#include<math.h> 
#include<stdio.h>
int main(){
	float a,b,c,x1,x2;
	
	float delta,real,imag;
	 
    printf("Enter a,b,c；");
	
	while(scanf("%f%f%f",&a,&b,&c ) !=EOF) {
		if(a==0)
		  printf("not quadratic equation.\n\n");
		else{
			delta=b*b-4*a*c;
			if(delta>=0) {
			  x1=(-b+sqrt(delta))/(2*a);
			  x2=(-b-sqrt(delta))/(2*a);
			  printf("x1=%.2f,x2=%.2f\n\n",x1,x2);
			}
			else{
				real=-b/(2*a);
				imag=sqrt(-delta)/(2*a);
				printf("x1=%.2f+%.2fi,x2=%.2f-%.2fi\n\n",real,imag,real,imag);
				
			}
		}
		printf("Enter a,b,c；");
	}
	return 0;
}

#include<stdio.h> 
#include<stdlib.h>
#include<time.h>
#define N 5
int main(){
	int x,n;
	srand(time(0));
	n=0;
	do{
		n++;
		x=rand()%10;
		printf("%3d",x);
	
	}while(n<N);
	printf("\n");
	return 0;
}

#include<stdio.h> 
#include<math.h>
int isprime(int n);
int main() {
	int i;
	for(i=101;i<200;i++)
	
	{
	   if(isprime(i))
	     printf("%4d",i);
	}
	return 0;
}
int isprime(int n)
{
	int k;
	for(k=2;k<=sqrt(n);k++)
	   if(n%k==0)
	     return 0;
	    return 1;
}

#include<stdio.h>

int main() {
	int s,t=0,n;
	float a=0.1;	
	printf("Enter a number:",s);

	while(scanf("%d",&s)!=EOF){
		
		while(s>0){
		n=s%10;
		s=s/10;
		if(n%2==1){
			a=a*10;
			t+=n*a;
			} 
	}
		printf("the number is:%d\n",t);
		printf("Enter a number:",s);
	
	}
	return 0;

}

將其每個位上的數取出，除二取餘是否為1，高位就是乘以十再加上新的數

#include<stdio.h>
#include<math.h>
#include<stdlib.h> 
int main(){
	double s;
	int n,sum=1,i;
	scanf("%d",&n);
	if(n<=0||n>=11)
	 
	  exit (0);
	else
	   
		{
		 for(i=n;i>0;i--)
	    sum*=i;
	    s=pow(-1,n-1)/sum;
		
		
	
	
	
	printf("Enter n(1~10):%d\nn=%d,s=%d\n",n,n,s);
	return 0;
}
  
}

#include<stdio.h> 
#include<stdlib.h>
int main(){
	int n,m=1,t;
	t=1+rand() %30;
    printf("猜猜2020年12月那一天是你的luck day\n");
    printf("\n開始嘍，你有三次機會，猜吧(1~31):",n);
    m++;
    scanf("%d",&n);
 //   m++;
    while(m<=3){
	
    if(n<t){
 

	printf("\n你猜的日期早了，luck day還沒到呢！\n");
    printf("\n再猜猜：\n");
	m++;
	scanf("%d",&n);
      // m++;
	
		}
    else if(n>t){
	
    printf("\n你猜的日期晚了，luck day悄悄溜到後面啦！\n");
      printf("\n再猜猜：\n");
        m++;
        scanf("%d",&n);
  //    m++;
        
    }   
    else if(n=t)
        printf("\n恭喜你，猜對啦！\n");
        return 0;
		} 
    
    printf("\n次數用完啦。偷偷告訴你:12月，你的luck day是%d號\n",t);

    return 0;
}