比较两个std :: lists的内容

Question

What i'm trying to do here is to compare two lists of structures like this one below. And if two persons share at least for example 3 intrests they should be paired together and put into list of pairs. I'm starting with the first girl in the list and compare it to the boys, if a pair is found it puts them in aa pairlist and remove them from their respective boylist/girllist.

 struct Person {
        char name[30];
        enum gendertype gender;
        TableOfIntrests intrests; //The tableofintrests consists of an array with 6 containters representing six diffrent intrests.
    };

Anyway, the problem I'm having is that the program works maybe ~50% of the times matching the persons and creating pairs. The other ~50% the I get an error message saying "List iterator not dereferancable". I have google the error message but I can't figure out what to do. Maybe I'm thinking completely wrong or it can be done in a much better way, I don't know but any feedback is appreciated.

void pair_together(Personlist *girllist, Personlist *boylist, Pairlist *pairlist, int            least_number_of_intrests)
{

int equal_intrests = 0; 
Pair pair;
Person p, p2;
int testcount3=0;
std::list<Person>::iterator i = girllist->begin();
std::list<Person>::iterator end = girllist->end();

std::list<Person>::iterator i2 = boylist->begin();
std::list<Person>::iterator end2 = boylist->end();
while ((i  != end))
{
    testcount3=0;
    if(i2==end2)
        break;

    equal_intrests = number_of_equal_intrests(i->intrests, i2->intrests);   //number_of_equal_intrests return the number of intrests that the two persons shares.   




    if(equal_intrests >= least_number_of_intrests)
    {           
        printf("%s + %s, ", i->name, i2->name);
        printf("%d\n", equal_intrests);
        equal_intrests =0;

        create_person(&p, i->name, i->gender);
        create_person(&p2, i2->name, i2->gender);
        create_pair(&pair, p, p2);
        pairlist->push_back(pair);
        i =girllist->erase(i);
        i2 =boylist->erase(i2);//--
        i2=boylist->begin();



        testcount3=1;

    }

     else if(testcount3!=1)
    {
        i2++;

    }

     if((i2==end2) && (equal_intrests < least_number_of_intrests))
    {
        i++;
        i2=boylist->begin();

    }

      if(number_of_intrests(i->intrests) <least_number_of_intrests)//number_of_intrests returns how many intrests a person have, so if the person have less intrests than least_number_of_intrests the program just skips to the next person.
    {           
        i++;            
    }




}

}

Answer 1

Towards the end you have this

if((i2==end2) && (equal_intrests < least_number_of_intrests))
{
    i++;
    i2=boylist->begin();

}

if(number_of_intrests(i->intrests) <least_number_of_intrests)//number_of_intrests ...
{           
    i++;            
}

In the second if, you don't check if i!=end and it could, so i->intrests could well cause problems. Try this

if((i!=end) && number_of_intrests(i->intrests) <least_number_of_intrests)//number_of_intrests ...
{           
    i++;            
}

Answer 2

You're erasing from your lists while iterating over them, which confuses the iterators. Instead, make copies of your lists. Iterate over the originals, but erase from the copies. When you're done, discard the originals and keep the copies.

EDIT: You don't have to make copies; you're right about the way you're resetting your 'i' iterator: it is safe. But when you erase from the list, you DO need to set a new value for the 'end' variable.