2 ) ) + ( x2*k2 ) ) ) /m1 ;

By ensuring the accuracy of aforementioned details, you should be able to resolve the error.

I hope this helps!