Why do I get this error? "indices must either be real positive integers or logicals."

hi, why do I get this error ?

Subscript indices must either be real positive integers or logicals.

Error in sev (line 27)
N(g) = (q*b-a*d)./(e*(q^2)+g.*2*a)

here's my code..

syms q
a=2; b=2; d=0.5; e=0.9 ; q=0.4;
g=1:0.1:10
N(g) = (q*b-a*d)./(e*(q^2)+g.*2*a)
i(g)=(b-e*q*N)/a

m=N(g)
n=i(g)

figure(1)
plot (g,m,'b')
hold on 
plot (g, n,'red')

Answer 1

[KSSV]

You need not to use syms. there is no requirement of syms.

You need not to use indesing N(g), this is not reuqired also this is not valid.

a=2; b=2; d=0.5; e=0.9 ; q=0.4;
g=1:0.1:10
N = (q*b-a*d)./(e*(q^2)+g.*2*a)
i=(b-e*q*N)/a

m=N
n=i

figure(1)
plot (g,m,'b')
hold on 
plot (g, n,'r')

[Ani Asoyan]

It worked, thank you !.. also one question please. earlier I used syms with the same code, but it didn't give me error( I used different calibration) , I got values and plotted the function, was It wrong? , do I have to change it?

[KSSV]

You have used syms but, you have over written the variable. So it no more a symbolic variable. In the present case g has fractions. MATLAN array indices should always be positive integers. May be in the previous case your array might be integers and so it worked.

[Ani Asoyan]

No it wasn't integers. but ok I got it,, Thank you !

Answer 2

[Image Analyst]

This is the most faqqy of the error faqs, so see the FAQ:

https://matlab.fandom.com/wiki/FAQ#.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22

[Ani Asoyan]

thanks

Answer 3

[Maximilian Schönau]

You could use N(g) to create a symbolic function with the symbolic variable g. Since g is not a symbolic variable but a vector, Matlab trys to use the values of g as indices of N, wich does not work.

Use:

N = (q*b-a*d)./(e*(q^2)+g.*2*a)

To create a function N(g) use:

syms g
N(g) = (q*b-a*d)./(e*(q^2)+g.*2*a) % symbolic function

or

N = @(g) (q*b-a*d)./(e*(q^2)+g.*2*a) % function handle

[Ani Asoyan]

oh sorry I wrote syms q, but it had to be syms g.... but even in that case I still get that error

[Ani Asoyan]

It worked, thank you !