How to include a negative number in initial conditions for Eulers method code?

Question

Alex 2023-12-13

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2060184-how-to-include-a-negative-number-in-initial-conditions-for-eulers-method-code

编辑： Torsten 2023-12-13

clear
close all
h=0.1; % step size 
x=-1:h:2; % x interval define here
y=zeros(size(x));
y(-1)=8; % intial condition 
n=numel(y);
for i = 1:n-1
    dydx=((2.*x.*y.^2+4)/(2.*(3-y.*x.^2))) 
    y(i+1) = y(i)+dydx*h;
end
plot(x,y,'ro');
hold on 
f1=exp(x);
plot(x,f1,'b');
grid on;

I'm writing some code for Eulers method and my initial condition is y(-1)=8. However when I run this it comes up with the error message saying array indicies must be positive integers or logical values. Any advice on how i can include my intial condition without these error messages?

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Alex 2023-12-13

I don't fully understand what you're saying

Torsten 2023-12-13

编辑：Torsten 2023-12-13

在 MATLAB Online 中打开

y(i) means the value of the function y at x(i).

It does not mean the value of the function y at x = i.

Since

x=-1:h:2; % x interval define here

you thought you could use y(-1) for y (x=-1).

But you must use y(1) for y @ x(1) = -1.

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Answer 1

Les Beckham 2023-12-13

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2060184-how-to-include-a-negative-number-in-initial-conditions-for-eulers-method-code#answer_1370864

在 MATLAB Online 中打开

It is always better to post example code as text rather than a screenshot. Nevertheless...

Since the first element of x is -1 and you want to specify the value of y when x is -1, just define the first element of y, y(1), with your initial condition

y(1) = 8;