# Hello guys, I need help.

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Everton Silva on 27 Mar 2020
Commented: Rik on 28 Mar 2020
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?

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Everton Silva on 27 Mar 2020
Thanks for your suggest. It's my first time that I asked here, the next time I'll write in this right way. ;-)
Guillaume on 27 Mar 2020
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 on 27 Mar 2020
To compute the smallest values you can use the mink function.

Rik on 27 Mar 2020
Edited: Rik on 28 Mar 2020
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);

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Walter Roberson on 27 Mar 2020
[x1_ind,x2_ind,x3_ind]=ind2sub(size(z),ind);
Everton Silva on 27 Mar 2020
Thank you so much.
Rik on 28 Mar 2020
@Walter thanks for catching the mistake
@Everton you're welcome

Torsten on 27 Mar 2020
Edited: Walter Roberson on 28 Mar 2020
[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);

#### 1 Comment

Everton Silva on 27 Mar 2020
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