index must be a positive integer or logical

I am trying to solve differential using Euler's approach. When running my code I keep getting this error message "Attempted to access dy(0,1); index must be a positive integer or logical". What am I doing wrong, please?

for i=1:n-1
    iter=0;
    y(i+1)=y(i)+dy(x(i),y(i))*(x(i+1)-x(i));
    iter=iter+1;
end
disp([x(i),y(i+1)])

[Full code merged from duplicate question]

clc
clear all
close all

x=[0,4];
y=-0.5*x.^4+4*x.^3-10*x.^2+8.5*x+1;
dy=-2*x.^3+12*x.^2-20*x+8.5;
y_0=1;
h=0.5;
if nargin<4,
    error('at least 4 input arguments required')
end
xi=x(1);
xf=x(2);
if ~(xf>xi),
    error('upper limit must be greater than lower')
end
x=(xi:h:xf)';
n=length(x);
if x(n)<xf
    x(n+1)=xf;
    n=n+1;
end
y=y_0*ones(n,1);
for i=1:n-1
    iter=0;
    y(i+1)=y(i)+dy(x(i),y(i))*(x(i+1)-x(i));
    iter=iter+1;
end
disp([x(i),y(i+1)])

Answer 1

[James Tursa]

What are the values of the x vector? E.g., is x(1) equal to 0? This could cause the error.

[Adeyinka Ademola]

This is my code

clc
clear all
close all

x=[0,4];
y=-0.5*x.^4+4*x.^3-10*x.^2+8.5*x+1;
dy=-2*x.^3+12*x.^2-20*x+8.5;
y_0=1;
h=0.5;
if nargin<4,
    error('at least 4 input arguments required')
end
xi=x(1);
xf=x(2);
if ~(xf>xi),
    error('upper limit must be greater than lower')
end
x=(xi:h:xf)';
n=length(x);
if x(n)<xf
    x(n+1)=xf;
    n=n+1;
end
y=y_0*ones(n,1);
for i=1:n-1
    iter=0;
    y(i+1)=y(i)+dy(x(i),y(i))*(x(i+1)-x(i));
    iter=iter+1;
end
disp([x(i),y(i+1)])