how to apply if statement with equations

Hey everyone !

I would like to request assisstance on this coding of mine. What im trying to achieve here is that for my value of beta2, by using the if statement I would like Matlab to run the codes to achieve different values for beta2. Lets say if Wtheta2 > 0, then beta2 will be calculated using formula beta2 = -(abs(acosd(Cm2./W2))). Meanwhile, if Wtheta < 0, hence beta2 will be calculated using the formula beta2 = abs(acosd(Cm2./W2)). However, when i run the code, it seems matlab does not recognize the beta2 variable as it is supposed to.

Cm2 = [80.105 81.868 97.935 116.429];

W2 = [82.564 83.693 98.029 119.103]; %Relative Velocity Inlet Rotor

Wtheta2 = [19.99 17.382 -4.308 -25.094] ; %Tangential relative velocity inlet rotor

% Relative Flow Angle @ Inlet Rotor, beta2

if (Wtheta2 > 0)

beta2 = -(acosd(Cm2./W2));

elseif (Wtheta2 < 0)

beta2 = abs(acosd(Cm2./W2));

end

Answer 1

[Star Strider]

Using a ‘logical indexing’ approach instead of the if block —

Cm2 = [80.105 81.868 97.935 116.429];
W2 =  [82.564 83.693 98.029 119.103]; %Relative Velocity Inlet Rotor
Wtheta2 = [19.99 17.382 -4.308 -25.094] ; %Tangential relative velocity inlet rotor
beta2 = (Wtheta2<0) .* (-acosd(Cm2./W2)) + (Wtheta2 > 0) .* (abs(acosd(Cm2./W2)));                  % One-Line Statement
beta2_fcn = @(Wtheta2) (Wtheta2<0) .* (-acosd(Cm2./W2)) + (Wtheta2 > 0) .* (abs(acosd(Cm2./W2)));   % Anonymous Function 

figure
plot(Wtheta2, beta2)
grid
xlabel('Wtheta2')
ylabel('beta2')

figure
plot(Wtheta2, beta2_fcn(Wtheta2))
grid
xlabel('Wtheta2')
ylabel('beta2')

.

Answer 2

[Arif Hoq]

you have only 2 conditions. greater/equal to 0 or less than 0. so try to use if-else function, not if-elseif.

Cm2 = [80.105 81.868 97.935 116.429];
W2 =  [82.564 83.693 98.029 119.103]; %Relative Velocity Inlet Rotor
Wtheta2 = [19.99 17.382 -4.308 -25.094] ; %Tangential relative velocity inlet rotor
% Relative Flow Angle @ Inlet Rotor, beta2
if Wtheta2 >= 0
    beta2 = -(acosd(Cm2./W2)); 
else 
    beta2 = abs(acosd(Cm2./W2));
end 
disp(beta2)

   14.0186   11.9872    2.5093   12.1639

[Danish Iqhwan Rohaizad]

First of all thanks Arif for responding. If lets say the value vector of 2.5093 and 12.1639 should be negative, how should i progress from there ?

Thanks in advance !

[Arif Hoq]

i don't now why you are expecting that. but simply you can do that

output=[beta2(1) beta2(2) -beta2(3) -beta2(4)]

or

output=[beta2([1 2]) -beta2([3 4])]