# Misbehaving script

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Christopher on 3 Dec 2011
Hi, i'm writing a script to calculate diameters of pipes for me. However, Matlab is acting up on me (or i'm missing something).
I've set up a few parameters at the start of the script, defined a function and set up a for loop to get me my values.
Q_in=(2/9);
n_nozzle=10;
n_row=9;
d_in=250e-3;
Q_nozzle=2.2e-3;
Q=zeros(n_row,n_nozzle);
Q(1,1)=Q_in;
Q_R=@(r,c) Q(r-1,c)-(Q_nozzle+(n_nozzle*Q_nozzle))
for (i=2:n_row)
Q(i,1)=Q_R(i,1);
end
for the first iteration where i=2, the script works properly and produces the right answer. But starting from i=3, the Q(r-1,c) part of the Q_R function just keeps ending up as 0 where it should be referencing Q(2,1).
I have manually tried Q(2,1)-(Q_nozzle+(n_nozzle*Q_nozzle)) and in this case the Q(2,1) correctly references Q(2,1). The function just does not work properly if I use Q_R(3,1).
What is going on here ?

Teja Muppirala on 4 Dec 2011
In your function handle, the "Q" that it is referencing is the original Q at the time you created the function handle. It is not linked with whatever is in your workspace.
The following should make this clear.
A = 10;
F = @(x) A + x
F(7)
A = 0;
F(7)
F = @(x) A + x
F(7)
Even though I change A, the function handle does not reflect that change until I redefine the function handle.
Paulo Silva on 4 Dec 2011
That was a tricky one, thanks for finding and explaining the error, +1 vote

Paulo Silva on 3 Dec 2011
I see it working properly, see it by yourself with this code
clc
Q_in=(2/9);
n_nozzle=10;
n_row=9;
d_in=250e-3;
Q_nozzle=2.2e-3;
Q=zeros(n_row,n_nozzle);
Q(1,1)=Q_in;
Q_R=@(r,c) Q(r-1,c)-(Q_nozzle+(n_nozzle*Q_nozzle));
for i=2:n_row
Q(i,1)=Q_R(i,1);
i
Q_R(i,1)
pause(0.3)
end
Christopher on 3 Dec 2011
Yeah, i've already changed my code to work just like you have done. But it is still puzzling me why it does not work with the function ?
Paulo Silva on 3 Dec 2011
so am I, been away from such weird problems for some time, you can reproduce the original problem without the loop
Q_in=(2/9);
n_nozzle=10;
n_row=9;
d_in=250e-3;
Q_nozzle=2.2e-3;
Q=zeros(n_row,n_nozzle);
Q(1,1)=Q_in;
%for
n=2:n_row;
Q(n,1)=Q(n-1,1)-(Q_nozzle+(n_nozzle*Q_nozzle));
%end
Q(1:end,1)
very strange or I might be missing something simple :S