fix a const error in IndexSetIter::operator+()

[xichuang]

There exists an const error in the IndexSetIter::operator+(), which affect the following codes.

Index s1=Index("s1",4),s2=Index("s2",6); auto b=delta(s1,s2); auto c=b.inds().begin(); PrintData(*c); PrintData(*(c+1));

where the += operator in the diffinition of operaotr+() actually modifies the value of x.
The IndexSetIter::operator-() may also occurs this const error, but I haven't test it.

[emstoudenmire]

Hi, so thanks for catching this. However I think the correct fix is not to remove the const from the function argument, but rather to make it no longer a reference at all (i.e. pass by value). Then the copy of x inside the function can be modified by the += operation, and the resulting IndexSetIter returned. Can you test this alternate code and confirm?

[xichuang]

you mean
operator+(const IndexSetIter<T>& x, typename IndexSetIter<T>::difference_type d)
to
operator+(const IndexSetIter<T> x, typename IndexSetIter<T>::difference_type d)
?
That still doesn't work for me.

[emstoudenmire]

Hi, so I meant that x should just be a regular value, so I meant to remove both the const and the reference modifiers. I just made those changes to your patch and merged them both in - thanks for catching this bug.

[emstoudenmire]

Of course please let me know if it still doesn't work but I think it should now and I added a unit test based on your sample code.

[emstoudenmire]

One other comment: be careful about making a delta tensor with two indices of different size. I may actually forbid it because it's not a common use-case; theoretically if you use it to "shrink" an index it acts like a projector whereas if you use it to "grow" an index it pads with zeros.