Re: operator precedence differs from C language

[Josh Stone <jistone at redhat dot com>]

On 04/01/2010 09:29 AM, Josh Stone wrote:
> On 03/31/2010 07:36 PM, Neo Liu wrote:
>> I found that the operator precedence of "&" and "==" differed from C
>> language. Take a look at the following statements.
> 
> That's not true -- in C and in stap, relational operators bind more
> tightly than bitwise operators.  http://tinyurl.com/9q6szp

You might also find this interesting:
http://cm.bell-labs.com/cm/cs/who/dmr/chist.html

"""
Neonatal C

Rapid changes continued after the language had been named, for example
the introduction of the && and || operators. In BCPL and B, the
evaluation of expressions depends on context: within if and other
conditional statements that compare an expression's value with zero,
these languages place a special interpretation on the and (&) and or (|)
operators. In ordinary contexts, they operate bitwise, but in the B
statement

    if (e1 & e2) ...

the compiler must evaluate e1 and if it is non-zero, evaluate e2, and if
it too is non-zero, elaborate the statement dependent on the if. The
requirement descends recursively on & and | operators within e1 and e2.
The short-circuit semantics of the Boolean operators in such
`truth-value' context seemed desirable, but the overloading of the
operators was difficult to explain and use. At the suggestion of Alan
Snyder, I introduced the && and || operators to make the mechanism more
explicit.

Their tardy introduction explains an infelicity of C's precedence rules.
In B one writes

    if (a==b & c) ...

to check whether a equals b and c is non-zero; in such a conditional
expression it is better that & have lower precedence than ==. In
converting from B to C, one wants to replace & by && in such a
statement; to make the conversion less painful, we decided to keep the
precedence of the & operator the same relative to ==, and merely split
the precedence of && slightly from &. Today, it seems that it would have
been preferable to move the relative precedences of & and ==, and
thereby simplify a common C idiom: to test a masked value against
another value, one must write

    if ((a&mask) == b) ...

where the inner parentheses are required but easily forgotten.
"""