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Definition of prime numbers
1. Prime numbers are also called prime numbers. A prime number refers to "a number greater than 1 that is only divisible by 1 and the number itself."
2. A prime number can also be equivalently expressed as: "a number within the range of positive integers that is greater than 1 and has only two divisors: 1 and itself."
Find prime numbers between 100-200
提示:以下是本篇文章正文内容,下面案例可供参考
Here is a question, taking this question as an example:
Find the prime numbers between 100-200 and calculate how many prime numbers there are in this range?
The first option
# include <stdio.h>
int main()
{
int i;
//先求出100-200之间的所有的数
//这里给出一个for循环
int count = 0;
for(i = 100; i < 200; i++)
{
int j; //定义一个变量j用来表示因子
for( j = 2; j < i ;j++) //用 2 到 i-1 之间的因子去试除i
{
if(i % j ==0)
{
break; //若能整除,则说明一定不是素数,跳出循环
}
}
//当上一个for循环任意一个数i都不能被j(2到i-1)整除,也即j==i时,跳出for循环
if (j == i) //此时因子i等于一个数本身,则说明是素数
{
count++;
printf("%d ",i);//打印素数
}
}
printf("\ncount = %d\n",count);//所有素数求出来之后,打印素数的个数
return 0;
}
Second option
This solution is similar to the previous one. A flag is defined to mark it. If it is 1, it means a prime number, and if it is 0, it means it is not a prime number.
# include <stdio.h>
int main()
{
int i;
//先求出100-200之间的所有的数
//这里给出一个for循环
int count = 0;
for(i = 100; i < 200; i++)
{
int j; //定义一个变量j用来表示因子
int flag = 1;//此时定义一个flag标记,表示假设i是素数
for( j = 2; j < i ;j++) //用 2 到 i-1 之间的因子去试除i
{
if(i % j ==0)
{
flag = 0;若能整除,则说明一定不是素数,把flag置为0
break;
}
}
//当上一个for循环任意一个数i都不能被j(2到i-1)整除,也即j==i时,跳出for循环
if (flag == 1) //此时判断如果flag还是为1,则说明i是素数
{
count++;
printf("%d ",i);//打印素数
}
}
printf("\ncount = %d\n",count);//所有素数求出来之后,打印素数的个数
return 0;
}
Third option
# include <stdio.h>
# include <math.h>
int main()
{
int i;
//先求出100-200之间的所有的数
//这里给出一个for循环
int count = 0;
for(i = 101; i < 200; i += 2) // 因为偶数一定不是素数,这里缩小范围,在奇数中去找素数
{
int j; //定义一个变量j用来表示因子
int flag = 1;//此时定义一个flag标记,表示假设i是素数
//假设一个数m= a*b,那么这个数m它的因子a和b中一定至少有一个因子小于等于m的开平方数
//拿12这个数来说 它的因子所有表示如下: 12=1*12=2*6=3*4=4*3=6*2=12*1
//12的开平方数是2√3(2倍的根3), 2√3是大于3小于4的,那么用1,2,3去试除12之后,那么4,6,12就没必要再去试除12了
//所以这里的因子范围给到 3 到 sqrt(i) ,sqrt()是C语言的一个库函数,用来求给定值的平方根,需要引用头文件<math.h>
for( j = 2; j <= sqrt(i) ;j++)
{
if(i % j == 0)
{
flag = 0;若能整除,则说明一定不是素数,把flag置为0
break;
}
}
//当上一个for循环任意一个数i都不能被j(2到i-1)整除,也即j==i时,跳出for循环
if (flag == 1) //此时判断如果flag还是为1,则说明i是素数
{
count++;
printf("%d ",i);//打印素数
}
}
printf("\ncount = %d\n",count);//所有素数求出来之后,打印素数的个数
return 0;
}