Foro de elhacker.net

digitos = log10(num) + 1

  int digitos , n = 123456789 ;
  for ( digitos = 0 ; n ; digitos++ , n /= 10 ) ;
  printf ( "%d" , digitos ) ;

#include <iostream>
#include <chrono>
#include <math.h>
 
int DigitosV1( int numero )
{
  int digitos;
  for ( digitos = 0; numero > 0; digitos++, numero /= 10 ) ;
  return digitos;
}
 
int DigitosV2( int numero )
{
  return log( numero ) + 1;
}
 
const int NUMERO = 1234567890;
const int MAXBUCLE = 1000000000;
 
int main( )
{
  int temp;
  std::chrono::time_point<std::chrono::system_clock> start, end;
  start = std::chrono::system_clock::now( );
  for ( int i=0; i < MAXBUCLE; i++ )
  {
    temp = DigitosV1( NUMERO );
  }
  end = std::chrono::system_clock::now();
 
  std::chrono::duration<double> elapsed_seconds = end-start;
 
  std::cout << "Tiempo division: " << elapsed_seconds.count() << "s\n";
 
  start = std::chrono::system_clock::now( );
  for ( int i=0; i < MAXBUCLE; i++ )
  {
    temp = DigitosV2( NUMERO );
  }
  end = std::chrono::system_clock::now();
 
  elapsed_seconds = end-start;
 
  std::cout << "Tiempo logaritmo: " << elapsed_seconds.count() << "s\n";
}
 

Tiempo division: 43.1613s
Tiempo logaritmo: 29.538s

Tiempo division: 19.1429s
Tiempo logaritmo: 29.564s

#include <iostream>
#include <chrono>
#include <limits>
#include <math.h>
 
const int INT5 = 12345;
const int INT7 = 1234567;
const int INT10 = 1234567890;
 
const double DOUBLE5 = 12345.0;
const double DOUBLE7 = 1234567.0;
const double DOUBLE10 = 1234567890.0;
 
template< class T >
int DigitosDIV( T numero )
{
  int digitos;
  T divisor = 10;
  T min = 1; // para que el bucle funcione con decimales
  for ( digitos = 0; numero >= min; digitos++, numero /= divisor ) ;
  return digitos;
}
 
template< class T >
int DigitosLOG( T numero )
{
  return log( numero ) + 1;
}
 
template< class T >
std::chrono::duration< double > Bucle( T numero, int(*func)(T) )
{
  int temp;
 
  auto start = std::chrono::system_clock::now( );
 
  const int max = std::numeric_limits< int >::max( );
  for ( int i=0; i < max; i++ )
  {
    temp = func( numero );
  }
  auto end = std::chrono::system_clock::now( );
 
  return end - start;
}
 
int main( )
{
  auto DivInt5  = Bucle( INT5,  DigitosDIV< int > );
  auto DivInt7  = Bucle( INT7,  DigitosDIV< int > );
  auto DivInt10 = Bucle( INT10, DigitosDIV< int > );
 
  auto LogInt5  = Bucle( INT5,  DigitosLOG< int > );
  auto LogInt7  = Bucle( INT7,  DigitosLOG< int > );
  auto LogInt10 = Bucle( INT10, DigitosLOG< int > );
 
  auto DivDouble5  = Bucle( DOUBLE5,  DigitosDIV< double > );
  auto DivDouble7  = Bucle( DOUBLE7,  DigitosDIV< double > );
  auto DivDouble10 = Bucle( DOUBLE10, DigitosDIV< double > );
 
  auto LogDouble5  = Bucle( DOUBLE5,  DigitosLOG< double > );
  auto LogDouble7  = Bucle( DOUBLE7,  DigitosLOG< double > );
  auto LogDouble10 = Bucle( DOUBLE10, DigitosLOG< double > );
 
  std::cout << "Tiempo division (int 5):     " << DivInt5.count( )     << " s\n";
  std::cout << "Tiempo division (int 7):     " << DivInt7.count( )     << " s\n";
  std::cout << "Tiempo division (int 10):    " << DivInt10.count( )    << " s\n\n";
  std::cout << "Tiempo log (int 5):          " << LogInt5.count( )     << " s\n";
  std::cout << "Tiempo log (int 7):          " << LogInt7.count( )     << " s\n";
  std::cout << "Tiempo log (int 10):         " << LogInt10.count( )    << " s\n\n";
  std::cout << "Tiempo division (double 5):  " << DivDouble5.count( )  << " s\n";
  std::cout << "Tiempo division (double 7):  " << DivDouble7.count( )  << " s\n";
  std::cout << "Tiempo division (double 10): " << DivDouble10.count( ) << " s\n\n";
  std::cout << "Tiempo log (double 5):       " << LogDouble5.count( )  << " s\n";
  std::cout << "Tiempo log (double 7):       " << LogDouble7.count( )  << " s\n";
  std::cout << "Tiempo log (double 10):      " << LogDouble10.count( ) << " s\n\n";
}

Tiempo division (int 5):     23.6144 s
Tiempo division (int 7):     33.9994 s
Tiempo division (int 10):    49.798 s
 
Tiempo log (int 5):          60.28 s
Tiempo log (int 7):          60.415 s
Tiempo log (int 10):         60.8651 s
 
Tiempo division (double 5):  81.3541 s
Tiempo division (double 7):  113.471 s
Tiempo division (double 10): 162.253 s
 
Tiempo log (double 5):       60.7651 s
Tiempo log (double 7):       60.8521 s
Tiempo log (double 10):      60.7481 s

digitos = log10(num) + 1

digitos ( 3 * 5) =  log10 ( 3 * 5 ) = log10 (3) + log10 (5) + 1  = 0 + 0 + 1 = 1

digitos ( 3 * 5) =  log10 ( 3 * 5 ) = log10 (3) + log10 (5) + 1  ==  2.176091 

n_digitos 3 * 5 = 2.176091 ==> de aqui tomamos la parte entera :
n_digitos 3 * 5 = 2
 

n_digitos [ 1500! ] = 4115.682325
 
n_digitos [ 1500! ] = 4115 ---> PERFECTO <----
 

#include <stdio.h>
#include <math.h>
#define N  1500
 
int main( ) {
  double n_digitos = 1 ;
  int digitos , i ;
  for( i = 1 ; i <= N ; i++ )
    n_digitos += ( log10 ( i ) ) ;
  printf( "n_digitos [ 1500! ] = %lf\n\n" , n_digitos );
  digitos = ( int ) n_digitos ;
  printf( "n_digitos [ 1500! ] = %d\n\n" , digitos );
  return 0;
}

digitos ( 3 * 5) =  log10 ( 3 * 5 ) = log10 (3) + log10 (5) + 1  = 0 + 0 + 1 = 1

digitos ( 3 * 5 ) =  log10 ( 3 * 5 ) = log10 ( 15 ) =  1.17 + 1  = 2 

int digito = snprintf(NULL, 0, "%d", numero);

digitos ( 3 * 5 ) =  log10 ( 3 * 5 ) = log10 ( 15 ) =  1.17 + 1  = 2 

log10 ( 3 *  5 ) = log10 (3) + log10 (5) = 0 + 0 

log10(1500!) =  log10(1500) + log10(1499) +...+.log10(3) + log10(2)

.....log10(8) + log10(9) = 0

.....log10(8) + log10(9)= 1.85...==> 1

int digito = snprintf(NULL, 0, "%d", numero);

int digito = snprintf(NULL, 0, "%d", "123.314");

int digito = snprintf(NULL, 0, "%d", 123.314);

  int digitos = 0 , num = 123456.345 ;
  while ( num )
    digitos = ( num /= 100 ) >= 0 ? digitos + 2 :  digitos + 1 ;