Comparing the private member variables of class objects stored in a vector - 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

Here is a nice little blog about it: http://fusharblog.com/3-ways-to-define-comparison-functions-in-cpp/ — NathanOliver, Jan 18 '17 at 17:42
The way you tried to use max_element is the key to your question. Why did you leave it out of it? — DeiDei, Jan 18 '17 at 17:42
`I've tried using a max_element ` Then show us what you wrote so we can explain the correct usage. Else, just take a look [here](http://en.cppreference.com/w/cpp/algorithm/max_element). — kebs, Jan 18 '17 at 17:42
Never loop on eof() - see https://latedev.wordpress.com/2012/12/04/all-about-eof for why. — , Jan 18 '17 at 17:42
"I'm currently using C++ 98 as it is what my course requires." Find another course. — , Jan 18 '17 at 17:43
Take a look [here](http://stackoverflow.com/questions/7102246/sorting-a-vector-of-objects-in-c) — theVoid, Jan 18 '17 at 17:47

DeiDei · Accepted Answer · 2017-01-18T18:17:16.863

2

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 Atoms.

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&).

edited Jan 18 '17 at 18:17

answered Jan 18 '17 at 17:47

DeiDei

10,205
6
55
80

std::max_element returns an iterator, not a reference – RyanP Jan 18 '17 at 17:50
@RyanP Thanks. Fixed. – DeiDei Jan 18 '17 at 17:51
I'm now getting this error: - passing ‘const Atom’ as ‘this’ argument of ‘float Atom::getMass()’ discards qualifiers [-fpermissive] return lhs.getMass() < rhs.getMass(); – Daniel Jan 18 '17 at 18:13
You need to make your getter functions const. I will edit the answer in a bit. – DeiDei Jan 18 '17 at 18:15
@Daniel Great! Make sure you accept an answer to wrap up the question. – DeiDei Jan 18 '17 at 18:29

score 0 · Answer 2 · answered Jan 18 '17 at 18:01

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();
));

score 0 · Answer 3 · answered Jan 18 '17 at 18:05

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;
}

Comparing the private member variables of class objects stored in a vector - C++

3 Answers3