how to apply if statement with equations

Question

Danish Iqhwan Rohaizad 2022-3-22

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1678189-how-to-apply-if-statement-with-equations

回答： Star Strider 2022-3-23

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

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Answer 1

Star Strider 2022-3-23

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https://ww2.mathworks.cn/matlabcentral/answers/1678189-how-to-apply-if-statement-with-equations#answer_924669

在 MATLAB Online 中打开

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')

.

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Answer 2

Arif Hoq 2022-3-23

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1678189-how-to-apply-if-statement-with-equations#answer_924559

编辑：Arif Hoq 2022-3-23

在 MATLAB Online 中打开

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