输出素数的代码优化

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输出100到200的素数

#include<stdio.h>int main(){int num = 0;int count = 0;for (num = 100; num <= 200; num++){int number = 0;for (number = 2; number <= num - 1; number++){if (num%number == 0){break;}}if (num == number){printf("%d\n", num);count++;}}printf("count=%d", count);system("pause");return 0;}

</pre><pre class="objc" name="code">对代码进行的优化：根据数学分析，偶数都不是素数，所以只考虑奇数的情况

<pre class="objc" name="code">#include<stdio.h>int main(){int num = 0;int count = 0;for (num = 101; num <= 200; num+=2){int number = 0;for (number = 2; number <= num - 1; number++){if (num%number == 0){break;}}if (num == number){printf("%d\n", num);count++;}}printf("count=%d", count);system("pause");return 0;}

在只考虑奇数的情况下，奇数肯定不能被偶数整除，所以除数也可以只考虑在奇数情况下

#include<stdio.h>int main(){int num = 0;int count = 0;for (num = 101; num <= 200; num+=2){int number = 0;for (number = 3; number <= num - 1; number+=2){if (num%number == 0){break;}}if (num == number){printf("%d\n", num);count++;}}printf("count=%d", count);system("pause");return 0;}

如果一个数能被其他数整除，那么这个数的两个除数中肯定有一个小于等于这个数的开平方数。（16=2*8,16=4*4，）

#include<stdio.h>#include<math.h>int main(){int num = 0;int count = 0;for (num = 101; num <= 200; num+=2){int number = 0;for (number = 3; number <= sqrt(num); number+=2){if (num%number == 0){break;}}if (number > sqrt(num)){printf("%d\n", num);count++;}}printf("count=%d", count);system("pause");return 0;}

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