Re: compare a large number of variables


In article <pan.2005.08.17.14.30.11.267556@xxxxxxxxxxx>, Netocrat <netocrat@xxxxxxxxxxx> writes:
> On Tue, 16 Aug 2005 23:55:15 -0700, Einar wrote:
>
> > Yes, your suggestions work perfectly, but I was mor looking for a nice
> > bit operator to operate on all my variables and then to compare it with
> > the value. something like (a|b|c|d...z) != value (this wont work
> > however... ).
>
> How about (off the top of my head and untested):
>
> if ( !( ((a|b|c|d...z) == value) && ((a&b&c&d...z) == value) ) )
> /* one or more of a,b,c,d...z is not equal to value */

Or the similarly silly:

if (a^value|b^value|c^value|...|z^value)
/* one or more of a,b,c,...,z is not equal to value */

(Untested, but I think that's right.) Or similarly:

if (a^b|b^c|c^d|...|y^z|z^value)

All assuming that we're working with values that aren't going to
produce trap representations, or with unsigned types.

Of course this is just a bitwise version of (in the second case):

if (a!=b || b!=c || ... || z!=value)

except that the logical version can short-circuit, so fewer
operations are performed. (Though it's conceivable that on some
systems it'd be faster to perform all the bitwise operations than
to do the comparing and branching required to implement short-
circuiting.)

> I don't think that would generally be as efficient as using multiple
> comparisons against value because in that case the first mismatch will
> prevent the rest from being evaluated.

More important, it's lousy code, and performance is rarely as
important as maintainability. And if performance *is* crucial in
this case, then it's probably time to redesign - it's unlikely that
keeping a lot of independent variables and testing them all against a
single value is actually the best way to accomplish whatever it is
that the problem requires.

That said, in some code I might find it defensible to write this as
a multiline controlling expression in an if statement (using the
logical operators, not the bitwise ones), as long as it was clear.


--
Michael Wojcik michael.wojcik@xxxxxxxxxxxxxx

Ten or ten thousand, does it much signify, Helen, how we
date fantasmal events, London or Troy? -- Basil Bunting
.

Relevant Pages

  • Re: compare a large number of variables
    ... >>> nice bit operator to operate on all my variables and then to compare ... these bitwise operations on signed integers are not portable. ... > More important, it's lousy code, ... Both issues can be dealt with by appropriate commenting. ...
    (comp.lang.c)
  • Re: lahey lf95: option to use integer as logical?
    ... What you are really trying to use is logical operators as ... that can't be the only extension (logical operators ... as bitwise on integers). ... no deficiencies and the other way is to make it so complicated ...
    (comp.lang.fortran)
  • Re: "0= if" or simply "if" ???
    ... It might seem confusing at first to have to pay attention to ... whether flags are well-formed. ... that had separate operations for logical and bitwise operations, ... Your heads-up about the logical operators being all bitwise ...
    (comp.lang.forth)
  • Re: Advanced C
    ... Say you'd already attended a course, that had covered stuff like structs, pointers, bitwise and logical operators, multi-demensional arrays, *but* hadn't covered other stuff like recursion, unions, bit-fields ...
    (comp.lang.c)
  • Bitwise (flags) enums: how to compare?
    ... I tried hard to find on google some reference about bitwise (flags) ... What's the proper way to define and compare bitwise enums? ...
    (comp.lang.java.programmer)