Re: NSExpression is incorrect?
Re: NSExpression is incorrect?
- Subject: Re: NSExpression is incorrect?
- From: Roland King <email@hidden>
- Date: Sun, 21 Nov 2010 14:07:15 +0800
I agree with you that exponentiation is usually defined as right-associative.
How about a compatibility switch in your code which you can flip to make it work either way.
On 21-Nov-2010, at 11:25 AM, Dave DeLong wrote:
> Hi everyone,
>
> Let's say I have the string @"2 ** 3 ** 2". I can run this through NSPredicate to parse it into an NSExpression tree for me, like so:
>
> NSExpression * e = [(NSComparisonPredicate *)[NSPredicate predicateWithFormat:@"2 ** 3 ** 2 == 0"] leftExpression];
>
> When I log/evaluate this predicate, it gives me "(2 ** 3) ** 2", which evaluates to 64. In other words, NSExpression (or NSPredicate or whomever) has assumed that the power operator is a left-associative operator.
>
> According to Wikipedia (the source of all truth and knowledge), when no parentheses are present in the expression, "the order is usually understood to be top-down, not bottom-up." [1] In other words, "a ** b ** c" is understood to be "a ** (b ** c)", not "(a ** b) ** c". Put more formally, the power operator is supposed to be right associative. (rdar://problem/8692313)
>
> I'm currently working on some code that mimics much of the behavior of NSExpression. However, I'm now faced with a choice: do I make my code technically correct by making the power operator right associative, or do I maintain parity with NSExpression and make it left associative?
>
> Any suggestions you have would be welcome.
>
> Thanks,
>
> Dave DeLong
>
> [1]: http://en.wikipedia.org/wiki/Exponentiation#Identities_and_properties
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