How to change positive/ negative sign of a value in if condition loop ?

Hello MATLAB Community,

I have problem with my if condition statement.

After finding two values (namely phi & theta) randomly within a certain a range,

I need to also consider a third value (psi) which is the opposite of phi value and perform an iteration using for loop.

In order to do so, I considered "if condition" to say that 'if phi is positive then psi is negative' and 'if phi is negative then psi is positive'.

But when I ran 3 iterations, I wasn't getting what I expected to get.

I got these values for 3 iterations:

first iteration:

phi = 0.0299

theta = -0.3501

psi = -0.0299

second iteration:

phi = 0.1981

theta = 0.2599

psi = -0.1981

third iteration:

phi = -0.5111

theta =-0.1706

psi = -0.5111

As you can see, when phi is negative in third iteration, the psi value is also the same but I want the sign to be opposite.

here is my code:

d2r = pi/180;

g = 30; %upper limit phi
h = -30; %lower limit phi
j = 30; %upper limit theta
l = -30; %lower limit theta

n = 3;
for k = 1:n
    phi = (h + (g-h)*rand(1))*d2r %range of limit in phi
    theta = (l + (j-l)*rand(1))*d2r %range of limit in theta
    if phi > 0
        psi = -phi
    elseif phi < 0
        psi = +phi
    end
end

Can anyone please help me with any suggestions.

Thank you in advance!!

I really appreciate your help.

Kind regards,

Shiv

Answer 1

[KSSV]

Why you are worried about creating it in a loop with if condition? You create your phi vector and after just change the sign of phi to get psi.

d2r = pi/180;

g = 30; %upper limit phi
h = -30; %lower limit phi
j = 30; %upper limit theta
l = -30; %lower limit theta

n = 3;
phi = zeros(n,1) ;
theta = zeros(n,1) ; 
for k = 1:n
    phi(k) = (h + (g-h)*rand(1))*d2r ; %range of limit in phi
    theta(k) = (l + (j-l)*rand(1))*d2r ; %range of limit in theta
end
psi = -phi ;
[phi psi]

ans = 3×2

-0.0747 0.0747 -0.4408 0.4408 0.4802 -0.4802

If you want to go by yout loop method. You need to follow like shown below.

d2r = pi/180;

g = 30; %upper limit phi
h = -30; %lower limit phi
j = 30; %upper limit theta
l = -30; %lower limit theta

n = 3;
for k = 1:n
    phi = (h + (g-h)*rand(1))*d2r ;%range of limit in phi
    theta = (l + (j-l)*rand(1))*d2r ; %range of limit in theta
    if phi > 0
        psi = -phi ; 
    elseif phi < 0
        psi = -phi ;  % only a chnage here, as phi is already negative, give minus sign or use abs(phi)
    end
    [phi psi]
end

[Shiv Karpoor]

I am new to coding and MATLAB as well, there is no mandate to use the if loop condition.

I just thought it would work and tried it.

But I will keep this in my mind, thank you so much KSSV !! I appreciate your help.