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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]# Operator precedence problem

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On Monday, June 6, 2016 at 7:27:18 PM UTC+5:30, Random832 wrote: > On Mon, Jun 6, 2016, at 01:46, Lawrence D?Oliveiro wrote: > > On Monday, June 6, 2016 at 4:06:20 AM UTC+12, Uri Even-Chen wrote: > > > Never write expressions, such as 2 ** 3 ** 2 or even 2 * 4 > > > + 5, without parentheses. > > > > That leads to the code equivalent of > > <http://i.dailymail.co.uk/i/pix/2011/10/13/article-2048696-03F82C520000044D-302_634x332.jpg>. > > Okay, can we settle on, as a principle, "the basic arithmetic operators > (not to include **) are the only ones whose precedence can be presumed > to be obvious to all readers, and other expressions may/should have > parentheses to make it more clear, even when not strictly necessary to > the meaning of the expression"? > > Sure, it's obvious to _me_ that << and >> have higher precedence than & > and |, and that "and" has a higher precedence than "or", but can I > assume the other people know this? And I don't know offhand the relative > precedence of non-conceptually-related groups of operators, except that > I'm pretty sure "and" and "or" have very low precedence. > > [To keep this on-topic, let's assume that this discussion has a goal of > getting something along the lines of "always/sometimes/never use > "unnecessary" parentheses" into PEP7/PEP8. Speaking of, did you know > that C has lower precedence for the bitwise operators &^| than for > comparisons? That was something that tripped me up for a very long time > and undermined my confidence as to other aspects of the bitwise > operators] Kernghan/Thomson/Ritchie (dont remember which) actually admitted to the fact that these precedences are wrong I believe there are necessary (maybe not sufficient) conditions for a precedence table ? : A ? A ? B ? : B ? B ? C then ? should be lower than ? because otherwise (x ? y) ? z comes out type-wrong Of course these rules are usually more informal than rigorous -- both in programming and in math eg in C there is really not much of any type but int Still if we informally treat < : A ? A ? Bool (A is some kind of numeric) +,* : A ? A ? A &&, || : Bool ? Bool ? Bool then it follows that +,* < <= etc && || is the natural precedence table The mistake that C creators made was to treat bitwise operators as *logical* rather than as *arithmetic*

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