比较存储在向量中的类对象的私有成员变量-C ++

Question

So I have created a program which can read a .dat file of about 20 lines containing info about different atoms (name, symbol, mass etc) and added them all to a vector of class type I made called Atom.

How would I write a function to find the atom with the highest mass?

Here is my class:

class Atom
{
    string element, symbol;
    float number;
    float mass;
public:
    Atom(string e, string s, float n, float m){
        element = e; symbol = s; number = n; mass = m;
    }
    string getElement();
    string getSymbol();
    float getNumber();
    float getMass();
    float ratio();
    friend ostream& operator<<(ostream& os, Atom c);
};

and the information is added to a vector with the following statements

    ifstream fin("atoms.dat");  

    string E, S;
    float M, N;

    vector <Atom> periodic;

    while(!fin.eof()){
        fin >> E >> S >> M >> N;
        Atom Atom(E, S, M, N);
        periodic.push_back(Atom);
    }

I want to be able to write a function which finds which atom has the highest mass, I've tried using a max_element function but I keep getting errors. Is there a quick way of comparing the member variables of class objects stored in a vector?

I'm currently using C++ 98 as it is what my course requires.

Thanks

Answer 1

I don't know what you did wrong with std::max_element , as you haven't provided what you've tried with it.

struct CompareAtomMass
{
    bool operator()(const Atom& lhs, const Atom& rhs) {
        return lhs.getMass() < rhs.getMass();
    }
};

and then:

vector <Atom> periodic;
Atom max_atom = *max_element(periodic.begin(), periodic.end(), CompareAtomMax());

struct CompareAtomMass is called a function object. It's a class with operator() overloaded to return a bool . std::max_element requires just such a function object to spit out the max element as it needs a way to compare your Atom s.

EDIT: You should mark your getter functions as const since they don't change the inner state of the class.

string getElement() const;
string getSymbol() const;
float getNumber() const;
float getMass() const;

This will allow you to call them from a const object of type Atom just as the above function object requires ( const Atom& ).

Answer 2

Variation of DeiDeis answer: If you only do this at one place, and don't feel a need to keep a CompareAtomMass function class, you can use a lambda:

const auto maxIt = max_element(periodic.begin(), periodic.end(), 
   [](const Atom& lhs, const Atom& rhs) {
    return lhs.getMass() < rhs.getMass();
));
if(maxIt != periodic.end()){
  // use *maxIt ;
}

in C++14 and later you can also use auto in lambdas:

const auto maxIt = max_element(periodic.begin(), periodic.end(), 
   [](const auto& lhs, const auto& rhs) {
    return lhs.getMass() < rhs.getMass();
));

Answer 3

It is a good idea to make your member functions const . This will allow this code. Otherwise just remove all const from my code. In case your vector is empty, you will get null pointer.

struct AtomMassComparator
{
    bool operator()(const Atom& lhs, const Atom& rhs)
    {
        return lhs.getMass() < rhs.getMass();
    }
};

const Atom* getAtomWithHighestMass(const vector<Atom>& v)
{
    vector<Atom>::const_iterator it = max_element(
        v.begin(), v.end(), AtomMassComparator());
    return v.end() == it ? 0 : &*it;
}