extracting data from 4-D double

I am trying to use ndgrid to calculate values for an optimization problem. I was able to successfully use the function for my equation but I am unable to find the optimal points or retrieve data from it at all. Only thing that hasn't returned an error is trying a feather graph ( which crashed my computer). Any help is much appreciated.

n=1:20;
[nFin, d ,L ,Aw] = ndgrid(n(:).^2,.001:.001:.05,.001:.001:.05,.01:.01:.5);
%corrected length eqn
Lc = L + d./4;
%area of fin
Af = (pi./4).*d.^2;
%perimeter of fin
P = pi.*d;
% fin convection term m
m = ((h.*P)./(k.*Af)).^0.5;
q = dT.*h.*(Aw-(Af.*nFin)) + dT.*nFin.*tanh(m.*Lc).*(h.*k.*P.*Af).^0.5 ;

[Akira Agata]

The variables "h" and "k" are undefined in this program. What are these variables ?

[jourdan joyner]

sorry i just cut a segment of the code

%input for choosing h
hh = [8;20];
I = input ('heat transfer coefficient of 8 or 20?\n');
h = hh(floor(I/8));
%k of aluminum 
k = 204;

[Akira Agata]

Thank you for your answer, but next "dT" is still undefined...

[Akira Agata]

Thank you for providing a hole code. Then, let me clarify what exactly you would like to find (i.e. what 'optimal point' in your question means). If you want to find the minimum point of 4-D matrix 'a', then you can do as follows:

 [val, idx] =  min(a(:));
 [i,j,k,h] = ind2sub(size(a), idx);

Then, the minimum value and it's index can be found to a(i,j,k,h) = val.

[jourdan joyner]

Thank you. I am hoping to find the minimum values where q= 30, using the suggestion you give as written gives me the first value in the q matrix I tried to rewrite as

   [val, idx] =  min(q(:));
   [nFin,d,L,Aw] = ind2sub(size(q), q>30&&q<30.1);

but there is an error due to q being vector. Is it possible to still use this line of code in this manner?

Answer 1

[Akira Agata]

Thank you for your clarifications!

The following code will find the minimum value of 4-D matrix 'a' where 30 < q < 30.1.

% Generate 4-D logical array indicating 30<q<30.1
idx1 = (q > 30) & (q < 30.1);

% Find a minimum value of 'a' where 30<q<30.1
[val, idx2] = min(a(idx1));

% Returns subscripts from linear index
[i,j,k,l] = ind2sub(size(idx1), find(idx1, idx2));

Then, a(i,j,k,l) is the minimum value where 'q' is 30<q<30.1.

[jourdan joyner]

Thank you very much!