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

How do you state the first iteration to be used in a for loop? I am having trouble with the following error message:

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

Hello,

I am using the Crank-Nicholson method to solve for a linear pde, and I have a simple initial condition. I am at whits end to try to fix this problem that seems so simple. Please help me.

My problem is trying to apply the initial conditions as the first iteration for a for loop.

What I am trying to do in "real-speak":

Let Bb be the initial condition.
Let AA be the matrix used for the crank-nicolson scheme.This never changes.
w0=AA\Bb; This is the concentration profile at t=0.

Take this value w0, and let this be the new "b" vector called BB. at t=1, we have a vector BB and matrix AA

w=AA/BB

take this w, and it should be the new BB for t=2.

Continue for n iterations, what you should have is the function w changing over time.

A portion of my code *note C values are constants:

Bb=zeros(m,1); %this forms the initial b matix
Bb(1,1)=(G0); %this defines the initial conditions

w0=AA\Bb; %this solves the initial matrix, and we would get u at %t=0
BB=zeros(m,1)

%this is problem

BB(1)=Bb; %for the first iteration of BB, let it be initial condition vector Bb
w(1)=w0; %for the first iteration of w, let it be initial condition vector w0

for j=1:n,
    for k=2:m,
        BB(k)=(-1*C1*w(k))+(C4*w(k))-(C3*w(k))

          RHS=BB;
      end
      w=AA\(RHS); %the B vector should lead to the new u vector
      %the solution of the u vector from the previous step must used as the b solution for the future step
      %assuming that the A matrix is correct

      %repeat
   end

**

This is the error msg:

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

Error in crank5_linda (line 49)
BB(1)=Bb; %for the first iteration of BB, let it be initial condition vector Bb

Thank you in advance.

Cathleen

Answer 1

[Walter Roberson]

You define Bb as a vector, and then you try to store that entire vector in to BB(1) which is a single element of an array.

I do not know what the correct code should be for your situation, but probably one of

BB(1) = Bb(1);

or

BB = Bb;

Answer 2

[Charanraj]

I too get the same error.I have written a simple Matlab s-function program to calculate nearest coordinates in a parallelogram, . the code is as follows:

function sys = mdlOutputs(t,x,u)

g=u(1);

h=u(2);

global m;

global n;

global m1;

global m2;

global n1;

global n2;

m=0;

if (g<m)

    m1=m-1;
    m2=m;
else

    m=m+1;
end

n=0;

if (h<n)

    n1=n-1;
    n2=n;
else

    n=n+1;
end

M1=m1;

M2=m2;

N1=n1;

N2=n2;

sal(1)=M1;

sal(2)=M2;

sal(3)=N1;

sal(4)=N2;

sys=sal;

% End of mdlOutputs.