kemotoe kemotoe -4 years ago 64
C++ Question

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!

Answer Source

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().

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