How to print vector > in c++?

Question

I am trying to assign and print vector<vector<int>> dynamically , However i cannot figure out how to do this i am stuck here is my program

#include<iostream>
#include<vector>
#include<string>
using namespace std;
class A {
public:
    vector<int> getVector(int s) {

        vector <int> A(s);
        for (int j = 0; j < s; j++) {
            A.push_back(j);
        }
        return A;
    }
};
int main() {
    A obj;
    int n = 5;
    vector<vector<int>> A;
    
    A.push_back(obj.getVector(n));     // pushes a vector on vector A
    A.push_back(obj.getVector(n - 1));

    vector<vector<int>> ::iterator it;
    it = A.begin();
    for (it; it != A.end(); it++) {
        cout << *it;
    }


    return 0;
}

Answer 1

Consider vector<vector> like a dynamic 2D array.

You will require 2 loops for this.

Also, you need to change a few things:

1. vector <int> A(s); will initialise a vector of size s with all elements set to 0 and then you are pushing the first n elements into A. So, changed vector <int> A(s); to vector <int> A;.

2. using namespace std is not considered as a good practice. Find out why.

Have a look at the following implementation:

#include<iostream>
#include<vector>
#include<string>

class A {
    
    public:
        
        std::vector<int> getVector(int s) {

            std::vector <int> A; //Change 1

            for (int j = 0; j < s; j++) {
                A.push_back(j);
            }
            return A;
        }
};

int main() {

    A obj;
    int n = 5;
    std::vector<std::vector<int>> test_vector;
    
    test_vector.push_back(obj.getVector(n));     // pushes a vector on vector A
    test_vector.push_back(obj.getVector(n - 1));

    std::vector<std::vector<int>> :: iterator test_vector_iterator;
    
    test_vector_iterator = test_vector.begin();
    
    for (test_vector_iterator; test_vector_iterator != test_vector.end(); test_vector_iterator++) {
        
        std::vector<int> :: iterator inner_vector_iterator = (*test_vector_iterator).begin();

        for(inner_vector_iterator; inner_vector_iterator != (*test_vector_iterator).end(); inner_vector_iterator++){

            std::cout << *inner_vector_iterator <<" ";
        }

        std::cout << std::endl;
    }


    return 0;
}

Output:

0 1 2 3 4 
0 1 2 3 

Answer 2

Use range loop like

for( const auto &i : A )      // for elements in A, i is vector<int>
{
    for( const auto &j : i )  // for sub elements in A, j is int
    {
        std::cout<<j;
    }
}

or using iterator

for( vector<vector<int>>::iterator i = A.begin() ; i != A.end(); i++ )
{
    for( vector<int>::iterator j = i->begin(); j != i->end(); j++ )
    {
        std::cout<<*j;
    }
}

Thanks.

Answer 3

vector<vector<int>> ::iterator it;
    it = A.begin();
    for (it; it != A.end(); it++) {
        for(vector<int>::iterator it2 = it->begin(); it2 != it->end(); ++it2)
            cout << *it2;
    }

Answer 4

Nested std::for_each could do the trick

Applies the given function object f to the result of dereferencing every iterator in the range [first, last), in order.

We will make f take std::vector<int> as argument, and print it.

#include <iostream>
#include <vector>
#include <algorithm>


int main()
{
    std::vector<std::vector<int>> A{ {1,2}, {3, 4} };

    std::for_each(A.begin(), A.end(), [](const auto& v) {
        std::for_each(v.begin(), v.end(), [](auto const& it) {
            std::cout << it << std::endl;
        });
    });

    return 0;
}

---

Output

1
2
3
4

Answer 5

Vector of vectors is an analog of 2d array. There is no standard method to serialize vector, a nested loop will do.

#include<iostream>
#include<vector>
#include<string>
#include <algorithm>

using std::for_each;
using std::cout;
using std::vector;
using std::endl;

class A {
public:
    vector<int> getVector(int s) {

        vector <int> A(s);
        for (int j = 0; j < s; j++) {
            A.push_back(j);
        }
        return A;
    }
};
int main() {
    A obj;
    int n = 5;
    vector<vector<int>> A;
    
    A.push_back(obj.getVector(n));     // pushes a vector on vector A
    A.push_back(obj.getVector(n - 1));

    // classic for loop
    for (auto itV = A.begin(), itVE =  A.end(); itV != itVE; itV++)
    {
        for (auto itI = itV->begin(), itIE = itV->end(); itI != itIE; itI++) 
        {
             cout << *itI;
        }
    }
    cout << endl;
    
    // much simpler range-based loop
    for (auto& rowV : A )     // note that this a reference 
                              // - no copy of stored vector is made.
        for (auto el : rowV)
             cout << el;

    cout << endl;

    // a generic lambda to serialize vector
    auto print_vector =  [](const auto& v) {
        std::for_each(v.begin(), v.end(), [](auto const& it) {
            std::cout << it << std::endl;
        });
    };

    std::for_each(A.begin(), A.end(), print_vector );

    return 0;
}

There are several ways to do that: use classic for() which is quite mouthful but allows fie control over some aspects, a range-based for loop is the shortest and most concise variant. THird approach is use of idiomatic function from standard library like for_each, which would require creation of callable, which can be preferred if callable can be re-used of be swapped with something else, allowing some flexibility.