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

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Ani Asoyan on 7 Jun 2020
Commented: Ani Asoyan on 7 Jun 2020
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')

KSSV on 7 Jun 2020
Edited: KSSV on 7 Jun 2020
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 on 7 Jun 2020
No it wasn't integers. but ok I got it,, Thank you !

Image Analyst on 7 Jun 2020
Ani Asoyan on 7 Jun 2020
thanks

Maximilian Schönau on 7 Jun 2020
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
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Ani Asoyan on 7 Jun 2020
It worked, thank you !