For loop, Subscript indices must either be real positive integers or logicals ?

Hi,

I have been trying the following commands, but I could not, and got that error, (I got that error, Subscript indices must either be real positive integers or logicals.),

for i=[90,30,-30]

theta(i)=i*pi/180

end

& also, when type length of (i) it equals =-30, how ?, and I have 3 variables ?

actually, I hope to have 3 variables (one at i =90, and another =30, and so on ]

So please could you tell me how could I do this ?

Thanks,

Answer 1

[Walter Roberson]

https://www.mathworks.com/matlabcentral/answers/470959-loop-with-two-variables#answer_382585 shows a general structure you can use for looping with values that are not small consecutive integers.

[Walter Roberson]

thetaDegrees = [90,30,-30];
for K = 1 : 3
    thetaRadians = thetaDegrees(K) * sym(pi) / 180;
    T=[l1^2 m1^2 n1^2 2*m1*n1 2*l1*n1 2*l1*m1;
        l2^2 m2^2 n2^2 2*m2*n2 2*l2*n2 2*l2*m2;
        l3^2 m3^2 n3^2 2*m3*n3 2*l3*n3 2*l3*m3;
        l2*l3 m2*m3 n2*n3 m2*n3+n2*m3 l2*n3+n2*l3 l2*m3+m2*l3;
        l1*l3 m1*m3 n1*n3 m1*n3+n1*m3 l1*n3+n1*l3 l1*m3+m1*l3;
        l1*l2 m1*m2 n1*n2 m1*n2+n1*m2 l1*n2+n1*l2 l1*m2+m1*l2];
    T_n{K} = subs(T,{l1,m1,n1,l2,m2,n2,l3,m3,n3},{cos(theta_rad),sin(theta_rad),aij_z(1,3),...
        -sin(theta_rad),cos(theta_rad),aij_z(2,3),...
        aij_z(3,1),aij_z(3,2),aij_z(3,3)});
end

T_n{2}
T_n{3}

[Ali Tawfik]

Hey Walter,

Thanks for your answer.

It works finally :), that excatly, what I want!

but just I need to obtain another T_n normally without subsitutions, because it did not work with subs ,

just one more thing, how could I also, extract values for thetaradians ? like T_n {}, because when I assign thetaRadians{} , it did not work with the rest of commands,

hope you understand me, and help me .

Again,

I just need to extract specfic values also for thetaRadians (e.g. thetaradians{2@30})

Also, how can I apply loop with subs ?

Thanks,

Answer 2

[Geoff Hayes]

Ali - your for loop is iterating over the array [90, 30, -30] so your i is becoming each of these values. That is fine (for the equation) but fails when you use i as an index into your array and i is -30 (and so the error message makes sense). You can avoid the loop and just do

thetaDegrees = [90,30,-30];
thetaRadians = thetaDegrees * pi / 180;

[Ali Tawfik]

Thanks for your prompt reply.

I understand, but how can I obtain thetaRadians when it equals to 30 specially, or -30 ?

I need to obtain thetaRadians at a specific value from those three ???

Looking to hearing from you

Thanks,

[Geoff Hayes]

Ali - since you know the order of the original array, where 30 degrees is the first element and -30 is the third element, then

thetaRadians(1) % corresponds to 30 degrees
thetaRadians(3) % corresponds to -30 degrees

If that doesn't meet your needs, perhaps try describing how (in your code) you need to use these converted values.

[Geoff Hayes]

Or you can use a map as

degreeToRadianMap = containers.Map([90,30,-30],[90,30,-30] * pi / 180);

and so

degreeToRadianMap(30) % will return the 30 degrees in radians

and

degreeToRadianMap(-30) % will return the -30 degrees in radians

I haven't used maps before so you might want to be wary when using doubles as keys...

[Ali Tawfik]

Thanks again for your answer.

Now, it's working, but is there a way to do that using for loop, as I need that, for more commands, ??

Hope you could find a way to do the same via For loop

Thanks again :)

[Geoff Hayes]

Ali - what other commands should be added so that you need a for loop? Can't you just extend the above as

anglesDegrees = [-90 -60 -30 0 30 60 90];
degreeToRadianMap = containers.Map(anglesDegrees, anglesDegrees * pi / 180);

?

[Ali Tawfik]

Thanks for your answer,

Actually, I need to add the following commands,

T=[l1^2 m1^2 n1^2 2*m1*n1 2*l1*n1 2*l1*m1;
    l2^2 m2^2 n2^2 2*m2*n2 2*l2*n2 2*l2*m2;
    l3^2 m3^2 n3^2 2*m3*n3 2*l3*n3 2*l3*m3;
    l2*l3 m2*m3 n2*n3 m2*n3+n2*m3 l2*n3+n2*l3 l2*m3+m2*l3;
    l1*l3 m1*m3 n1*n3 m1*n3+n1*m3 l1*n3+n1*l3 l1*m3+m1*l3;
    l1*l2 m1*m2 n1*n2 m1*n2+n1*m2 l1*n2+n1*l2 l1*m2+m1*l2];
T_n=subs(T,{l1,m1,n1,l2,m2,n2,l3,m3,n3},{cos(theta_rad),sin(theta_rad),aij_z(1,3),...
    -sin(theta_rad),cos(theta_rad),aij_z(2,3),...
    aij_z(3,1),aij_z(3,2),aij_z(3,3)});

So I could have three different (T_n) based on each angle ?

It is not working with your previous command, It works with for loop, but how could again to extract values for a specfic values ?

Thanks,

[Geoff Hayes]

I think that you need to show more of your code. Are you using the Symbolic Toolbox? And please describe why you need to extract values for a specfic values.

Answer 3

[Image Analyst]

See the FAQ https://matlab.fandom.com/wiki/FAQ#How_do_I_fix_the_error_.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22.3F