Unknown error in my m.function

I have designed a m.function in a matlab scripts that calculates the angle between a vector you enter in it as an input and the horizontal line (0º).

The script is that:

function [theta]=angulodrecta(V)
% You put an input in a vector form (for example V=[...]
%And the function gives you the angle between that vector and the
%positive horizontal (0º)

%it works supposing that x1=y1=0


y2=V(2,2);
x2=V(1,2);
y1=V(2,1);
x1=V(1,1);

m=(y2-y1)/(x2-x1);


if y2==0
    if x2>0
        elangulo=atand(m);
        theta=elangulo
    elseif x2<0
        elangulo=atand(m);
        theta=180+elangulo
    end
elseif x2==0
    if y2>0
        elangulo=atand(m);
        theta=elangulo
    elseif y2<0
        elangulo=atand(m);
        theta=180-elangulo
    end
elseif x2<0&&y2>0
    elangulo=atand(m);
    theta=180+elangulo
elseif x2<0&&y2<0
    elangulo=atand(m);
    theta=180+elangulo
elseif x2>0&&y2<0
    elangulo=atand(m);
    theta=270-elangulo
end

end

The question here is that, when I try to execute the following code:

%------------------------BLOCK A------------------------
L1=[0 5;0 0];
L2=[0 5;0 5];
L3=[0 0;0 5];
L4=[0 -5;0 5];
L5=[0 -5;0 0];
L6=[0 -5;0 -5];
L7=[0 0;0 -5];
L8=[0 5;0 -5];
%-------------------------------------------------------

%------------------------BLOCK B------------------------
hold on
axis equal
grid
%-------------------------------------------------------

%------------------------BLOCK C------------------------
plot(L1(1,:),L1(2,:),'r');
plot(L2(1,:),L2(2,:),'r');
plot(L3(1,:),L3(2,:),'r');
plot(L4(1,:),L4(2,:),'r');
plot(L5(1,:),L5(2,:),'r');
plot(L6(1,:),L6(2,:),'r');
plot(L7(1,:),L7(2,:),'r');
plot(L8(1,:),L8(2,:),'r');
%-------------------------------------------------------

%------------------------BLOCK D------------------------
[th1]=angulodrecta(L1)
[th2]=angulodrecta(L2)
[th3=angulodrecta(L3)
[th4]=angulodrecta(L4)
[th5]=angulodrecta(L5)
[th6]=angulodrecta(L6)
[th7]=angulodrecta(L7) 
[th8]=angulodrecta(L8)
%-------------------------------------------------------

Matlab only calculates correctly the first angle (th1) and then it gives me the following error as you can see

What am I doing wrong? what is going wrong?

That error only occurs when I tell matlab to calculate one or more anglee, I mean, if the code introduced in the command window I only put one ([th4]=angulodrecta(L4) for example, or [th7]=angulodrecta(L7) or whatever) it works fine. The problem occurs when I put in the command window two or more of them.

Answer 1

[Stephen23]

There are a few combinations of values for which your function does not define the output argument. If you call the function with those values, it will throw an error becase the output is undefined. For example, consider y2==0 and x2==0. It might be that you expect the code evaluation to "fall through" like it does in some languages, but in MATLAB only one block of an if-elseif-...-else block will be executed, so once y2==0 is matched then that block will be executed, and if x2==0 then you do not define theta. Thus the error.

Tip1: note that the MATLAB approach would use simpler vectorized code, e.g. based on modulo and other vectorized operations. While your approach works for individual values, you should learn how to write code that handles entire vectors/matrices/arrays of data:

https://www.mathworks.com/help/matlab/matlab_prog/vectorization.html

Tip2: numbering variables like that is a sign that you are doing something wrong. The MATLAB approach is to use indexing, which is automatically expandable, generalizable, does not require copy-and-paste of code, and makes both looping and code vectorization easy.

[ErikJon Pérez Mardaras]

Thanks but I don't understand very well what you said. I mean, I made this m.function supposing that everytime is going to be x1=y1=0 and that at no time is going to be x2=0 AND y2=0. As you can see in vectors (L1,L2,L3...etc), no one has x2=0 AND y2=0.

L1=[0 5;0 0];
L2=[0 5;0 5];
L3=[0 0;0 5];
L4=[0 -5;0 5];
L5=[0 -5;0 0];
L6=[0 -5;0 -5];
L7=[0 0;0 -5];
L8=[0 5;0 -5];

However I still don't understand why it works one by one and not when are putted all of them at the same time. I mean, executing A,B, C blocks and only ONE line (whichever) of block D, it works perefectly. The issue comes when two or more lines from block D are putted.

[Stephen23]

"However I still don't understand why it works one by one..."

It doesn't. When I try L2 I get the same error (undefined output).

The cause is easy to find: you did not define the output theta if both x2>0 and y2>0.

I have not given you a complete, exhaustive list of all of the cases for which your function does not define theta, but so far I have shown you two cases, at least one of which actually occurs in your L data matrices. You need to revise the logic of your function yourself (I cannot tell you how because I do not know what the algorithm is).

I strongly recommend that you write vectorized code (e.g. using mod, etc) and avoid these verbose and buggy ifs.

[ErikJon Pérez Mardaras]

Okay, I will check it out again

Great Thanks Cheburashka!!