Error: Inner Matrix dimensions must agree

INPUT:
mg=4000;                   
L=4;                        
r_min=1.2;                  
r_max=2.2;                  
r=1.2:0.05:2.2;             
theta_min=-20;              
theta_max=80;               
theta=-20:5:80;            
phi=0:9:180;                
gamma=phi+20;               
c_2=((r_max^2)-(r_min^2))./(cos(gamma+theta_min)-cos(gamma+theta_max));
c_1=(r_min^2)+c_2.*cos(gamma+theta_max);
a=0.5*(((c_1+c_2).^(1/2))+((c_1-c_2).^(1/2)));
b=c_2/(2*a);
F=(r*mg*L*cos(theta))/(b*a*sin(gamma+theta));         

WORKSPACE: 
phi: 1x21 double
r: 1x21 double
theta: 1x21 double

OUTPUT:
>> MatLab_Code_1
Error using  * 
Inner matrix dimensions must agree.

Error in MatLab_Code_1 (line 37)
F=(r*mg*L*cos(theta))/(b*a*sin(gamma+theta));

Answer 1

[Manoj]

Try using the dot operator for b and F as well

    b=c_2./(2*a); 
F=(r*mg*L.*cos(theta))./(b.*a.*sin(gamma+theta));

[michel hardy]

heu Manjo

how i can find out when it s neccessary using .* or not? or give me suggestion for more self-study about it .

it is your favor if reply me on gmail. d.physic54

[DGM]

An account that's been inactive for the last six years isn't likely to spring to life to answer your question.

https://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html

Answer 2

[Ced]

First things first... please try to display your question in a more accessible form.

1. Use the Code {} button to insert code

2. explain what this is about, what the question is.

3. Have you tried debugging it yourself? What have you tried? What worked/didn't?

Then, to your question: "Inner matrix dimensions must agree" means that you are applying an operator to two matrices which don't match in their number of required rows/columns. Remember that matlab always does matrix operations unless stated otherwise.

Concretely in your example:

a) you probably want to do element-wise operations, e.g. compute the force for different values of r, theta, etc. You can specify element-wise operations by adding a "." to your operator, e.g. "./" instead of "/" and ".*" instead of "*".

b) If you have a number of vectors you want to combine in a formula, it may be a good idea to use linspace(a,b,N) instead of e.g. 10:5:100. That way, you'll be sure to have the same number of elements in all vectors.