八度错误：返回列表中的元素未定义

Question

I am writing the following octave code:

function p = predict(Theta1, Theta2, X)
m = size(X, 1);
num_labels = size(Theta2, 1);
global a=zeros(size(Theta2, 2), m);
global delta=zeros(m, 1);

p = zeros(size(X, 1), 1);

X=[ones(size(X,1),1) X];
a=sigmoid(Theta1*X');
a=[ones(1,size(X,1));a];
[delta p]=max(sigmoid(Theta2*a))';

It gives me the error: "element number 2 undefined in return list". The error occurs when I use delta in the last line to store max values. I have searched a lot but couldn't find any relevant answer.

Answer 1

The line

[delta p] = max( sigmoid( Theta2*a ) )'; # transpose operator over the result

is equivalent to

[delta p] = transpose( max( sigmoid( Theta2*a ) ); # transpose function over the result

which means you are trying to get a "two-output" result out of this transpose operation, which fails, since the transpose function only returns one output, therefore octave is informing you that it cannot find a second output in the 'results' list.

Presumably you either meant to do something along the lines of:

[delta p] = max( sigmoid( Theta2*a )' );

and misplaced the transpose operator, or you actually did want to obtain the maxima and their indices as a column vector, in which case you need to do this in two steps, ie

[delta p] = max( sigmoid( Theta2*a ) );
ColVector = [delta p]';

PS. Incidentally, you should use .' instead of ' as the transpose operator. ' is not the transpose operator, it's the "conjugate transpose" one.