how to write this function

I have this problem:

%  A      B        C    obj
P =[1  4  4  0.1
    2  6  6  0.1
    4  7  7  0.4
    5  8  2  0.3
    7  9  3  0.2
    8  0  9  0.5
    3  2  2  0.1
    4  1  5  0.2
    6  6  4  0.3
    7  7  1  0.4
    8  8  3  0.5]
A=P(:,1);
B=P(:,2);
C=P(:,3);
obj=P(:,4);
n=size(P,1);

I want to write code to find min (obj) subject to: 1<A<4;1<B<3; 1<C<4 and show the result on the first row of P. I tried to write it but alway give me a wrong results. Could you help me to solve it? Thanks.

[Stephen23]

@Quynh tran: can you please explain your request "...show the result on the first row of P": How do you want the result to be shown in P ? Can you please give an example of this.

Answer 1

[Torsten]

minimum = inf;
index = -1;
for k=1:numel(A)
  if A(k)>1 && A(k)<4 && B(k)>1 && B(k)<3 && C(k)>1 && C(k)<4 && obj(k)<minimum
    index = k;
    minimum = obj(k);
  end
end
index
minimum

Best wishes

Torsten.

[Stephen23]

You use a loop for that!!!?

This is MATLAB, not C++. Why use an ugly loop?

See my answer for a much simpler method using logical indexing.

[Torsten]

I like for-loops because they clearly show what you are doing :-).

The comment of the OP seems to show that he doesn't know what you are doing.

Best wishes

Torsten.

[Stephen23]

"I like for-loops because they clearly show what you are doing"

Just to clarify: you are saying that this is too complicated to understand?:

idx = A>1 & A<4 & B>1 & B<3 & C>1 & C<4;
min(obj(idx))

Especially when compared to this:

minimum = inf;
index = -1;
for k=1:numel(A)
  if A(k)>1 && A(k)<4 && B(k)>1 && B(k)<3 && C(k)>1 && C(k)<4 && obj(k)<minimum
    index = k;
    minimum = obj(k);
  end
end

I just wanted to understand how three temporary variables, a for loop, an if, and and several "magic numbers" are easier to understand than one simple logical index (as we all know, logical indexing is so fundamental to using MATLAB efficiently, as it is usually the fastest way to access data in an array).

Do you have a programming background?

[Torsten]

Do you have a programming background?

Well, yes, it's the classical background you mentioned above (Fortran & C).

Maybe that's why MATLAB as a high-level language often appears quite "unnatural" to me. But things might be different if one grew up with MATLAB.

Best wishes

Torsten.

Answer 2

[Stephen23]

>> idx = A>1 & A<4 & B>1 & B<3 & C>1 & C<4;
>> min(obj(idx))
ans =
          0.1

[Quynh tran]

Sorry, could you show the result of A,B,C because we can see have many value of A,b,c get min(obj)=0.1. Thanks

[Stephen23]

@Qunyh tran: your request is not clear: if you want to see the indices for each column then this is easy:

>> idA = A>1 & A<4
idA =
 0
 1
 0
 0
 0
 0
 1
 0
 0
 0
 0
>> min(obj(idA))
ans =  0.10000

and of course you can do the same for B and C. It is easy to combine them as well (just imagine how difficult this would be with a for loop!)

>> min(obj(idA&idB&idC))
ans =  0.10000