#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(void){
    int n,m,p=0;
    for(n=101;n<=200;n++){
    m=2;
    while(m<=sqrt(n)){
        if(n%m==0)
        break;
        m++;
    }    
    if(m>sqrt(n)){
    printf("%d",n);
    printf(" ");
    p++;}
    }
    printf("\n");
    printf("101~200之間共有%d個素數。",p);
    return 0;
}

#include<stdio.h>
#include<math.h>
int main(void){
    long int s;
    int t=0,i,m;
    printf("Enter a number:");
    scanf("%ld",&s);
    for(i=0;s!=0;){
        m=s%10;
    if(m%2!=0){
    t=m*pow(10,i)+t;
    i++;
    }    
    s=s/10;
    }
    printf("New number is:%d",t);
    return 0;
} 

Key 1：運用與2餘數不為零來篩選出奇數，if語句跳過偶數。

Key 2：定義變數i，遇到奇數加一，讓取出的奇數乘10^i，迴圈相加，結果便是自高位到低位（類似於反位數取法）。

#include<stdio.h>
#include<math.h>
int main(void){
    int n,i=1,m=1;
    double s=0;
    printf("Enter n(1~10):");
    scanf("%d",&n);
    while(i<=n){
        m=m*i;
        s=s+pow(-1,i-1)/m;
        i++;
    }
    printf("n=%d,s=%lf",n,s);
    return 0;
}

#include<stdio.h> 
#include <stdlib.h>
#include <time.h>
 
 int main(){
    int day,guess;
     int chance = 3;
     
     srand(time(0));
     day = rand() % 31;
     
     printf("猜猜2020年12月的哪一天會是你的lucky day");
     getchar();
     printf("\n開始嘍,你有三次機會，猜吧（1~31）:");
     
 judge:while(chance--){
         scanf("%d",&guess); 
         if(guess==day) return 0;
         if(guess>day){
             printf("\n\n你猜的日期晚了，lucky day悄悄溜到前面啦:(\n");
             if(chance==0){
                 printf("\n\n次數已經用完了，偷偷告訴你，12月,你的lucky day是%d號\n",day);
                 
             }
             
         }
         if(guess<day){
             printf("\n\n你猜的日期早了，lucky day還沒到\n");
             if(chance==0){
                 printf("\n\n次數已經用完了，偷偷告訴你，12月.你的lucky day是%d號\n",day);
                 
             }
           
         }
     }
     return 0; 
 }