#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>
int main(){
    int i,j;
    int count = 0;

    for(i = 101;i <= 200;i++)
    {for(j = 2;j <= i;j++)
        {if(i % j == 0)
           {break;}
    }
    if(i == j)
    {printf("%5d",i);

    count++;
    if(count % 5 == 0)
    {printf("\n");}
    }
    }
    printf("\n101`200之間共有%d個素數\n",count);
    return 0;
}

#include <stdio.h>
#include<stdlib.h>
int main(){
    long s,t = 0;
    int i = 1,j;
    printf("Enter a number: ");

    while(scanf("%ld",&s)!= EOF)
    {      i=1,t=0;

            while(s > 0)
            {
                j = s % 10;
                s = s / 10;
                if(j % 2 ==1)
                {
                    t = t + j * i;
                    i = i * 10;
                }
            }
            printf("new number is:%ld\n\n ",t);

            printf("Enter a number: ");
    }
   return 0;

    }

通過if語句篩選出奇數，再通過迴圈和乘10運算依次排位

#include<stdio.h>
#include<math.h>
int main(){
    int n,j,t = 1;
   double s = 0,m;
    printf("Enter n(1~10): ");
    while(scanf("%d",&n)!=EOF)
         {t = 1;
          j = 1;
          int b = -1;
         double s = 0;
          for(t = 1;t <= n;t++)
          {
              j *= t;
              b = b*(-1);
              m = b*1.0/j;
              s = s+m;
          }


         printf("n = %d,s = %f\n",n,s);
         printf("\nEnter n(1~10): ");
         }
         return 0;
         }

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
int main(){
    int x,i,date = 6;
    srand((unsigned)time(NULL));
    x = rand()%31 + 1;
    printf("猜猜2020年12月哪一天會是你的lucky day\n");
    printf("開始嘍，你有三次機會，猜吧(1~31): ");
    for(i = 1;i <= 3;i++)
       { scanf("%d",&x);
          if(x > date)
          {printf("\n你猜的日期晚了，lucky day悄悄溜到前面啦\n"); }

           else if(x < date)
           {printf("\n你猜的日期早了，lucky day還沒到呢\n");}
           if(i != 3){ printf("\n再猜(1~31): ");}
        }

        printf("次數用完啦，偷偷告訴你：12月，你的lucky day是6號");
        return 0;
        }

通過這章瞭解到三種語句的運用方法以及其的巢狀使用，通過過這些可以使計算變得簡便。