如何计算小于N的素数？ [英] How do I count the number of primes less than N?

问题描述

我似乎无法弄明白如何计算一定数量后的素数数量。



我尝试了什么：

I can't seem to figure out exactly how to count the amount of prime numbers after a certain number.

What I have tried:

public class Solution {
    public int countPrimes(int n) {
        if(n <= 2){
            return 0;
        } else if(n == 3){
            return 1;
        }
        int count = 0;
        for(int i = 2; i <= n; i++){
            if(IsPrime( i )){
                count++;
            }
        }
        return count;
    }
    public boolean IsPrime(int num) {
        for(int i=2;i<=num/2;i++){
            if(num % i == 0){
                return false;
            }
        }
        return true;
    }
}

推荐答案

您应该学习尽快使用调试器。而不是猜测你的代码在做什么，现在是时候看到你的代码执行并确保它完成你期望的。



调试器允许你跟踪执行逐行检查变量，你会看到它有一个停止做你期望的点。

调试器 - 维基百科，免费的百科全书 [ ^ ]

掌握Visual Studio 2010中的调试 - A初学者指南 [ ^ ]

http： //docs.oracle.com/javase/7/docs/technotes/tools/windows/jdb.html [ ^ ]

https://www.jetbrains.com/idea/help/debugging-your -first-java-application.html [ ^ ]



调试器在这里向您展示您的代码正在做什么，您的任务是与它应该做什么进行比较做。

当代码没有达到预期的效果时，你就接近一个bug。



应该是什么结果？ n = 0或1？

n = 2应该是什么结果？

查看你的代码。

You should learn to use the debugger as soon as possible. Rather than guessing what your code is doing, It is time to see your code executing and ensuring that it does what you expect.

The debugger allow you to follow the execution line by line, inspect variables and you will see that there is a point where it stop doing what you expect.
Debugger - Wikipedia, the free encyclopedia[^]
Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
http://docs.oracle.com/javase/7/docs/technotes/tools/windows/jdb.html[^]
https://www.jetbrains.com/idea/help/debugging-your-first-java-application.html[^]

The debugger is here to show you what your code is doing and your task is to compare with what it should do.
When the code don't do what is expected, you are close to a bug.

What should be the result for n=0 or 1 ?
What should be the result for n=2 ?
Look at your code.

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