CODEWITHSUNDEEP
c++listpointersstl
kemotoe
kemotoe

Reputation: 1777

How do i search a list of object pointers? C++

Firstly, this is an assignment with artificial restrictions. The assignment tasks me to use STL, inheritance, and polymorphism. I also must use iterators to find, print and delete items from the list based on the object id.

I'm using a list of pointers to objects. These objects are derived from a abstract base class Sequence, and are being dynamically allocated and stored in the list.

My abstract base class

class Sequence{
public:
    virtual void print() = 0;
    virtual int getId() = 0;


protected:
    std::string m_label;
    int m_id;
    std::string m_sequence;
    int m_length;

};

The print() and getId() functions are overridden in the derived classes. The data is being read in from a file and parsed by commands on each line.

SequenceDatabase::SequenceDatabase(){

    std::list<Sequence*> myList;
}

// function reads in the filename creates a data stream and performs the requested actions
void SequenceDatabase::importEntries(std::string inputFile){
    std::ifstream dnaFile(inputFile);
    char command;
    std::string label, sequence, type;
    int id, length, index, orf; 
    while(dnaFile >> command){
        Sequence* s;
        // if the command = D this allocates memory for a dna object and pushes the object onto the list
        if(command == 'D'){
            dnaFile >> label >> id >> sequence >> length >> index;
            std::cout << "Adding " << id << " ...\n\n";
            s = new DNA(label, id, sequence, length, index);
            myList.push_back(s);
        }
        // if the command = R this allocates memory for a RNA object and pushes the object onto the list
        if(command == 'R'){
            dnaFile >> label >> id >> sequence >> length >> type;
            std::cout << "Adding " << id << " ...\n\n";
            s = new RNA(label, id, sequence, length, type);
            myList.push_back(s);
        }
        // if the command = A this allocates memory for an AA object and pushes the object onto the list
        if(command == 'A'){
            dnaFile >> label >> id >> sequence >> length >> orf;
            std::cout << "Adding " << id << " ...\n\n";
            s = new AA(label, id, sequence, length, orf);
            myList.push_back(s);
        }
        // if the command = O this searches the list for the id and either outputs that the object doesn't exist or it deletes it
        if(command == 'O'){
            dnaFile >> id;
            std::cout << "Obliterating " << id << " ...\n\n";
            // problem
        }
        // if the command = P this searches the lists for the id and either outputs that the object doesn't exist or it prints out the info of the object 
        if(command == 'P'){
            dnaFile >> id;
            std::cout << "Printing " << id << " ...\n";
            // problem  
        }
        // if the command = S this outputs the number of entries in the list
        if(command == 'S')
            std::cout << "Entries: " << myList.size() << " total\n";
    }
    dnaFile.close();
}

The list is being built correctly. My problem arises when trying to search the list for an object with a certain id. I created the findId() function because I know i have to compare that to the id read in.

I'm not sure how to use the std::find or std::find_if functions when dealing with object pointers. I've tried for hours and every thing i tried doesn't compile.

Any help is appreciated. Thank you!

Upvotes: 1

Views: 2270

Answers (3)

Shreevardhan
Shreevardhan

Reputation: 12641

In case you don't have access to a C++11 compiler or you don't want to use lambda functions, you could define a comparator

struct Comparator {
    id_type find_id;

    Comparator(id_type id) : find_id(id) {}

    inline bool operator ()(const Sequence * obj) {
        return find_id == obj->getId();
    }
}

. . .

typename std::list<Sequence *>::iterator it;
it = std::find_if(List.begin(), List.end(), Comparator(id));
return it != List.end();

Upvotes: 1

R Sahu
R Sahu

Reputation: 206567

If you have access to a C++11 compiler, you can use:

auto iter = std::find_if(myList.begin(),
                         myList.end(),
                         [](Sequence* s) -> bool { return (s->getId() == id); });
if ( iter != myList.end() )
{
   return *iter;
}
else
{
   return nullptr;
}

std::find_if searches for an element for which predicate returns true. Additional information can be found at http://en.cppreference.com/w/cpp/algorithm/find.

In this case, the predicate is a lambda function that returns true if the given ID matches in the ID of one of the Sequence* objects.

If the list doesn't contain a matching Sequence*, find_if returns myList.end().

Upvotes: 5

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

You could use something like

  for (Sequence* p : myList)
    if (p->getId()>50) dosomething(p);

BTW, you may want to use a list of smart pointers (e.g. std::shared_ptr) so 

std::list<std::shared_ptr<Sequence>> myList;

Upvotes: 1

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