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1. 输入年份和月份判断天数（包含闰年的判断）

#include<stdio.h>int judgeleapyear(int y)
{int lp = 0;if (y%4==0 && y%100!=0 || y%400==0){lp = 1;}else{lp = 0;}return lp;
}int judgedays(int y, int m)
{int days = 0;if (m==4 || m==6 || m==9 || m==11){days = 30;}else if (m==1 || m==3 || m==5 || m==7 || m==8 || m==10 || m==12){days = 31;}else if (judgeleapyear(y) == 1){days = 29;}else{days = 28;}return days;
}int main(void)
{int y = 0;int m = 0;printf("Input year and month:\n");scanf("%d%d",&y,&m);printf("%d\n",judgedays(y,m));return 0;
}

2. 打印100以内的素数

#include<stdio.h>int judgeprime(int a)
{int i = 0;int j = 1;for (i = 2; i < a-1; ++i){if (a%i == 0)   //不是素数的条件{j = 0;break;}}return j;
}void printfprime(int n)
{int i = 0;for (i = 2; i <= n; ++i){if (judgeprime(i) == 1){printf("%d\n",i);}}
}int main(void)
{int n = 100;printfprime(n);return 0;}

3. 比较三个数的最大值

#include<stdio.h>int numMax3(int a, int b, int c)
{int max = 0;if (b > a){max = b;}else if (b == a){max = b;}else{max = a;}if (c > max){max = c;}else if (c == max) {max = c;}return max;
}int main(void)
{int a = 0;int b = 0;int c = 0;printf("Input three num:\n");scanf("%d%d%d",&a,&b,&c);printf("max = %d\n",numMax3(a,b,c));return 0;}

4. 递归实现   n！(n的阶乘)

#include <stdio.h>int fact (int n)
{if (n == 1){return 1;}else{return fact (n-1) * n;}
}int main(void)
{int n = 0;printf("Input a num:\n");scanf("%d",&n);printf("%d\n",fact(n));return 0;
}

5. 递归实现斐波那契数列前n项和计算

#include <stdio.h>int fibo (int n)
{if (n==1 || n==2 ){return 1;}else{return fibo (n-1) + fibo (n-2) ;}
}int fibosum (int n)
{if (n==1 || n==2){return n;}else{return fibosum (n-1) + fibo(n);}
}int main(void)
{int n = 0;printf("Input a num:\n");scanf("%d",&n);printf("fibo(%d) = %d\n",n,fibo(n));printf("sum = %d\n",fibosum(n));return 0;
}

6.  数组逆序

#include<stdio.h>  //数组逆序void arrayRever (int a[], int len)
{int t = 0;int i = 0;for (i = 0; i < len/2; ++i){t = a[i];a[i] = a[len-1-i];a[len-1-i] = t;  }
}void printArray(int a[],int len)//    数组输出
{int i = 0;for (i = 0; i < len; ++i){printf("a[%d] = %d\n", i, a[i]);}
}int main(void)
{int a[6] = {9,8,7,6,5,4};       int len = 6;        //计算给出数组长度为lenarrayRever(a,len);printArray(a,len);  return 0;
}