In an assignment A(I) = B, the number of elements in B and I must be the same.

I have a problem with my current Matlab code

if true
  % code
end
if true
  Nt=T/dt+1;
t=0:dt:T;
phi=zeros(Nt,1);
b=zeros(Nt,1);
f=zeros(Nt,1);
p(1)=3;
b(1)=10;
f(1)=l;
for i=1:Nt-1
b(i)=(12*pi(i))/(2*r)^2;
f(i)=l*p(i)^(3/2)/(2*r^2*((p(i)-1)^(1/2)));
p(i+1)=p(i)+dt*(p(i)-p(i)*((1-f/(sqrt(b*p(i))*x))*sinh(sqrt(b*p(i))*x)));
end

For p(i+1) Matlab keeps saying "In an assignment A(I) = B, the number of elements in B and I must be the same." Any ideas?

[Adam]

You should probably presize p rather than have it grow in the loop.

Put a breakpoint in on the line giving the error and put:

size( p(i)+dt*(p(i)-p(i)*((1-f/(sqrt(b*p(i))*x))*sinh(sqrt(b*p(i))*x))) )

on the command line at that point. From a quick glance at your code without time to run it this must be

ans =

       1     1

in order for it to be able to be assigned to p(i+1).

[butz]]

it says

if true
ans =
601    1
end

it looks like the problem is the variable f. if i put a fixed value for f the code works. but i don't really know what to do.

[Adam]

You can use syntax like:

p(i+1,:) = ...

to enter values into a 2d array whose second dimension can be 601, but this only works if every set of results being entered into the array has size 601 which is not the case because you assign:

p(1) = 3;

at the start and I don't know if your results are the same size every time round the loop either.

[butz]]

if true
clear all 
clc
l=1;
r=1*10^-6;                                                                                                                                    
x=2;                                          
dt=0.005;                                       
T=3;  
Nt=T/dt+1;
t=0:dt:T;
p=zeros(Nt,1);
b=zeros(Nt,1);
f=zeros(Nt,1);
p(1)=3;
b(1)=10;
f(1)=l;
for i=1:Nt-1
b(i)=(12*p(i))/(2*r)^2;
f(i)=l*p(i)^(3/2)/(2*r^2*((p(i)-1)^(1/2)));
p(i+1)=p(i)+dt*(p(i)-p(i)*((1-f/(sqrt(b*p(i))*x))*sinh(sqrt(b*p(i))*x)));
end

Answer 1

[Azzi Abdelmalek]

Nt=T/dt+1;
t=0:dt:T;
phi=zeros(Nt,1);
b=zeros(Nt,1);
f=zeros(Nt,1);
p(1)=3;
b(1)=10;
f(1)=l;
for i=1:Nt-1
  b(i)=(12*p(i))/(2*r)^2;
  f(i)=l*p(i)^(3/2)/(2*r^2*((p(i)-1)^(1/2)));
  p(i+1)=p(i)+dt*(p(i)-p(i)*((1-f(i)/(sqrt(b*p(i))*x))*sinh(sqrt(b*p(i))*x)));
end

[butz]]

Unfortunatelly this didn't solve the problem

[Azzi Abdelmalek]

What problem?

[butz]]

That matlab keeps saying "In an assignment A(I) = B, the number of elements in B and I must be the same." If i just replace the f by f(i) this will pop up "Warning: Rank deficient, rank = 0, tol = NaN. "

[Azzi Abdelmalek]

We can't test your code if we don't know what is T, and other parameters

[butz]]

Ok, posted the whole code now.