#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 main(){
    int n,i,m;
    double k;
    m=0;
    for(n=101;n<=200;n++){
       k=sqrt(n);
       for(i=2;i<=k;i++){
        if(n%i==0)
        break;
       }
       if(i>k){
           printf("%5d",n);
           m++; 
       }
    }
    printf("\n");
    printf("101~200之間共有%d個素數\n",m);
    return 0;
}

#include <stdio.h>
#include <math.h>
int main()
{
    long int s,n=0,m=0,t=0;
    printf("Enter a number: ");
    while(scanf("%ld",&s)!=EOF)
    {
        while(s!=0)
        {
            m=s%10;
            if(m%2==1)
            {
                n++;
                t=m*pow(10,n-1)+t;
            }
            s=s/10;
        }
        printf("new number is: %ld\n\n",t);
        printf("Enter a number: ");
        
        t=0;
        n=0;
    }
    return 0;
}

判斷奇偶，再判斷大小

#include <stdio.h>
double fun(int n)
{
    int i;
    double sum=1;
    for (i=1;i<=n;i++)
        sum*=i;
    return sum;
}
int main()
{
    int n,i;
    double s=0;
    printf("Enter n(1~10):");
    while(scanf("%d",&n)!=EOF)
    {
        s=0;
        for (i=1;i<=n;i++)
        {
            if(i%2==0)
                s-=1.0/fun(i);
            else
                s+=1.0/fun(i);
        }
        printf("n=%d,s=%lf\n",n,s);
        printf("Enter n(1~10):");
    }
}

#include<stdio.h>
#include<stdlib.h>
#include<time.h>
int main(){
    int n,x,m;
    printf("猜猜2020年12月哪一天會是你的luck day\n");
    srand(time(0));
    m=(rand()%(31-1+1))+1;
    x=1;
    printf("%d\n",m);
    printf("開始嘍。你有三次機會，猜吧(1~31):");
    do{
        scanf("%d",&n);
        if(n==m)
           break;
        else if(n<m){
            printf("你猜的日期早了，luck day還沒到呢\n");
            x++;
        }
        else{
            printf("你猜的日期晚了，luck day悄悄溜到前面啦\n");
            x++; 
        }   
        if(x<3)    
            printf("再猜(1~31):");
    }while(x<=3);
    if(x>3){
        printf("次數用完啦。偷偷告訴你：12月，你的luck day是%d號",m);
    }
    return 0;
}