"theta" is defined inside of the statement, but when I run my code it gives me an error saying that " 'theta' is not a recognized function or variable'. Any solutions?

Question

Brendan 2023-1-28

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1902380-theta-is-defined-inside-of-the-statement-but-when-i-run-my-code-it-gives-me-an-error-saying-that

评论： cdawg 2023-1-28

x = [2 2 0 -3 -2 -1 0 0 2];
y = [0 1 3 1 0 -2 0 -2 2];
r = (sqrt((x.^2)+(y.^2)));
if x > 0 
  theta = atan(y./x);
  
elseif x < 0 
    theta = atan(y./x);
    
    if y > 0
        theta = atan(y./x) + (pi);
        
    elseif y < 0 
        theta = atan(y./x) - (pi);
        
    elseif y == 0 
        theta = pi;
        
    end
elseif x == 0
    if y > 0 
        theta = (pi./2);
        
    elseif y < 0 
        theta = (-pi./2);
        
    elseif y == 0
        theta = 0;
      
    end
end
theta = rad2deg(theta); % Changing theta from radians to degrees
w = [x' y' r' theta']; % A matrix containing all of the values

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Brendan 2023-1-28

编辑：Brendan 2023-1-28

When I go and define 'theta' outside of the if statment, Matlab just bypasses the if statement entirely and only uses the defined value.

Stephen23 2023-1-28

在 MATLAB Online 中打开

""theta" is defined inside of the statement"

Nope. As the IF documentation states, "An expression is true when its result is nonempty and contains only nonzero elements". Lets check which of your logical expressions are considered true:

x = [2,2,0,-3,-2,-1,0,0,2];
y = [0,1,3,1,0,-2,0,-2,2];
all(x>0)
ans = logical
   0
all(x<0)
ans = logical
   0
all(x==0)
ans = logical
   0
all(y>0)
ans = logical
   0
all(y<0)
ans = logical
   0
all(y==0)
ans = logical
   0

So none of them are true, none of your IF/ELSEIF will run, and THETA is never defined inside them.

In any case, you should be using indexing for this task, not IF/ELSEIF. Or ATAN2.

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Answer 1

cdawg 2023-1-28

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1902380-theta-is-defined-inside-of-the-statement-but-when-i-run-my-code-it-gives-me-an-error-saying-that#answer_1158495

在 MATLAB Online 中打开

x is a vector so when you say "if x > 0" it is returning a logic vector of the locations where that is the case. You need to loop through:

X = [2 2 0 -3 -2 -1 0 0 2];
Y = [0 1 3 1 0 -2 0 -2 2];
R = (sqrt((X.^2)+(Y.^2)));
for ii = 1:length(X)
x = X(ii);
y = Y(ii);
r = R(ii);
if x > 0 
  theta = atan(y./x);
 
elseif x < 0 
    theta = atan(y./x);
    
    if y > 0
        theta = atan(y./x) + (pi);
        
    elseif y < 0 
        theta = atan(y./x) - (pi);
        
    elseif y == 0 
        theta = pi;
        
    end
elseif x == 0
    if y > 0 
        theta = (pi./2);
        
    elseif y < 0 
        theta = (-pi./2);
        
    elseif y == 0
        theta = 0;
      
    end
end
THETA(ii) = theta;
end
THETA = rad2deg(THETA); % Changing theta from radians to degrees
w = [X' Y' R' THETA']; % A matrix containing all of the values

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Stephen23 2023-1-28

编辑：Stephen23 2023-1-28

在 MATLAB Online 中打开

"You need to loop through"

No, you don't need a loop. This is easy to solve using exactly one command and no loop. Lets try:

X = [2,2,0,-3,-2,-1,0,0,2];
Y = [0,1,3,1,0,-2,0,-2,2];
R = (sqrt((X.^2)+(Y.^2)));
for ii = 1:length(X)
    x = X(ii);
    y = Y(ii);
    r = R(ii);
    if x > 0 
        theta = atan(y./x); 
    elseif x < 0 
        theta = atan(y./x);
        if y > 0
            theta = atan(y./x) + (pi);
        elseif y < 0 
            theta = atan(y./x) - (pi);
        elseif y == 0 
            theta = pi;
        end
    elseif x == 0
        if y > 0 
            theta = (pi./2);
        elseif y < 0 
            theta = (-pi./2);
        elseif y == 0
            theta = 0;
        end
    end
    THETA(ii) = theta;
end
T1 = rad2deg(THETA) % Changing theta from radians to degrees
T1 = 1×9
         0   26.5651   90.0000  161.5651  180.0000 -116.5651         0  -90.0000   45.0000
T2 = atan2d(Y,X) % the simple MATLAB approach
T2 = 1×9
         0   26.5651   90.0000  161.5651  180.0000 -116.5651         0  -90.0000   45.0000

There you are: one simple function that gives the same output (to within floating point precision).

No loops or complex IF/ELSEIF required.

cdawg 2023-1-28

Thanks Stephen. I guess I shouldn’t have said you *need* to loop through. In fact that’s something I’m trying to work on as it’s usually my “goto” method, but is often unnecessary :-)

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