How can I add a value to my array?

This is the Forward Euler script:

y=input('initial value of y: ');
x=input('initial value of x: ');
h=input('value of h: ');
xmax=input('value of xmax: ');
while x<xmax
    n=n+1;
    y=y+h*fun(x,y);   %apply Forward Euler to y
    x=x+h;            %update value of x
    truth=exp(-0.5*x^2); %true value of y
    diff=abs(truth-y);    %difference between truth and estimate
    xx(n)=x;
    yy(n)=y;
    disp([x,y,diff]);
    plot(xx,yy);
end

I have the initial conditions such that y(0)=1, however I can't figure out how to add this information into the two arrays xx and yy, such that the graph begins at x=0, rather than x=0.1.

Thanks.

Answer 1

[Walter Roberson]

Before the "while",

xx(1) = x;
yy(1) = y;
n = 1;

Answer 2

[Ced]

To answer your question, you could just concatenate them, e.g.

x = [ 2 3 ];

% now add a 1:
x = [ 1 x ];

BUT The better way of solving your problem is the following:

1. determine how many time steps you will have (which you know, since you have xmax and h), e.g.

Nt = ...

2. preallocate the full vectors, i.e.

xx = zeros(Nt,1); 
yy = zeros(Nt,1);

3. Include your initial condition right there

xx(1) = ...
yy(1) = ...

3. use a for loop instead of while

for i = 2:Nt
xx(i) = xx(i-1) + h;
yy(i) = yy(i-1) + ...

plot(xx(1:i),...)
end

As a side note, you actually know the whole vector xx beforehand, no need to "update it" in each loop iteration.

Hope this helps.

Cheers