# I am trying to plot the each value of C at different theta values, but I found an error. Error: Invalid expression. When calling a function or indexing a variable, use parenth

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Seifeldin Mohamed on 5 Dec 2022
Answered: Alan Stevens on 5 Dec 2022
clear all
clc
c=[0.5,0.75,1,1.1];
theta=[-pi:0.01:pi];
a=zeros(1,629);
for k=c
G(k)=(1+4*(a(k)).^2-a(k)).*(sin(theta/2)).^2);
plot(G(k),theta);
end
Walter Roberson on 5 Dec 2022
Are you trying to define G as being a piecewise formula? Or is G to be a matrix of values? Matrices need to be indexed with positive integers.

Walter Roberson on 5 Dec 2022
you have too many ) on that line.

Alan Stevens on 5 Dec 2022
Do you want something more along the following lines?
c=[0.5,0.75,1,1.1];
theta=-pi:0.01:pi;
%a=zeros(1,629);
for k=1:numel(c) % numel is number of elements in c
% There are four values of c and all the a's are zero
% so I've replaced a by c in the following.
G(k,:)=(1+4*c(k).^2-c(k)).*sin(theta/2).^2;
end
plot(theta,G);
xlabel('\theta'), ylabel('G')