# "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?

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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

##### 2 Comments

Stephen23
on 28 Jan 2023

""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)

all(x<0)

all(x==0)

all(y>0)

all(y<0)

all(y==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.

### Accepted Answer

cdawg
on 28 Jan 2023

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

##### 3 Comments

Stephen23
on 28 Jan 2023

Edited: Stephen23
on 28 Jan 2023

"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

T2 = atan2d(Y,X) % the simple MATLAB approach

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

No loops or complex IF/ELSEIF required.

cdawg
on 28 Jan 2023

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