c04a.cpp

《C++程序设计习题及解答》配套代码VC版

// c04a.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"

#include <iostream.h>
#include <iomanip.h>
#include <math.h>
#include <stdlib.h>

int getN();
int getBitNum( int n );
char * init( int size );
void calc( char* a, int n );
void display( char* s, int size );

int main(int argc, char* argv[])
{
	int n = getN();

	int size = getBitNum( n );
	char * pa = init( size );

	calc( pa, n );

	display( pa, size );

	delete[] pa;

	return 0;
}

int getN()
{
	int n;
	cout <<"请输入n!中的n:";
	cin >> n;
	while( n<0 )
	{
		cout << "输入有错，请重输:";
		cin >> n;
	}
	
	if ( n==0 )
		exit(1);

	return n;
}

int getBitNum( int n )
{
	double sum = 1.0;
	for ( int i = 1; i <= n; ++i )
		sum += log10( i );

	return (int)sum;
}

char* init( int size )
{
	char* pa = new char[size];
	if (!pa)
	{
		cout <<"too large factor of "<<size<<endl;
		exit(1);
	}

	pa[0] = 1;
	for ( int i = 1; i < size; ++i )
		pa[i] = 0;

	return pa;
}

void calc( char* a, int n )
{
	double bitcount = 1;
	int begin = 0;
	for ( int i = 2; i <= n; ++i )
	{
		long and = 0;
		bitcount += log10(i);
		if ( a[begin] == 0 )
			begin++;

		for ( int j = begin; j < int(bitcount); j++ )
		{
			and += i * a[j];
			a[j] = char( and % 10 );
			and /= 10;
		}
	}
}

void display( char* a, int size )
{
	int bit = 0;
	for ( int i = size - 1; i >= 0; i-- )
	{
		if ( bit % 50 == 0 )
			cout << endl << "第" << setw(3) << (bit/50+1) << "个50位:";
		cout << (int)a[i];
		//cout << a[i]; // DON'T DO THIS, THE RESULT IS SEVERE!
		bit++;
	}
	cout << endl;
}