C Primer Plus 练习 9-7

题目：在程序清单6.20中，函数power()的功能是返回一个double类型数的某个正整数次幂。现在改进该函数，使其能正确地计算负幂。同时，用该函数实现0的任何次幂为0，任何数的0次幂为1.使用循环的方法编写该函数并在一个程序中测试它。

//9-7
#include<stdio.h>
double power(double n, int p);
int main(void)
{
	double x, xpow;
	int n;

	printf("Enter a number and the integer power");
	printf(" to which\nthe number will be raised. Enter q");
	printf(" to quit.\n");
	while (scanf("%lf%d", &x, &n) == 2)
	{
		xpow = power(x, n);       
		printf("%.3g to the power %d is %.5g\n", x, n, xpow);
		printf("Enter next pair of numbers or q to quit.\n");
	}
	printf("Hope you enjoyed this power trip -- bye!\n");
	
	return 0;
}

double power(double n, int p)
{
	double pow=1;
	int i;

	if (n == 0)
		pow = 0;
	else if (n != 0 && p == 0)
		pow = 1.0;
	else if (p > 0){
		for (i = 1; i <= p; i++)
			pow *= n;
	}
	else{
		pow = 1.0 / power(n, -p);
	}

	return pow;
}