循环未按预期运行

Question

我在这里找不到逻辑中断但是在运行时我获得了3125的输出，因为它应该是最大的素数因子。 3125显然不是素数，也不是最大的非素因子。这不是设计成找到素因子的有效方法，只是试图解决这个问题。

long long modulo= 99999;
long long LrgPrimeFactor=0;
long long currentfactor=0;
long long tempmodulo=0;
int break1 =0;
int break2 =0;

while (modulo>0&&break2==0)
{
    if ((100000%modulo) ==0)
    {
        currentfactor=modulo;

        for (tempmodulo=currentfactor-1;tempmodulo>0;tempmodulo--)
        {
            if (currentfactor%tempmodulo==0)
            {
                LrgPFactor=currentfactor;
                break1=1;
                break;
            }
            else if(break1==1)
            {
                break2=1;
                break;
            }
        }
    }
    else 
    {
        modulo-=2;
    }
}

Answer 1

如果我要使用你的方法，那么我将如何对其进行编码，这种方法的开始时间比你想要的数字少一些。我确认这可以在CodePad.org中使用。

#include <stdio.h>

int main() 
{
    int numberToFactor = 100000;
    int largestPrimeFactorCandidate = numberToFactor;
    int testDivisor = 0;
    int stillLookingForPrime = 1;
    int stillCheckingPrimeness = 1;

    while (largestPrimeFactorCandidate >= 1 && stillLookingForPrime)
    {
        if (numberToFactor != largestPrimeFactorCandidate)
        {
            // First we need a factor of the number.  The first time
            // around, though, we skip this, because the number itself
            // could be prime.

            while (numberToFactor % largestPrimeFactorCandidate != 0)
            {
                largestPrimeFactorCandidate--;
            }
        }

        // Now that we have a factor, we check to see if it's prime.

        testDivisor = largestPrimeFactorCandidate - 1;
        stillCheckingPrimeness = 1;

        while (stillCheckingPrimeness)
        {
            if (largestPrimeFactorCandidate % testDivisor != 0)
            {
                testDivisor--;
            }
            else
            {
                stillCheckingPrimeness = 0;
            }
        }

        if (testDivisor != 1)
        {
            // It's not prime, so we keep looking.
            largestPrimeFactorCandidate--;
        }
        else
        {
            // Largest factor besides itself is 1, so it's prime.
            stillLookingForPrime = 0;
        }
    }

    printf("%d", largestPrimeFactorCandidate);

    // At this point, largestPrimeFactorCandidate is 5, which is your answer.

    return 0;
}

Answer 2

我对您的代码进行了一些更改，现在它给出了正确的结果。希望这会有所帮助。

long long modulo = 99999;
long long lrgPFactor = 0;
long long currentfactor = 0;
long long tempmodulo = 0;

while (modulo > 0)
{
    if (100000 % modulo == 0)
    {
        for (tempmodulo = modulo - 1; tempmodulo > 1; --tempmodulo)
        {
            if (modulo % tempmodulo == 0)
                break;
        }
        if (tempmodulo == 1)
        {
            lrgPFactor = modulo;
            break;
        }
    }
    modulo -= 2;
}

Answer 3

我认为你应该从2开始向上工作，而不是从N向下工作。此代码实现了评注中概述的算法（评注中粗略概述）：

#include <stdio.h>
#include <inttypes.h>

typedef unsigned long long Number;

#define PRI_uNumber "llu"

static Number largest_prime_factor(Number n)
{
    if (n <= 1)
        return n;
    while (n % 2 == 0)
    {
        n /= 2;
        if (n == 1)
            return 2;
    }

    /* When f is composite, its factors have already been eliminated from n */
    for (Number f = 3; f < n; f += 2)
    {
        while (n % f == 0)
        {
            n /= f;
            if (n == 1)
                return f;
        }
    }
    return n;
}

int main(void)
{
    Number numbers[] =
    {
        1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
        15, 19, 21, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99,
        100, 101, 102, 103, 100000, 100001, 100003, 100007, 100009,
    };

    for (size_t i = 0; i < sizeof(numbers) / sizeof(numbers[0]); i++)
        printf("%6" PRI_uNumber ": LPF = %" PRI_uNumber "\n",
                numbers[i], largest_prime_factor(numbers[i]));

    return 0;
}

输出：

     1: LPF = 1
     2: LPF = 2
     3: LPF = 3
     4: LPF = 2
     5: LPF = 5
     6: LPF = 3
     7: LPF = 7
     8: LPF = 2
     9: LPF = 3
    10: LPF = 5
    11: LPF = 11
    12: LPF = 3
    13: LPF = 13
    15: LPF = 5
    19: LPF = 19
    21: LPF = 7
    90: LPF = 5
    91: LPF = 13
    92: LPF = 23
    93: LPF = 31
    94: LPF = 47
    95: LPF = 19
    96: LPF = 3
    97: LPF = 97
    98: LPF = 7
    99: LPF = 11
   100: LPF = 5
   101: LPF = 101
   102: LPF = 17
   103: LPF = 103
100000: LPF = 5
100001: LPF = 9091
100003: LPF = 100003
100007: LPF = 1031
100009: LPF = 157

请注意PRI_uNumber小心地避免为<inttypes.h>标题保留名称空间：

  

7.31.5整数类型<inttypes.h>

的格式转换      

¶1以PRI或SCN开头且以小写字母或X开头的宏可能是   添加到<inttypes.h>标题中定义的宏。

下划线表示它是安全的。