How can I add a value to my array?

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Callum C
Callum C 2016-3-23
回答： Ced 2016-3-23
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.

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采纳的回答

Walter Roberson
Walter Roberson 2016-3-23
Before the "while",
xx(1) = x;
yy(1) = y;
n = 1;

更多回答（1 个）

Ced
Ced 2016-3-23
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

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