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Re: Very strange dates - BUG
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Re: Very strange dates - BUG


  • Subject: Re: Very strange dates - BUG
  • From: "Dennis W. Manasco" <email@hidden>
  • Date: Fri, 31 Jan 2003 05:17:01 -0600

At 9:31 pm +0100 1/30/03, Thomas wrote:

That is clearly a bug, -2^2 should return -4 of course.


NO.

Unary minus always takes precedence. (At least in any language I have ever studied.)

Thus -2^2 is equivalent to (-2)^2.

The Standard Rules of Precedence (as best I remember them):

1. Unary minus
2. Power (^)
3. Multiplication and division (which have equivalent precedence --> beware if you don't understand both equivalence and the localized order of evaluation...).
4. Everything else may be arbitrary, undefined, illogical or just plain confusing.

The moral is:

If you have the slightest doubt about the evaluation of a function _use parentheses_; they don't take much time, they provide documentation for those who must later follow your code and they give explicit instructions to the compiler or interpreter.

Question:

How is the following interpreted ?

-a*b+c*d/e-f^g/h+i*j/k

Answer:

It's a trick question. Even if you look up all of the precedence rules for the language you are working with you can't give a definitive answer. You still have to know whether the implementation-dependant compiler/interpreter is evaluating the equivalent expressions from left-to-right or right-to-left. Sometimes even this isn't enough; sub-evaluations may change direction.

Take some pro-active steps to make your equations non-arbitrary. You won't confuse yourself if you have to read your code later and the parser will give you unambiguous results.


=-Dennis
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 >Re: Very strange dates - BUG (From: Thomas <email@hidden>)

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