How to stop crashes in a recursion loop-CodePudding

I made this simple function, but it crashes at p=29. It makes a stopped working error window. Please Help Me

#include <iostream>
using namespace std;

char *primality(unsigned long,unsigned long i=0);

int main()
{
    for(int i=0;i<1000;i  )
        cout<<i<<": "<<primality(i)<<endl;
};

char *primality(unsigned long p,unsigned long i)
{
    if(i==0)
    {
        if(p<=1)
            return "NEITHER PRIME NOR COMPOSITE";
        else if(p==2||p==3)
            return "\tPRIME";
        else if(p%2==0||p%3==0)
            return "\tCOMPOSITE";
    }
    i=5;
    if(i*i<=p)
        if(p%i==0||p%(i 2)==0)
            return "\tCOMPOSITE";
        else
            return primality(p,i 6);
    else
        return "\tPRIME";
}

The output after 29 is stopped and causes a crash Output:

0: NEITHER PRIME NOR COMPOSITE
1: NEITHER PRIME NOR COMPOSITE
2:      PRIME
3:      PRIME
4:      COMPOSITE
5:      PRIME
6:      COMPOSITE
7:      PRIME
8:      COMPOSITE
9:      COMPOSITE
10:     COMPOSITE
11:     PRIME
12:     COMPOSITE
13:     PRIME
14:     COMPOSITE
15:     COMPOSITE
16:     COMPOSITE
17:     PRIME
18:     COMPOSITE
19:     PRIME
20:     COMPOSITE
21:     COMPOSITE
22:     COMPOSITE
23:     PRIME
24:     COMPOSITE
25:     COMPOSITE
26:     COMPOSITE
27:     COMPOSITE
28:     COMPOSITE
29:
--------------------------------
Process exited after 2.855 seconds with return value 3221225725
Press any key to continue . . .

Also, note that I got a return value. Please tell the meaning of this error message

I'm also instructed to use char arrays instead of strings.

CodePudding user response：

29 is the first number, for which you enter into an endless recursion. As a consequence, the application crashes eventually.

I did not look at your algorithm in much detail, but I assume that i=5 is not really working as expected.

CodePudding user response：

In your primality() function,


char *primality(unsigned long p,unsigned long i)
{
    if(i==0)
    {
        if(p<=1)
            return "NEITHER PRIME NOR COMPOSITE";
        else if(p==2||p==3)
            return "\tPRIME";
        else if(p%2==0||p%3==0)
            return "\tCOMPOSITE";
    }
    i=5;
    if(i*i<=p)
        if(p%i==0||p%(i 2)==0)
            return "\tCOMPOSITE";
        else
            return primality(p,i 6);
    else
        return "\tPRIME";
}

You're assigning the value 5 to i every time primality() runs. So the recursion has no stop. Eventually, your stack is too tired to hold all the different primality() calls, it will cause a stack overflow.

Change that to:

char *primality(unsigned long p,unsigned long i = 5);

char *primality(unsigned long p,unsigned long i)
{
    if(p<=1)
        return "NEITHER PRIME NOR COMPOSITE";
    else if(p==2||p==3)
        return "\tPRIME";
    else if(p%2==0||p%3==0)
        return "\tCOMPOSITE";

    if(i*i<=p)
        if(p%i==0||p%(i 2)==0)
            return "\tCOMPOSITE";
        else
            return primality(p,i 6);
    else
        return "\tPRIME";
}