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;
}

• David G: Looks like you want emplace_back() rather than push_back().

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.

• wuno: ohhhh ok god that makes sense. So because I am doing it as object I need to push back a object. Is it possible to add 50 as I want and create a object to push back then? What would be the logical way to handle this?
• Daniel: @NDiaz Well you haven't shown your Input class definition, so I don't really know what you can do with that. If you show that class definition, I could try to help.
• Daniel: @NDiaz Now that you have added your Input code, could you say what you are trying to accomplish?
• Daniel: @NDiaz Are you trying to push_back a new Input object with the same group and total_pay but different quantity, or are you just trying to change the quantity of an Input object that is already in the vector?
• Daniel: @iwolf TAing has allowed me to recognize mappings of (things novice programmers do) -> (things novice programmers are trying to do) that regular professional software developers might never think of.