nest loop

Hello,

Here is my code:-

clear all

x = 1:3;

y = 1:3;

a = 1:3;

b = 1:3;

for i = 1:3;

    for j = 1:3;
        a_pred = a(:,i);
        b_pred = b(:,j);
        y_pred = a_pred + b_pred.* x;
        chi2 = (y-y_pred).^2;    
    end

end

how would I amend the nested loop so that every single combination of a and b values are calculated in y_pred please? i.e. a(1) and b(1) then a(2) and b(1) all the way to a(3) and b(3).

Thanks

Answer 1

[Matt Fig]

Why do you think all aren't being calculated? Here is a modified version of your code, not changing the mechanics, which shows what you are getting.

a = 1:3;
b = 1:3;

for i = 1:3;
    for j = 1:3;
        a_pred = a(:,i);
        b_pred = b(:,j);
        disp([a_pred b_pred])    
    end  
end

.

.

EDIT

.

Here is how to store all the results in chi2:

x = 1:3;
y = 1:3;
a = 1:3;
b = 1:3;
chi2 = zeros(9,3);
cnt = 0;
for i = 1:3;
    for j = 1:3;
        cnt = cnt + 1;
        a_pred = a(:,i);
        b_pred = b(:,j);
        y_pred = a_pred + b_pred.* x
        chi2(cnt,:) = (y-y_pred).^2;
    end  
end
chi2

And here is a vectorized version, using NPERMUTEK:

IDX = npermutek(1:3,2);
y_pred = bsxfun(@plus,a(IDX(:,1)), bsxfun(@times,b(IDX(:,2)),x.'));
chi2 = bsxfun(@minus,y,y_pred.').^2

Not that it should be faster here...

[Dominic Lawson]

Dear Matt,

Thanks for getting back to me so quickly. Is it the case they are being calculated but are being over written and not stored in a matrix? The [] help store the numbers in a matrix?

[Walter Roberson]

chi2(i,j) = (y-y_pred).^2;

[Matt Fig]

As Walter shows, you are overwriting chi2 every loop iteration. For best results, pre-allocate your chi2 matrix before the loops.

chi2 = zeros(3);

for ...

for ..

...

...

chi2(i,j) = ...

end

end

Also, I recommend you get in the practice of _not_ using i and j as loop indices as they will mask the built-in MATLAB functions i and j.

[Walter Roberson]

To nit-pick: the built-in values for i and j are variables, not functions.

[Matt Fig]

But, they are functions too!

which i

which j

Also, see the help...

[Matt Fig]

Actually, since (y-y_pred).^2 is a row vector, storing in chi2(i,j) will not work. You will have to use a cell array and/or do some fancy indexing to save all these rows. See my edit above.

[Walter Roberson]

You are right, Matt. I got confused because class(i) returns 'double'. But of course with them being functions, naming them is equivalent to invoking them with no arguments, so that is what is done when I ask for the class.

Another way to see that it is built in is,

functions(@i)

the "file" field of which reads 'MATLAB built-in function' which would not be the case if i was a plain variable.

[Matt Fig]

I didn't know about the FUNCTIONS function, thanks Walter. That could come in handy!

[Matt Fig]

BTW, what does the second field, 'type' mean here? I see REPMAT is simple, so is PLOT!

[Walter Roberson]

I found it by using methods(@i)

[Walter Roberson]

Good question about 'type'. I was not able to find anything that did not indicate 'simple' there. I did find, though, that functions will use an empty file name for methods that do not have a file of their own. For example, functions(@func2str) where which shows func2str as if it is part of a file that does not actually exist.