实验三

#include<stdio.h>
#include<math.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, n;
    int count = 0; 
    
    for(N = 101; N <= 200; N++)
    {
        for(n = 2; n <= sqrt(N); n++)
        {
            if(N%n == 0) 
                break;        
        }
        if(n > sqrt(N)) 
        {
            printf("%4d", N);
            
            count++;//统计素数的个数 
            
            if(count%5 == 0)// 每行输出五个数字 
                printf("\n");
        }
    }
    printf("\n");
    printf("101~200之间有%d个素数\n", count);
    
    return 0;
} 

#include<stdio.h>
#include<math.h>

int main(){
    int n, i;
    int k;
    double s;
    
    printf("Enter n(1~10): ");
    
    while(scanf("%d", &n) != EOF) {
        k = 1;
        s = 0;
        for(i = 1; i <= n; i++)
        {
            k *= i;
            s += 1.0*pow(-1,i-1)/k;
        }    
        
        printf("n = %d, s = %lf\n\n", n, s);
        printf("Enter n(1~10): ");
    }
    
    return 0;
}

#include<stdio.h>
#include<stdlib.h>
#include<time.h>

int main()
{
    int x, i;
    int date;
    
    srand(time(0));
    x = rand()%31;
    
    printf("猜猜2020年12月哪一天会是你的lucky day\n\n");
    printf("开始喽，你有三次机会，猜吧（1~31）：");
    
    for(i = 1; i <= 3; i++)
    {
        scanf("%d", &date);
        printf("\n");
        if(x == date)
        {
            printf("恭喜你猜对了！\n");
            break;
        }
        else if(x > date)
            printf("你猜的日期早了，lucky day还没到呢\n\n");
        else
            printf("你猜的日期晚了，lucky day悄悄溜到前面啦\n\n");
        
        if(i != 3)
            printf("再猜（1~31):");
    }
    if(i == 4)
        printf("次数用完啦。偷偷告诉你，你的lucky day是%d号\n", x);
    
    return 0;
 }