Unexpected result related to whitespace and operators

[zcohan]

I'm running into an issue where the absence of a whitespace character before a '+' operator in some cases has an affect on the result. For instance:

Expression("3 * 4 +5").evaluate() // 27.0
Expression("3 * 4 + 5").evaluate() // 17.0
Expression("3*4+5").evaluate() // 17.0

I believe this to be a bug.

[nicklockwood]

@zcohan you're right, this isn't expected behavior. I'll take a look, thanks.

[zcohan]

Thanks. I was also surprised by some results related to spaces and subtracting negative numbers.

Many expression based calculators accept all the following syntaxes:

Expression("20 - -20").evaluate() //40.0
Expression("20--20").evaluate() //error 
Expression("20 -- 20").evaluate() //error
Expression("20 - - 20).evaluate() //error

[nicklockwood]

@zcohan those latter examples are expected. Expression allows the caller to define custom operators, which means it can't distinguish between an infix -- operator vs an infix - operator followed by a prefix - operator unless the whitespace makes it unambiguous.

[zcohan]

Fair enough. To minimise surprise, perhaps it would make sense to set some "sensible defaults" if there are no custom operators defined (given those expressions are mathematically equivalent).

[nicklockwood]

I'm running into an issue where the absence of a whitespace character before a '+' operator in some cases has an affect on the result. For instance:

@zcohan this is now fixed in 0.12.12.

To minimise surprise, perhaps it would make sense to set some "sensible defaults" if there are no custom operators defined.

I did experiment with this, but is was a big unsatisfactory for various reasons. E.g. if you overrode the "+" operator with a custom method, and then someone wrote "a+-b" it would call the internal "+-" operator instead of your overridden "+".

It's probably not insurmountable, but if you want this behavior you can already implement "+-", "--", etc yourself as custom operators.