- Using #define in the construction of an array - 3 Updates
| scott@slp53.sl.home (Scott Lurndal): Nov 09 09:12PM >BitsSetTable[k] = number of bits set in the binary expansion of k. >No clue why this works, though. It's neat. >Paul Look at the number of bits required to represent 0-f 0 1 1 2 1 2 2 3 1 2 2 3 2 3 3 4 Note the pattern: 0112 N 1223 N+1 1223 N+1 2334 N+2 look familiar? |
| "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Nov 09 10:52PM +0100 On 09.11.2018 18:12, Paul wrote: > # define B6(n) B4(n), B4(n+1), B4(n+1), B4(n+2) > B6(0), B6(1), B6(1), B6(2) > }; Others have already explained about the preprocessor. Below is one way to avoid using it for this task in C++. I don't know if it's worth it, though, but it may perhaps bring some insight about the numbers (they're a mystery to me so far). # define B2(n) n, n+1, n+1, n+2 # define B4(n) B2(n), B2(n+1), B2(n+1), B2(n+2) # define B6(n) B4(n), B4(n+1), B4(n+1), B4(n+2) static const unsigned char BitsSetTable256[256] = { B6(0), B6(1), B6(1), B6(2) }; //-------------------------------------------------------------- #include <array> using Byte = unsigned char; template< int level > constexpr auto b( const int n, const int i ) -> int { const int sequence_length = 1 << level; const int subsequence_length = sequence_length/4; const int subseq_i = i/subsequence_length; return b<level-2>( n + subseq_i - (subseq_i >= 2), i%subsequence_length ); } template<> constexpr auto b<2>( const int n, const int i ) -> int { return n + i - (i >= 2); } constexpr auto bits_set_256( const int i ) -> Byte { return b<8>( 0, i ); } //-------------------------------------------------------------- #include <iostream> #include <stdlib.h> auto main() -> int { using namespace std; for( int i = 0; i < 256; ++i ) { const int a = BitsSetTable256[i]; const int b = bits_set_256( i ); if( a != b ) { cout << "At index " << i << ": " << "BitSetTable256 " << a << " != " << "bits_set_256 " << b << endl; return EXIT_FAILURE; } } cout << "Alles in ordnung!" << endl; return EXIT_SUCCESS; } Cheers!, - Alf |
| Jorgen Grahn <grahn+nntp@snipabacken.se>: Nov 09 11:22PM On Fri, 2018-11-09, Paul wrote: > Below is part of some code for counting how many bits of an integer are set, > using a lookup table. ... > static const unsigned char BitsSetTable256[256] = ... FWIW, I wouldn't do it like that: it's cryptic, and I suspect it's slow. Lookup tables made more sense in the 1990s, before CPUs became much faster than RAM. /Jorgen -- // Jorgen Grahn <grahn@ Oo o. . . \X/ snipabacken.se> O o . |
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