Subscript indices must either be real positive integers or logicals.

I am trying to run eulers method for a first order differential equation. However recieve the above error when I try to plot the results.

See my code blow:

x=zeros(400,1);

y=zeros(400,1);

x(1)=2;

y(1)=4;

for n=1:400

x(n+1)=x(n)+0.005;

y(n+1)=x(n)+0.005.*(((y).^2)+(7*y)+2)/(3*x((y).^2)+((1-x)*(3*((y)^2)+7)));

end

plot(x,y)

title('eulers')

grid on

What am I doing wrong?

Answer 1

[KL]

when you write,

 y(n+1)=x(n)+0.005.*(((y).^2)+(7*y)+2)/(3*x((y).^2)+((1-x)*(3*((y)^2)+7)));
                                             ^

you're trying to access the zeroth element of x (since y is vector with all zeroes except its first element). You need to rewrite that equation. How does the equation in your book look like?

Anyway, if you want to create a vector with 0.005 increaments use linspace or colon(:) operator.

[Harry Price]

This is my equation on paper.

[KL]

if you have access to symbolic math toolbox, it's a lot easier. Go through the example here: https://www.mathworks.com/help/symbolic/solve-a-single-differential-equation.html#f1-7555