function not returning values in vector

[kamarjit]

I have a class that has a function that should return a vector. The code runs correctly but nothing is returned. When I check the returned vector size it is zero. When I check the size of the vector in the class it has 11 items. I am new to c++ but have used java. Any help would be appreciated. thank in advance.


class Person
{
string name; //private variables for this class
vector<string> personpref;
public:
//public methods that this class will have
Person::Person()
{}
void setName(string n)
{
name=n;
}
string getName()
{
return name;
}
vector<string> getPersonPref()
{
return personpref;
}
};

Code snippet that should get the vector. Person csuitor.getName() is returning the name from the class csuitor.getPersonPref() is returning an empty vector.

while (!cbachelorlist.empty())//iterate over the bachelors list
{
Person csuitor = cbachelorlist.front();
cout <<"csuitor "<< csuitor.getName() <<"\n";
vector<string> cpref=csuitor.getPersonPref();
std::cout <<"pref size" <<cpref.size() <<"\n";
//iterator over his preference
for (vector<string>:: const_iterator it=cpref.begin(); it !=cpref.end();++it)

[vlad from moscow]

I think that the error somewhere else. This code snip is not enough to make conclusion.

[kamarjit]

OK, I am posting the entire code.

class Person
{
string name; //private variables for this class
vector<string> personpref;
public:
//public methods that this class will have
Person::Person()
{}
void setName(string n)
{
name=n;
}
string getName()
{
return name;
}
vector<string>& getPersonPref()
{
return personpref;
}
void addToList(string n)
{//will add a bride to preflist of groom
personpref.push_back(n);
//std::cout <<"add to list "<<n<<"\n";
std::cout << "person pref in class size " <<personpref.size() << "\n";
}
bool isNameInList(string n)
{
//iterate over groompref

for (vector<string>::iterator it=personpref.begin();it!=personpref.end();++it)
{
if(*it==n)
{
return true;
}
return false;
}
}
bool cPrefers(string first, string second)
{
for (vector<string>::iterator it=personpref.begin();it!=personpref.end();++it)
{
if(*it==first) return true;
if (*it==second)return false;
}
return false;
}


};
//get the preferencelist, first preference, second preference












const char *men_data[][11] = {
{ "abe", "abi","eve","cath","ivy","jan","dee","fay","bea","hope","gay" },
{ "bob", "cath","hope","abi","dee","eve","fay","bea","jan","ivy","gay" },
{ "col", "hope","eve","abi","dee","bea","fay","ivy","gay","cath","jan" },
{ "dan", "ivy","fay","dee","gay","hope","eve","jan","bea","cath","abi" },
{ "ed", "jan","dee","bea","cath","fay","eve","abi","ivy","hope","gay" },
{ "fred", "bea","abi","dee","gay","eve","ivy","cath","jan","hope","fay" },
{ "gav", "gay","eve","ivy","bea","cath","abi","dee","hope","jan","fay" },
{ "hal", "abi","eve","hope","fay","ivy","cath","jan","bea","gay","dee" },
{ "ian", "hope","cath","dee","gay","bea","abi","fay","ivy","jan","eve" },
{ "jon", "abi","fay","jan","gay","eve","bea","dee","cath","ivy","hope" }
};

const char *women_data[][11] = {
{ "abi", "bob","fred","jon","gav","ian","abe","dan","ed","col","hal" },
{ "bea", "bob","abe","col","fred","gav","dan","ian","ed","jon","hal" },
{ "cath", "fred","bob","ed","gav","hal","col","ian","abe","dan","jon" },
{ "dee", "fred","jon","col","abe","ian","hal","gav","dan","bob","ed" },
{ "eve", "jon","hal","fred","dan","abe","gav","col","ed","ian","bob" },
{ "fay", "bob","abe","ed","ian","jon","dan","fred","gav","col","hal" },
{ "gay", "jon","gav","hal","fred","bob","abe","col","ed","dan","ian" },
{ "hope", "gav","jon","bob","abe","ian","dan","hal","ed","col","fred" },
{ "ivy", "ian","col","hal","gav","fred","bob","abe","ed","jon","dan" },
{ "jan", "ed","hal","gav","abe","bob","jon","col","ian","fred","dan" }
};

typedef map<string, Person> CPersonlist;
typedef map<string, string> CCouples;

int main()
{
//my code starts here----
//init data structures
CPersonlist cbachelors;
CPersonlist cbachelorettes;
queue<Person>cbachelorlist;
for (int i = 0; i < 10; ++i) // person
{
//make a groom
Person g=Person();
g.setName(men_data[i][0]);
//make a bride
Person b=Person();
b.setName(women_data[i][0]);

cbachelors[men_data[i][0]]=g; //add to bachelors list
cbachelorlist.push(g); //list of bachelors that we will use to iterate
cbachelorettes[women_data[i][0]]=b;//add to bachelorettes list
}

//get their preferences
string boyname;
string girlname;


for (int i = 0; i < 10; ++i) // person
{
//make a groom
Person g=Person();
g.setName(men_data[i][0]);
//make a bride
//std::cout << "print groom name" <<g.getName() <<"\n";
Person b=Person();
b.setName(women_data[i][0]);

cbachelors[men_data[i][0]]=g; //add to bachelors list
cbachelorlist.push(g); //list of bachelors that we will use to iterate
cbachelorettes[women_data[i][0]]=b;//add to bachelorettes list
for (int j = 1; j < 11; ++j) // preference
{
g.addToList(women_data[i][j]);
b.addToList(men_data[i][j]);

}
}

CCouples cengaged;
std::cout<<"Class matchmaking:\n";
while (!cbachelorlist.empty())//iterate over the bachelors list
{
Person &csuitor = cbachelorlist.front();
cout <<"csuitor "<< csuitor.getName() <<"\n";
vector<string> &cpref=csuitor.getPersonPref();
std::cout <<"pref size" <<cpref.size() <<"\n";
//iterator over his preference
for (vector<string>:: const_iterator it=cpref.begin(); it !=cpref.end();++it)
{
//can't get this to print out
cout <<"cpref "<<*it<<"\n";
Person cbride=cbachelorettes[*it];//it is a string so find person in bachelorettes list that matches the string it, return object Person
std::cout << "print name " <<cbride.getName() <<"\n";
if (cengaged.find(cbride.getName())==cengaged.end()) //look in cengaged, if she is not in there then she is availabe.
{
std::cout << "\t" << cbride.getName() << "and " << csuitor.getName() << "\n";
cengaged[cbride.getName()]=csuitor.getName(); //hook up
break;
}
string cgroom=cengaged[cbride.getName()];


if (cbride.cPrefers(csuitor.getName(),cgroom))
{
std::cout << "\t" << cbride.getName() << "dumped " << cgroom << " for " << csuitor.getName() << "\n";
//get object that matches the name of groom from cbachelors
Person cgroompush=cbachelors[cgroom];
cbachelorlist.push(cgroompush); // dump that zero
cengaged[cbride.getName()] = csuitor.getName(); // get a hero
break;
}
}
cbachelorlist.pop(); // pop at the end to not invalidate suitor reference
}
}

[JLBorges]

> I am new to c++ but have used java.

C++ makes a distinction between values, references, and pointers (pointers are the closest to references in Java).

In Person::getPersonPref() returns a vector by value; a copy of the personpref member is made and returned. After that, if elements are added to the personpref member, the copy will not be affected.

#include <vector>
#include <iostream>

struct A
{
    std::vector<int> seq ;

    std::vector<int> get_copy_of_seq() const { return seq ; }

    std::vector<int>& get_reference_to_seq() { return seq ; }
};

int main()
{
    A a ;

    auto val = a.get_copy_of_seq() ;
    auto& ref = a.get_reference_to_seq() ;

    std::cout << val.size() << '\n' ; // 0
    std::cout << ref.size() << '\n' ; // 0

    a.seq.resize(1234) ;
    std::cout << val.size() << '\n' ; // 0
    std::cout << ref.size() << '\n' ; // 1234
}

http://ideone.com/rYuuNG

[kamarjit]

All the items were added before I called the function that returned the vector.This is what I can't understand. I want to get this working before I start looking at returning the vector as a reference.

[JLBorges]

> All the items were added before I called the function that returned the vector

Saw your complete code only now.

Change this
//vector<string> &cpref=csuitor.getPersonPref();

To
vector<string> cpref = csuitor.getPersonPref();

The object returned by csuitor.getPersonPref(); is an anonymous temporary.

[kamarjit]

I tried that and it did not work. I am still getting a vector with size 0. Thanks for your help. This has been frustrating me for the last few days. I will appreciate it if you can suggest something else.

[JLBorges]

Add this line to getPersonPref()

vector<string>& getPersonPref()
{
        std::cout << "getPersonPref() is returning a vector of size " <<  personpref.size() << '\n' ;
        return personpref;
}

What does it print out?

[kamarjit]

It is printing the size as zero.


Now I am totally confused because the the following is printing the correct sizes as I am adding the elements to the list

void addToList(string n)
{//will add a bride to preflist of groom
personpref.push_back(n);
//std::cout <<"add to list "<<n<<"\n";
std::cout << "person pref in class size " <<personpref.size() << "\n";
}

I am making each person, setting their name and preflist, adding them to the cbachelors queue
After that, I am getting a person from the cbachelors queue and trying to retrieve their name and preflist. I can retrieve the name but not the preflist. So it seems that the name is stored with my person object but not the preflist.
Don't know what to do......

[kamarjit]

Got it. Dumb Dumb mistake. I was adding the person to the bachelors list and then adding their preference. Thanks for all the help. You guys & gals are great!!!!!