ex2

 #include <stdio.h>
 #include <math.h>
 int main(){
 
 int sum, n, num;
 
 printf("请输入求和项数：") ;
 scanf("%d",&num);
 
 if(num>=1 && num<=10 ){
     
      int n=1,sum=1;
     
     while(n<=num){
      
       sum=sum+pow(2,n);
       
       n++; 
       
    }
    
     printf("当n=%d时，sum=%d",num,sum);
          
 }
 else
   printf("输入数据错误\n");
   
   return 0 ;
       
}

#include <stdio.h>
int main() {    
char x;    
    
x = getchar();    
    
if( x>='0' && x<='9')         
    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是其它字符\n", x);            
return 0;
}

#include<stdio.h>
int main(){
    
    double x,y;    
    char c1,c2,c3;    
    int a1,a2,a3;    
     
    scanf("%d%d%d",&a1,&a2,&a3);    
    printf("%d，%d，%d\n",a1,a2,a3);    
    
    scanf("%c%c%c",&c1,&c2,&c3);    
    printf("%c%c%c\n",c1,c2,c3);
        
    scanf("%lf,%lf",&x,&y);    
    printf("%.1lf,%.1lf\n",x,y);        
    
    
    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("\n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() {    
 int x=1234;
 int y=3, z=4;
 float f=123.456;
 double m=123.456;    
 char ch='a';    
 char a[]="Hello, world!";     

 printf("%d %d\n", y, z);    
 printf("y=%d, z=%d\n", y,z);    
 printf("%8d,%2d\n", x,x);    
 printf("%f, %8f, %8.1f, %0.2f, %.2e\n",f,f,f,f,f);    
 printf("%lf\n",m);    printf("%3c\n", ch);    
 printf("%s\n%15s\n%10.5s\n%2.5s\n%.3s\n",a,a,a,a,a);        

return 0;}

#include<stdio.h>
int main (){
    int num ;
    
    scanf("%d",&num);
    printf("2020%04d\n",num);
    
    scanf("%d",&num);
    printf("2020%04d\n",num);
    
    scanf("%d",&num);
    printf("2020%04d\n",num);
    
    return 0 ;
}

#include<stdio.h>
int main(){
int x=0;
int num;    
    while(x==0){
    
    scanf("%d",&num);
    printf("2020%04d\n",num);
    }
    
    return 0 ;
}