How to write a function in m. file?
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Ragini Ravichandren
le 4 Avr 2021
Modifié(e) : Ragini Ravichandren
le 5 Avr 2021
I've been trying to write the following function in an m.file, that will yield values t, and y as outputs:
function [t,y]=eulerMethod(f3, dt, Tf, t0, y0)
syms n;
n=(Tf-t0)/dt;
nf=round(n,1);
yp=y0;
for n=0:nf
tp=t0+n*dt;
f3p=f3(tp,yp);
yn=yp+dt*f3p;
yp=yn;
end
t=tn;
y=yp;
end
However, upon running the fuction
[t,y]=eulerMethod(f3, 1, 10, 0, 1)
I get nothing. Do I need to declare t and y at the beginning, or return them at the end?
2 commentaires
Walter Roberson
le 4 Avr 2021
You do not need to declare those or return them.
I do not see anything obviously wrong with the code. What is your f3 that you are passing in?
It is confusing that you use n for three different purposes. The syms n is completely unnecessary in this context.
Réponse acceptée
Walter Roberson
le 4 Avr 2021
f3=@(t,y) exp(t)-t;
eulerMethod(f3, 1, 10, 0, 1)
function [t,y]=eulerMethod(f3, dt, Tf, t0, y0)
syms n;
n=(Tf-t0)/dt;
nf=round(n,1);
yp=y0;
for n=0:nf
tp=t0+n*dt;
f3p=f3(tp,yp);
yn=yp+dt*f3p;
yp=yn;
end
t=tn;
y=yp;
end
and indeed, you assign t=tn but tn does not exist.
Are you intended to return scalar t, and y corresponding only to the final time? Or are you intended to return a vector of times and each yn for each time ?
5 commentaires
Walter Roberson
le 5 Avr 2021
f3p=f3(tp,yp);
You are passing all of yp in, instead of the "current" yp
yn(n)=yp+dt*f3p;
You are calculating using all of yp, instead of the "current" yp
yn(n)=yp+dt*f3p;
The left hand side is scalar, so this code assumes that the result of f3p is scalar. That might be true for the particular function you are testing with, but your code needs to be able to handle the case where you are dealing with multiple values (for example a second order system dealing in f'' and f' )
yp(n+1,:) = yn(n+1,:);
Here you assumed yn(n+1,:) is a vector even though a second ago you assumed that yn(n) is a scalar.
yn(n+1,:) also does not exist at this point -- or rather it does, but only because you initialized to 0.
for n=0:nf
You will find the code a lot easier to write if you start n from 1 and use
tp(n)=t0+(n-1)*dt;
Or better yet, initialize tp(1) to t0, and at each step add dt to the previous tp value, instead of calculating it using n.
... You will probably find it easier to start n from 2 instead of from 1 or 0, really.
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