#include<stdio.h>  //6章7题 
#include<string.h>
int main()
{
	char arr[20] = { 0 };
	scanf("%20s", arr);
	int i = 0;
	for (i = strlen(arr)-1; i >= 0; i--)
	{
		printf("%c", arr[i]);
	}
	return 0;
}

#include<stdio.h>//6章8题 
int main()
{
	double f, s;
	while (scanf("%lf %lf", &f, &s) == 2)
	{
		printf("%f\n", (f - s) / (f * s));
		printf("enter the two next number:\n");
	}
	printf("break\n");
}

#include<stdio.h> //6章9题 
double YB(double x, double y)
{
	double z = (x - y) / (x * y);
	return z;
}
int main()
{
	double f, s;
	while (scanf("%lf %lf", &f, &s) == 2)
	{
		double i = YB(f, s);
		printf("%g\n", i);
		printf("please enter again(q ot quit):");
	}
	
	return 0;
}

#include<stdio.h>//6章10题
int main()
{
	int i = 0;
	int j = 0;
	printf("please enter lower and upper integer limits:");
	while (scanf("%d %d", &i, &j)==2 && (i < j))
	{
		int sums = 0;
		int d = 0;
		for (d = i; d <= j; d++)
		{
			sums += d * d;
		}
		printf("The sums is the squares from %d to %d is %d\n", i * i, j * j, sums);
		printf("Enter next set of limits\n");
		
	}
	return 0;
}


#include<stdio.h> //6章11题
#define limit 8
int main()
{
	int arr[limit] = { 0 };
	int i = 0;
	for (i = 0; i <= limit-1; i++)
	{
		scanf_s("%d", &arr[i]);
	}
	for (i = limit - 1; i >= 0; i--)
	{
		printf("%d ", arr[i]);
	}
	return 0;
}

//第6章12题
#include<stdio.h>
int main()
{
	int n = 0;
	while (scanf("%d", &n) == 1 && n > 0)
	{
		int i = 0;
		double sum1 = 0.0;
		double sum2 = 0.0	;
		int j = 1;
		for (i = 1; i <=n; i++)
		{
			sum1 += 1.0 / i;
		}
		for (i = 1; i <= n; i++)
		{
			sum2 += (1.0 / i)*j;
			j *= -1;
		}
		printf("%g %g", sum1, sum2);
	}
	
	return 0;
}

//第6章12题
#include<stdio.h>
int main()
{
	int n = 0;
	int i = 0;
	while (scanf("%d", &n)==1 && n > 0)
	{
		double res1 = 0.0;
		double res2 = 0.0;
		for (i = 1; i <= n; i++)
		{
			res1 += 1.0 / i;
			if (i % 2 == 1)
			{
				res2 += 1.0 / i;
			}
			else
			{
				res2 -= 1.0 / i;
			}
		}
		printf("%g %g %g", res1, res2,res1+res2);
	}
	return 0;
}

//第6章13题
#include<stdio.h>
int main()
{
	int arr[8] = { 0 };
	int i = 0;
	int t = 2;
	for (i = 0; i < 8; i++)
	{
		arr[i] = t;
		t *= 2;
	}
	for (i = 0; i < 8; i++)
	{
		printf("%d ", arr[i]);
	}
	return 0;
}

//第6章14题
#include<stdio.h>
int main()
{
	double arr1[8] = { 0 };
	double arr2[8] = { 0 };
	int i = 0;
	int j = 0;
	//for (i = 0; i < 8; i++) 嵌套循环实现
	//{
	//	scanf("%lf", &s);
	//	arr1[i] = s;
	//	for (j = 0; j <= i; j++)
	//	{
	//		arr2[i] +=arr1[j] ;
	//	}
	//}
	for (i = 0; i < 8; i++) 单循环实现
	{
		scanf("%lf", &arr1[i]);
		arr2[0] = arr1[0];
		if (i >= 1)
		{
			arr2[i] = arr1[i] + arr2[i - 1];
		}
	}
	for (i = 0; i < 8; i++)
	{
		printf("%-3g ", arr1[i]);
	}
	printf("\n");
	for (i = 0; i < 8; i++)
	{
		printf("%-3g ", arr2[i]);
	}
	return 0;
}

//第6章15题
#include<stdio.h>
#define len 255
int main()
{
	int i = 0;
	char input[len];
	while (i < len)
	{
		scanf("%c", &input[i]);
		if (input[i] == '\n')
		{
			break;
		}
		i++;
	}
	for (i=i-1; i >= 0; i--)
	{
		printf("%c", input[i]);
	}
	return 0;
}

//第6章16题
#include<stdio.h>
int main()
{
	double De = 100;
	double Da = 100;
	int year = 0;
	while (De <= Da)
	{
		Da += 100 * 0.1;
		De += De * 0.05;
		year++;
	}
	printf("after %d year %g beyond %g",year,De,Da);
	return 0;
}

//第6章17题
#include<stdio.h>
int main()
{
	double i = 100.0;
	int year = 0;
	do 
	{
		i += i * 0.08;
		i -= 10;
		year++;
	} while (i > 9);
	printf("%d %g\n",++year, i);
	return 0;
}

//第6章18题
#include <stdio.h>
int main(void)
{
	int i = 1;
	int friends = 5;

	while (friends < 150)
	{
		printf("At %d weeks, Rabnud has", i);
		printf("%4d friends.\n", friends);
		friends = 2 * (friends - i++);
	}
	printf("At %d weeks, over Dunbar's number(150).\n", i);

	return 0;
}