Hello guys, I need help.

I need to evaluate the function in each combination of (x1, x2, x3), that is:

for i=1:11
    for j=1:11
        for k=1:11
            z(i,j,k)=feval(f,x1(i),x2(j),x3(k));
        end
    end 
end

This evaluation gives us an NxNxN matrix (cell?). I don't know what it's called hahahaha

[sfx, ind]=sort(z(:));

we selected the t smallest function values

sfx=sfx(1:t);

And the indexes corresponding to the t smallest function values

ind=ind(1:t);

Does anyone have any idea how I can return the values of x1, x2 and x3 corresponding to these t smallest function values?

[Rik]

This time I edited your question for you. Next time, please use the tools explained on this page to make your question more readable.

[Everton Silva]

Thanks for your suggest. It's my first time that I asked here, the next time I'll write in this right way. ;-)

[Guillaume]

Notes:

z(i,j,k)=feval(f,x1(i),x2(j),x3(k));

can be written more simply as

z(i,j,k)=f(x1(i),x2(j),x3(k));

assuming that f is a function handle.

Also, depending on f you may not need the loops at all.

[Steven Lord]

To compute the smallest values you can use the mink function.

Answer 1

[Rik]

The indices you get in your last operation can be converted to subscripts, which will correspond to your three inputs.

ind=ind(1:t);
[x1_ind,x2_ind,x3_ind]=ind2sub(size(z),ind);
x1=x1(x1_ind);
x2=x1(x2_ind);
x3=x1(x3_ind);

[Everton Silva]

When I write this commands it's doesn't work. Appears this message: ''Error using sub2ind

Too many output arguments.''

[Walter Roberson]

[x1_ind,x2_ind,x3_ind]=ind2sub(size(z),ind);

[Everton Silva]

Thank you so much.

[Rik]

@Walter thanks for catching the mistake

@Everton you're welcome

Answer 2

[Torsten]

[X1,X2,X3]=ndgrid(x1,x2,x3);
Z=f(X1,X2,X3);   %assumes f is vectorized
M=[X1,X2,X3,Z];
[~,idx]=sort(M(:,4));
sortedM=M(idx,:);
X1s=sortedM(1:s,1);
X2s=sortedM(1:s,2);
X3s=sortedM(1:s,3);

[Everton Silva]

For example, if I write in the command window

t=11;
f=@(x1,x2,x3)(x1+x2+x3);
x1=0:0.1:1;
x2=0:0.1:1;
x3=0:0.1:1;
[X1,X2,X3]=ndgrid(x1,x2,x3);
Z=f(X1,X2,X3);
M=[X1,X2,X3,Z];
[~,idx]=sort(M(:,4));
sortedM=M(idx,:);
X1s=sortedM(1:t,1);
X2s=sortedM(1:t,2);
X3s=sortedM(1:t,3);

X=[X1s,X2s,X1s]

returns

X=
        0         0         0
    0.1000    0.1000    0.1000
    0.2000    0.2000    0.2000
    0.3000    0.3000    0.3000
    0.4000    0.4000    0.4000
    0.5000    0.5000    0.5000
    0.6000    0.6000    0.6000
    0.7000    0.7000    0.7000
    0.8000    0.8000    0.8000
    0.9000    0.9000    0.9000
    1.0000    1.0000    1.0000