#include <stdio.h>
int main()
{
    int num;
    
    scanf("%d",&num);
    printf("2049%04d\n",num);
    
    scanf("%d",&num);
    printf("2049%04d\n",num);
    
    scanf("%d",&num);
    printf("2049%04d\n",num);
    
    return 0;
     
 } 

控制双精度浮点型位数

#include <stdio.h>
int main()
{
    int num;
    while(1) 
    {
            scanf("%d",&num);
    printf("2049%04d\n",num);
        
        
    }
    

    

    
    return 0;
     
 } 

#include <stdio.h>
int main()
{
    char ans1,ans2;
    printf("复习了没？(输入y或Y表示复习了，输入n或N表示没复习):");
    ans1=getchar();
    
    getchar();
    printf("\动手敲代码了没？(输入y或Y表示敲了，输入n或N表示木有敲):");
    ans2=getchar();
    
    if ((ans1 == 'y'||ans1 == 'Y')&&(ans2 == 'y'||ans2 == 'Y'))
       printf("\n罗马不是一天建成的:)\n");
    else
       printf("\n罗马不是一天毁灭的。。。\n");
       
       
    

    

    
    return 0;
     
 } 

#include <stdio.h>
int main()
{
    char x;
    
    x = getchar();
    
    if(x<='9'&&x>='0')
      printf("%c是数字字符\n",x);
      else if((x>='a'&&x<='z')||(x>='A'&&x<='Z'))
        printf("%c是英文字母\n",x);
        else
        printf("%c是其他字符\n",x);
        
    return 0;
     
 } 

#include <stdio.h>
int main()
{
    int n,a,b,c;
    float sum;
    while(scanf("%d%d",&n,&a)!=EOF){
        sum = 0;
        b = 1;
        c = a;
    
        while(b<=n)
        {
            sum = sum + (float)b/a;
            a = a*10 + c;
            b++;
        }
          printf("n = %d,a = %d,sum = %f\n",n,c,sum);
        }
    return 0;
     
 } 

#include <stdio.h>
#include <stdlib.h>
 #include <time.h>
 const int N = 5;
 
int main()
{
    int x, n;
    
    srand(time(0));
    
    for (n = 1;n<=N;n++)
    {
        x = rand() % 100;
        printf("%3d",x);
    }
    
    printf("\n");
    
    return 0;
     
 } 

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
int main()
{
    int x, n,i;
    
    srand(time(0));
    x = rand()%31 +1;
    printf("猜猜十二月哪一天是你的luck day\n");
    printf("开始喽，你有三次机会，猜吧(1`31) ： \n");
    
    for (i = 1;i<=3;i++)
    {
    scanf ("%d", &n);
    if (x==n)
    {
        printf("猜对了，恭喜\n");
        break;
    }
    else if (n>x)
    printf("你猜的日期晚了，luck day还没到呢\n");
    else if (n<x)
    printf("你猜的日期早了，luck day 还没到呢\n");
     
    }
    if (i==4);
    printf("次数用完了。偷偷告诉你：十二月，你的luck day 是%d号\n",x);
    
    
    return 0;
     
 } 

#include <stdio.h>

int main()
{
    int a,b;
    scanf("%d",&a);
    b=1;
    while (b<=a)
    {
        printf(" o \n");
        printf("<H>\n");
        printf("| |\n");
        printf("\n");
        b++;
     } 

    return 0;
     
 }