Why does my plot go blank when I run my program?

The code is fairly fine syntaxwise but when I run it there is no plot or simulation appearing

clear all; %#ok<CLALL> 
close all;


L_0=7;
L_1=3;
L_2=7.5;
L_3=4.5;
L_PA=4;

alpha=35;
w2=5;

theta2=0:2:360;
figure(1);

for i=1:length(theta2(i))
    theta2(i)=0:2:360;
    AC(i)= sqrtL_0^2+L_1^2-2*L_0*L_1*cosd(theta2(i));  
    beta(i)=acosd(L_0^2+AC(i)^2-L_1^2)/ 2*L_0*AC(i);  
    psi(i)=acosd(L_2^2+AC(i)^2-L3^2)/ 2*L_2*AC(i);
    lambda(i)=acosd(L_3^2+AC(i)^2-L_2^2)/ 2*L_3*AC(i); %#ok<*SAGROW> 
    
    theta3(i)= psi(i)-beta(i);
    theta4(i)= 180-lambda(i)-beta(i);
    if theta2(i)> 180
        theta3(i)=psi(i)+beta(i);
        theta4(i)=180-lambda(i)+beta(i);
    end
    %Define joint positions
    Ox(i)=0;   
    Oy(i)=0;
    
    Ax(i)=Ox(i)+L_1*cosd(theta2(i));
    Ay(i)=Ox(i)+L_1*sind(theta2(i));
    
    Bx(i)=Ox(i)+Ax(i)+L_2*cosd(theta3(i));
    By(i)=Oy(i)+Ay(i)+l_2*sind(theta3(i));
    
    Cx(i)=L_0;
    Cy(i)=0;
    
    Px(i)=Ax(i)+L_PA*cosd(alpha+theta3);
    Py(i)=Ay(i)+L_PA*sind(aplha+theta3);
    
    
    plot ( [Ox(i) Ax(i)], [Oy(i) Ay(i)], [Ax(i) Bx(i)], [Ay(i) By(i)],...
        [Bx(i) Cx(i)], [By(i) Cy(i)],'LineWidth',3);
    plot([Ax(i) Px(i)], [Ay(i) Py(i)], [Px(i) Bx(i)], [Py(i) By(i)], 'LineWidth',3);
    
    hold on;
    grid on;
    axis equal;
    axis( [-5 15 -5 10] );
    drawnow;
    hold off; 
    
end

Array indices must be positive integers or logical values.

Answer 1

[Voss]

There is an error that occurs before the loop is entered:

clear all; %#ok<CLALL> 
close all;
 
 
L_0=7;
L_1=3;
L_2=7.5;
L_3=4.5;
L_PA=4;
 
alpha=35;
w2=5;
 
theta2=0:2:360;
figure(1);

for i=1:length(theta2(i))
    % ...
    % stuff
    % ...
end

Array indices must be positive integers or logical values.

i is not defined before the for loop, so MATLAB uses the imaginary unit 1i in

theta2(i)

which causes an error because 1i is not a valid index. I imagine that was a typo - clearly length(theta) was intended there.

If you make that change, next you'll run into an error on the first line inside the for loop:

theta2(i)=0:2:360;

because theta2(i) is one element and 0:2:360 is a vector with more than one element. That line can be removed.

Then you'll run into an error because sqrtL_0 is not defined. That one I don't know how to fix.

Answer 2

[Walter Roberson]

for i=1:length(theta2(i))

At the point in your code where you are constructing the bounds of the for loop, the bounds to the right of the = are evaluated before any assignment is done to the loop control variable. Therefore, your theta2(i) needs to be evaluated before anything gets assigned to i because of the for loop. But you have not assigned any value to i before that point, so i is going to have its default value, which is sqrt(-1) ... which is not a valid index.

If i were a valid scalar index at that point, then theta2(i) would refer to a single location, and length() of that would be 1. length(theta2(i)) would always be the same as length(i) provided that all values in i are within the bounds of theta2 . So you might as well code the clearer

for i = 1 : length(i)

after having assigned something to i

... Or perhaps what you wanted was

for i = 1 : length(theta2)