Problem solved. The problem here is the value of n. Since you wrote that n = 600, the resulting number is too big for the computer to process, which assigns an Inf value to it. This is turns makes the result be a NaN as an output.
To sum up, the operation:
x.^(2*i)
should never be allowed to have an inf value. If you try to do:
5^(2*600); % My matlab gives me an infinite value here, its too big!
So the only thing that you need is to pay attention to the value of n. I set it to 100 and it is working properly for me now:
x = [-5:0.1:5];
y = cos(x); % Matlab
n = 100;
Y = cosin(n,x); % Own
figure
hold on
plot(x,y,'.-');
plot(x,Y);
legend('cos(x)','cosin(x)');
hold off
function [ Y ] = cosin( n,x)
s=size(x,2);
Y=zeros(1,s);
for i=0:n-1
%Y=Y+((((-1)^(i-1))*(x.^((2*i)-2)))/factorial((2*i)-2));
Y=Y+(((-1).^(i)).*(x.^(2.*i)))./factorial(2.*i);
end
end
