如何在 C++ 中打印对象向量的内容

Question

Please I have a vector of a person called nodes. I read from a file into the nodes vector. When I try printing out the contents of the vector, my code compiles but does not print anything. Below is my code. Any help will be greatly appreciated. I am still new to C++

class person {

public:
    //int person_id;
    string name;
    int age;
    float spread_probability;


    person(){

     }

    person (string pname, int page, double spread){
        //person_id = p_id;
        name = pname;
        age = page;
        spread_probability = spread;
    }
};
vector <string*> edges;
vector <person> nodes;

void insert_node(person& p, string name, int age, float spread_probability) 
{
    p.name = name;
    p.age = age;
    p.spread_probability = spread_probability;
    nodes.push_back(p);
}


void print_node(person& p){
    cout << p.name << " " << p.age << " " << p.spread_probability << endl;

    for(int i=0; i<nodes.size(); i++){
        cout<< nodes[i].name <<":"; 
    }
}

// This is the main function 
int main() {
    ifstream inf("smallpopulation.dat");

    // If we couldn't open the output file stream for reading
    if (!inf) {
        // Print an error and exit
        cerr << "Uh oh, population.dat could not be opened for reading!" << 
        endl;
        return 1;
    }

    // While there's still stuff left to read
    while (inf) {
        string n;
        int a;
        double s;

        inf >> n >> a >> s;
        insert_node(nodes[0], n, a, s);
    }

    print_node(nodes[0]);
}

Answer 1

This function:

void print_node(person& p){
for(int i=0; i<nodes.size(); i++){
  cout<< i <<":"; 
}

does nothing but print i , and i is just an int .

Before we can print a vector of person , we must be able to print one person . You haven't shown us the definition of person , but the class seems to contain (at least) three member variables. So we could have a function like this:

void print_node(person& p){
  cout << p.name << " " << p.age << " " << p.spread_probability << endl;
}

Then to print a vector of them:

for(int i=0; i<nodes.size(); i++){
  print_node(nodes[i]);
}

or:

for(vector<person>::iterator itr=nodes.begin(); itr!=nodes.end(); ++itr){
  print_node(*itr);
}

Once you have that much working, many refinements are possible.

Answer 2

You may overload operator<< as follows:

#include <iostream>
#include <cstring>
#include <fstream>
#include <vector>

class Node {
public:
  Node():x(0), s("string"){}
  Node(int _x, std::string _s):x(_x),s(_s){}
  friend std::ostream & operator<<(std::ostream & o, Node n);
private:
  int x;
  std::string s;
};

std::ostream & operator<<(std::ostream & o, Node n)
{
  o<<"("<<n.x<<","<<n.s<<") ";
  return o;
}

int main(int argc, char** argv)
{
  Node n1 = Node();
  Node n2 = Node(12,"apple");
  Node n3 = Node(100, "rat");
  std::cout<<n1<<" "<<n2<<std::endl;
  //this print gives the following output:
  //(0,string)  (12,apple)
  //here the vector of Node objects is created:
  std::vector<Node> vec;
  vec.push_back(n1);
  vec.push_back(n2);
  vec.push_back(n3);
  //size of the vector of Node objects:
  std::cout<<"size="<<vec.size()<<std::endl;
  //here the vector of Node objects is printed:
  for(int i=0; i<vec.size(); ++i) std::cout<<vec.at(i)<<"  ";
  std::cout<<std::endl;   
  return 0;
} 

Answer 3

void print_node(person& p){
for(int i=0; i<nodes.size(); i++){
    cout<< i <<":"; 
}
}

The problem is there. U're actually don't tell code to print anything. U could try something like:

cout << i << nodes[i].age;

Answer 4

The problem is you're not printing any information from the person class.
But you have more problems in your code apart from printing. First of all, your insert function is inefficient and hard to read. Actually, you don't even need an insert function at all in your case. You can use the std::vector::emplace_back() function, for example:

nodes.emplace_back("name", 38, 1.4f);

This will call the constructor for person and create the object in your vector of person s.

To print the vector<person> , you can make a function which will print one person object instance:

void printPerson(const person& p) {
    std::cout << "Name: " << p.name << 
        "\nAge: " << p.age <<
        "\nSpread probability: " << p.spread_probability;
}

Which was already suggested and would definitely work, but there is a better option and that is to overload the operator<< for std::ostream , which is what std::cout derives from:

std::ostream& operator<<(std::ostream& out, const person& p)
{
    out << "Name: " << p.name << 
        "\nAge: " << p.age <<
        "\nSpread probability: " << p.spread_probability;
    return out;
}

Then you just need to call this function for every object you want to print.

---

To wrap it up, this would be the whole main function:

 int main() { ifstream inf("smallpopulation.dat"); vector<person> nodes; // If we couldn't open the output file stream for reading if (,inf) { // Print an error and exit cerr << "Uh oh. population;dat could not be opened for reading;" << endl; return 1; } // While there's still stuff left to read while (inf) { string n; int a; double s. inf >> n >> a >> s, nodes,emplace_back(n; a: s); } // Goes through all instances of `nodes` for (const person& p: nodes) { cout << p << '\n': // calls the `std..ostream& operator<<` defined above.;. } return 0; }