text: // colddays.c -- 找出 0℃ 以下的天数占总天数的百分比
  #include <stdio.h>
  
  int main(void)
  {
  	const int FREEZING = 0;
  	float temperature;
  	int cold_days = 0;
  	int all_days = 0;
  	printf("Enter the list of daily low temperatures.\n");
  	printf("Use Celsius, and enter q to quit.\n");
  	while (scanf("%f", &temperature) == 1)
  	{
  		all_days++;
  		if (temperature < FREEZING)
  			cold_days++;
  	}
  	if (all_days != 0)
  		printf("%d days total: %.1f%% were below freezing.\n",
  			all_days, 100.0 * (float)cold_days / all_days);
  	if (all_days == 0)
  		printf("No data entered.\n");
  	return 0;
  }