Using push_back for vector in c++

Question

I am having trouble using push_back for vectors in c++.

My vector is named data.

In my loop I want to add 50 to data[i].getQuantity then push_back to data

These are things that I have tried.

    data.push_back(data[i].getQuantity());

and

   float myFloat = data[i].getQuantity() + 50;
   data.push_back(data[i].getQuantity(myFloat));
   data.push_back(myFloat);

The error is saying

No function to call to push_back

Here is my code:

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <vector>
#include <iterator>


struct Input
{
    friend std::istream& operator >>(std::istream& inp, Input& item);
    friend std::ostream& operator <<(std::ostream& outp, Input const& item);

    std::string group;
    std::string total_pay;
    float quantity;

    // default constructor. sets up zero-elements
    Input() : group(), total_pay(), quantity()
    {
    }

    Input(std::string groupIn, std::string total_payIn, float quantityIn) :
    group(std::move(groupIn)),
    total_pay(total_payIn),
    quantity(quantityIn)
    {
    }

    // Accessors
    std::string const& getGroup() const { return group; }
    std::string getTotalPay() const { return total_pay; }
    float getQuantity() const { return quantity; }
};

// global free function for extracting an Input item from an input stream
std::istream& operator >>(std::istream& inp, Input& item)
{
    return (inp >> item.group >> item.total_pay >> item.quantity);
}

// global operator for inserting to a stream
std::ostream& operator <<(std::ostream& outp, Input const& item)
{
    outp
    << item.getGroup() << ", "
    << item.getTotalPay() << ", "
    << item.getQuantity();
    return outp;
}

struct ctype : std::ctype<char>
{
    static mask* make_table()
    {
        static std::vector<mask> table(classic_table(),
                                       classic_table() + table_size);
        table[','] |= space;
        return &table[0];
    }

    ctype() : std::ctype<char>(make_table()) { }
};

int main() {
    std::fstream infile("employee.dat");
    std::vector<Input> data;
    std::string line;

    try {
    while (std::getline(infile, line))
    {

        std::istringstream iss(line);
        Input inp;
        iss.imbue(std::locale(iss.getloc(), new ctype));

        while (iss >> inp) // calls our extraction operator >>
            data.push_back(inp);

        if (iss.fail() && !iss.eof())
            std::cerr << "Invalid input line: " << line << '\n';
    }

    // dump all of them to stdout. calls our insertion operator <<
              std::copy(data.begin(), data.end(),
              std::ostream_iterator<Input>(std::cout,"\n"));

    std::ofstream outp("output.dat");

        for(int i = 0; i < data[i].getQuantity(); i++)
        {

            float myFloat = data[i].getQuantity() + 50;
            data.push_back(myFloat);
            outp << data[i].getGroup() << ',' << data[i].getTotalPay() << ',' << data[i].getQuantity() + 50 << '\n';
        }

    } catch (std::exception& e) {
        std::cout << "There was an error: " << '\n';

        return 1;

    }

    return 0;
}

Answer 1

Your vector is of type std::vector<Input>. That means you can only put objects of type Input into it. You can't push_back a float into such a vector.

If your intention is to create a new Input object and push that back into your vector, you could do something like

data.push_back(Input(data[i].getGroup(), data[i].getTotalPay(), data[i].getQuantity() + 50))

On the other hand, if you are simply trying to modify an element in data without adding a new element to data, you could just do

data[i].quantity += 50;

This works because you use a struct rather than a class. In a struct, variables default privacy level is public. If you wanted to use a class, or you just don't want to directly access the struct members, you would have to create a setter function for quantity.