c - 为什么我们在这里使用 bool ？

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我不明白为什么我们在这里使用bool isPrime以及该函数的作用。

{
    bool    isPrime;
    int     startingPoint, candidate, last, i;

    startingPoint = 856;
    if ( startingPoint < 2 )
    {
    candidate = 2;
    }
    else
    if ( startingPoint == 2 )
    {
     candidate = 3;
    }
    else
    {
    candidate = startingPoint;
    if ( candidate % 2 == 0 )
    candidate--;
    do
    {
            isPrime = true;
            candidate += 2;
            last = sqrt( candidate );
            for ( i = 3; (i <= last) && isPrime; i += 2 )
            {
                if ( (candidate % i) == 0 )
                    isPrime = false;
            }
        } while ( ! isPrime );
    }

    printf( "The next prime after %d is %d. Happy?\n",
           startingPoint, candidate );
    return 0;
}

最佳答案

isPrime is being used to exit from the for loop & do while loop by conditionally.

1. in For Loop

    Our goal is to find out the next greater prime number of candidate.

    If we have the 25 as candidate, we are going to execute the for loop upto 3 to 5(sqrt of 25).

    if we found that any number between that value(3 , 5)
         divides the candidate and gave the remaining as 0.
         then the candidate was not prime number.
         So exit the from the for loop by flag setting isPrime to false.
         next time the for loop will not be executed because ,
         for loop condition fails.

    Control moves to while loop and checks condition,
            still we didn't find out the prime number & do while condition
            is true by (!isPrime) where isPrime is false (!false == Yes)
            so while loop again executing from the starting of the loop.


2. in Do while loop

    To exit from the do while loop ,
    the condition should be failed,
    that means isPrime should be always true for 3, 5 in for loop.

    So for loop will entirely run upto the maximum of last value.

    No chance of setting up the isPrime to false on the for loop.

    so it will exit from the do while loop.
    Otherwise it will never sleep until find out the next prime number.
    it will be infinity.

I hope it will help you to understand the
code sequence & why we are using the isPrime flag on the code sequence.

代码序列摘要：

What is Prime Number：

质数（或素数）是大于1的自然数，除1及其本身外，没有除数。大于1的自然数（不是素数）称为复合数。

例如，5是质数，因为只有1和5将其均匀除，而6是复合数，因为它除1和6外还有除数2和3。

bool    isPrime;
int     startingPoint, candidate, last, i;

startingPoint = 24;

//If start point less than 2
if ( startingPoint < 2 ) {

    //take candidate as 2
    candidate = 2;
}
//If start point equals 2

else if ( startingPoint == 2 ) {
    //take candidate as 3
    candidate = 3;
}
else {


//if none of the above condition then have startingPoint as candidate

    candidate = startingPoint;
    //candiate 24

    if (candidate % 2 == 0)             /* Test only odd numbers */
        candidate--;//if the candidate is even number then make it to odd number by -1 which will be 23

    do {

        isPrime = true;                 /* Initially we are assuming that the Number 23 is prime number.*/

        candidate += 2;                 /* Bump to the next number(23+2 =25) to test */
        //candidate 25

        last = sqrt( candidate );
        //last 5


        Everytime we will do process the 'for' loop upto sqrt of the candidate.
        //candidate 25
        //last 5

        So If we found the any number between the 3 to 5 not prime number, then we no need to run the for loop till the end.

        Because we need to find out the nearest next prime number. So no need to waste our CPU usage.

        // Both 3<=5 && isPrime should be true.

        //last 3<=5 && true(we assumed it will be a prime number initially)
        for ( i = 3; (i <= last) && isPrime; i += 2 ) {
            if ( (candidate % i) == 0 )
                isPrime = false;
        //here increase the 3 to 5 by (i+2), So again it will be executed. But third time it will fail the first condition.
        //So go to start of do while loop, and so on it will work
        //Next time candidate will be increased by 2 which will be (25+2) = 27 and failed to find out the prime Number.
        //Next time candidate will be increased by 2 which will be (27+2) = 29 and success 29 is prime number.

        }

    } while ( ! isPrime );// if the condition true then go to "do" statement.
}

printf( "The next prime after %d is %d. Happy?\n",
       startingPoint, candidate );
return 0;

关于c - 为什么我们在这里使用 bool ？ ，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/17925638/