CoCoA » CoCoALib

Currently permitted expressions require that an exponent be an unsigned integer literal or a possible signed integer literal inside (round) brackets.
It is probably not worth trying to allow more general exponents.

Since CoCoA-5 interprets the exponentiation operator ("hat") as right-associative, "ReadExpr" should not treat it as left-associative.
So RingElem(QQ,"2^3^4") must give an error; but the actual message produced could be more helpful.

What should the error mesg say?

I have added a simple extra test: if the first char after the end of the expr is '^', then it prints out a more helpful message:

/**/ RingElem(QQ, "2^3^4");
--> ERROR: Double exponent not allowed; use brackets like (a^b)^c
--> [CoCoALib] ReadExpr(string)
--> /**/ RingElem(QQ, "2^3^4");
-->      ^^^^^^^^^^^^^^^^^^^^^

Are there any other special cases?
Perhaps the general message should at least say what the first unexpected char is?

How about RingElem(P, "(x+y)(x-y)")?
Also RingElem(P,"2x")?
Perhaps if the extra char is one which could appear after a + or *, the err mesg could suggest that an operator is missing.
Such chars are digits, alpha, open-bracket.
An excess close-bracket could suggest that an open-bracket was missing earlier...

Mmmm, not so easy to guess what the mistake might be.

I have added a new error mesg for the case that the first extra char is open-bracket or letter or digit.
The err mesg append a hopefully helpful comment "missing operator?"

/**/ RingElem(R,"2x");
--> ERROR: Extra chars (first is 'x') after ringelem expr -- missing operator?
--> [CoCoALib] ReadExpr(string)
--> RingElem(R,"2x");
--> ^^^^^^^^^^^^^^^^
/**/ RingElem(R,"(x+1)(x-1)");
--> ERROR: Extra chars (first is '(') after ringelem expr -- missing operator?
--> [CoCoALib] ReadExpr(string)
--> RingElem(R,"(x+1)(x-1)");
--> ^^^^^^^^^^^^^^^^^^^^^^^^

I'm not too convinced by the parenthetical comment (first is `(`). Any better ideas?

I think I improved these error messages (hard to spot in string otherwise):

RingElem(CurrentRing, "18*x^4*y*z +720*x^3*y^2*z -135*x^4z^2 -10*x^3*y*z^2");
--> ERROR: Unexpected char 'z' in string from "..z^2 -10*x^.." -- maybe forgotten '*'?
--> [CoCoALib] ReadExpr(string)
--> RingElem(CurrentRing, "18*x^4*y*z +720*x^3*y^2*z -135*x^4z^2 -10*x^3*y*z^2");
--> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

RingElem(CurrentRing, "18*x^4*y*z +720*x^3*y^2*z -135*xx^4z^2 -10*x^3*y*z^2");
--> ERROR: symbol "xx" not in ring
--> [CoCoALib] ReadFactor: symbol
--> RingElem(CurrentRing, "18*x^4*y*z +720*x^3*y^2*z -135*xx^4z^2 -10*x^3*y*z^2");
--> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

What confused me at the start was that the "real" CoCoA interpreter is so much better is indicating these errors.

Anna Maria Bigatti wrote:

Still to do: check other cases.

I cannot remember what I meant.
Now seems pretty good, and in case we'll make a new issue.
Closing.