素数程序 [英] Prime numbers program

问题描述

我目前正在尝试一些问题，目的只是为了练习我的编程技能.(我还没有上过课，这是自学成才的.)我遇到了这个问题，我需要从一个给定的txt文件中读取一个数字.该数字将为N.现在，我想找到N <= 10000的第N个质数.找到它之后，我想将其打印到另一个txt文件中.现在，对于问题的大部分内容，我都能够理解和设计一种获取N的方法.问题是，我正在使用数组来保存先前找到的素数，以便使用它们来检查将来的数.即使我的数组的大小为100，只要输入整数大致<15日，程序崩溃.

I'm currently trying out some questions just to practice my programming skills. ( Not taking it in school or anything yet, self taught ) I came across this problem which required me to read in a number from a given txt file. This number would be N. Now I'm suppose to find the Nth prime number for N <= 10 000. After I find it, I'm suppose to print it out to another txt file. Now for most parts of the question I'm able to understand and devise a method to get N. The problem is that I'm using an array to save previously found prime numbers so as to use them to check against future numbers. Even when my array was size 100, as long as the input integer was roughly < 15, the program crashes.

#include <cstdio>
#include <iostream>
#include <cstdlib>
#include <fstream>
using namespace std;

int main() {
    ifstream trial;
    trial.open("C:\\Users\\User\\Documents\\trial.txt");
    int prime;
    trial >> prime;
    ofstream write;
    write.open("C:\\Users\\User\\Documents\\answer.txt");
    int num[100], b, c, e;
    bool check;
    b = 0;
    switch (prime) {
        case 1:
        {
            write << 2 << endl;
            break;
        }
        case 2:
        {
            write << 3 << endl;
            break;
        }
        case 3:
        {
            write << 5 << endl;
            break;
        }
        case 4:
        {
            write << 7 << endl;
            break;
        }
        default:
        {
            for (int a = 10; a <= 1000000; a++) {
                check = false;
                if (((a % 2) != 0) && ((a % 3) != 0) && ((a % 5) != 0) && ((a % 7) != 0)) // first filter
                {
                    for (int d = 0; d <= b; d++) {
                        c = num[d];
                        if ((a % c) == 0) {
                            check = true; // second filter based on previous recorded primes in array
                            break;
                        }

                    }
                    if (!check) {
                        e = a;
                        if (b <= 100) {
                            num[b] = a;
                        }

                        b = b + 1;
                    }
                }
                if ((b) == (prime - 4)) {
                    write << e << endl;
                    break;
                }
            }
        }
    }
    trial.close();
    write.close();
    return 0;
}

我完全是根据我的虚拟指南进行此操作的，所以我原谅了我的算法在代码效率低下和通用性方面的不足.同样，最多可以正确显示15个素数.

I did this entirely base on my dummies guide and myself so do forgive some code inefficiency and general newbie-ness of my algorithm. Also for up to 15 it displays the prime numbers correctly.

有人可以告诉我如何改进当前代码吗?我正在考虑使用txt文件代替数组.那可能吗?感谢您的帮助.

Could anyone tell me how I should go about improving this current code? I'm thinking of using a txt file in place of the array. Is that possible? Any help is appreciated.

推荐答案

#include <cstdio>
#include <iostream>
#include <cstdlib>
#include <fstream>

using namespace std;

int main()
{
ifstream trial;
trial.open("C:\\Users\\User\\Documents\\trial.txt");
int prime, e;
trial>>prime;
ofstream write; 
write.open("C:\\Users\\User\\Documents\\answer.txt");
int num[10000], currentPrime, c, primePrint;
bool check;
currentPrime=0;
num[currentPrime] = 2;
currentPrime=1;

for(int currentInt=2; currentInt<=1000000; currentInt++) 
{check = false; 
for( int arrayPrime=0; arrayPrime<currentPrime; arrayPrime++) 
        { c=num[arrayPrime];
            if ((currentInt%c)==0) { check = true;// second filter based on previous recorded primes in array
                    break;}
               }


 if (!check)
    { e=currentInt;  
    if( currentInt!= 2 ) { 
    num[currentPrime]= currentInt;}
    currentPrime = currentPrime+1;}
    if(currentPrime==prime)
    {
    write<<e<<endl;
    break;}
    }
trial.close();
write.close();
return 0;
}

这是基于我的原始代码的最终版本.它可以完美地工作，如果您想增加素数的范围，只需增加数组号即可.感谢您的帮助=)

This is the finalized version base on my original code. It works perfectly and if you want to increase the range of prime numbers simply increase the array number. Thanks for the help =)

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