How is it possible that matlab solves two simultaneous equations which have relation to each other? I am very new with matlab and can not get it done. I want to solve several equations with the euler method bot always get errors.

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steve
steve 2015-3-12
评论： Andrew Newell 2015-3-13
main code :
clear all
a=0;
b=10;
N=10;
y0=0;
h=(b-a)/N;
t(1)=a;
R(1)=1;
Pg(1)=1;
P1(1)=1;
c(1)=12;
for n=1:N;
t(n+1)=t(n)+h;
c(n+1)=c(n)+h*g(t(n),c(n));
Pg(n+1)=1.9*g(t(n),c(n));
P1(n+1)=8*R(n);
R(n+1)=R(n)+h*f(t(n),R(n)) ;
end
plot(t,R)
function g:
function g=g(t,c)
g=(45*c)/74;
function f:
function f=f(t,R)
f=(((Pg-P1)*R)/152)-60;
this is the result:
Error :Undefined function or variable 'Pg'
Error in f (line 2)
f=(((Pg-P1)*R)/152)-60
how can I asign the variable Pg and P1 to the equation in the main code
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Andrew Newell
Andrew Newell 2015-3-12
Brendan, I think I may have caused some confusion by reformatting the code incorrectly (I have fixed the error). I think they already are in separate files. Sorry about that.
steve
steve 2015-3-13
Thanks for the comments! The functions were in different files, but thanks for the detailed response and links. I realize, I'm still stuck with many problems here, but unfortunately I'm very impatient:-) Iam happy for every help I can get.

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采纳的回答

Andrew Newell
Andrew Newell 2015-3-12
You could just write
Pg = 1;
but a better way, since you know how large the vector will be, is to preallocate the memory:
Pg = ones(1,N+1);
  2 个评论
steve
steve 2015-3-13
编辑：steve 2015-3-13
First off, thanks for the reply. The code calculates a result when I sat Pg to 1 in the function f . However, my goal would be that after each cycle of the loop the Pg value from the large file gets transfered to the function file. When I change the constant in
Pg(n+1)=1.9*g(t(n),c(n)) ...
the R(n+1) does not change. Thx
Andrew Newell
Andrew Newell 2015-3-13
Sorry, I realize now that I didn't really answer your question. The reason for the error is that you are not passing the values of Pg and P1 to f. One approach is to add them as input parameters. Or you could combine them as below:
R(n+1)=R(n)+h*f(t(n),(Pg(n)-P1(n))*R(n)) ;
The function is:
function f=f(~,x)
f=(x/152)-60;
Note the tilde. It's good practice to do this when an input variable (in this case t) is not used.
I ran this modified code and it produced a plot.

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