How to iterate through a Vector without using Iterators in C++

Prerequisite: C++ STL, Iterators in C++ STL

The iterator is not the only way to iterate through any STL container. There exists a better and efficient way to iterate through vector without using iterators. It can be iterated using the values stored in any container. Below is the syntax for the same for vectors:

Syntax:

for(auto itr : vector_name)

Explanation: Here itr is the value stored in vector which is used to traverse vectors. Below is the program to illustrate the same:

// C++ program to illustrate the above
// topic
#include <bits/stdc++.h>
using namespace std;

// Driver Code
int main()
{

    // Declare the vector
    vector<int> arr = { 1, 2, 3, 4 };

    // Traversing the vector using
    // values directly
    for (auto& it : arr) {

        // Print the values
        cout << it << ' ';
    }
    return 0;
}

Output:

1 2 3 4

Updating values in vector: For updating values in a vector without using iterators traverse the values stored in vector using reference and updated the value. Below is the syntax for the same:

Syntax:

for(auto &itr : vector_name)

Explanation: Here itr is an address to the value stored in vector which is used to traverse vectors. Below is the program to illustrate the same:

// C++ program to illustrate the updation
// in vector without using iterator
#include <bits/stdc++.h>
using namespace std;

// Function to update the value in vector
void updateVector(vector<int> arr)
{

    cout << "Vector Before Update: ";
    for (auto& it : arr) {
        cout << it << ' ';
    }

    // Traverse using the reference to value
    // and multiply each value by 2
    for (auto& it : arr) {
        it *= 2;
    }

    cout << "\nVector After Update: ";
    // Print vector elements
    for (auto& it : arr) {
        cout << it << ' ';
    }
}

// Driver Code
int main()
{

    // Declare the vector
    vector<int> arr = { 1, 2, 3, 4 };

    // Function Call
    updateVector(arr);
    return 0;
}

Output:

Vector Before Update: 1 2 3 4 
Vector After Update: 2 4 6 8

Advantages:

• Simple and easy to write code.
• Better and efficient than using iterators method.

Disadvantages:

• It iterates only in forward direction.
• Keeps no counter i.e., We cannot find the index of any element with this traversal. For counting the element, the counter have to taken explicitly.

We can also iterate using the same traversal in many different Containers in C++. Below are the illustration for the same:

1. Map:

   // C++ program to illustrate the iteration
   // in Map without using iterator
   #include <bits/stdc++.h>
   using namespace std;
   
   // Driver Code
   int main()
   {
   
       // Declare the map
       map<int, int> Mp;
   
       // Inserting values in Map
       Mp[1] = 1;
       Mp[2] = 2;
       Mp[3] = 3;
   
       // Iterate using value in Map
       for (auto it : Mp) {
   
           // Print the elements
           cout << it.first << ' '
                << it.second << endl;
       }
   
       return 0;
   }
   Output:

   1 1
   2 2
   3 3
   

2. Map of Vectors:

   // C++ program to illustrate the iteration
   // in Map of vectors without using iterator
   #include <bits/stdc++.h>
   using namespace std;
   
   // Driver Code
   int main()
   {
   
       // Declare the map of vectors
       map<int, vector<int> > Mp;
   
       // Temporary vector
       vector<int> temp = { 1, 2, 3 };
   
       // Inserting values in Map
       Mp[1] = temp;
   
       temp = { 2, 3, 8, 9 };
       Mp[2] = temp;
   
       temp = { 10, -2 };
       Mp[3] = temp;
   
       // Iterate using value in Map of vectors
       for (auto it : Mp) {
   
           // Print the elements
           cout << it.first << " -> ";
   
           // Traverse each vector map
           // with it.first and print the
           // elements
           for (auto jt : it.second) {
               cout << jt << ' ';
           }
   
           cout << endl;
       }
   
       return 0;
   }
   Output:

   1 -> 1 2 3 
   2 -> 2 3 8 9 
   3 -> 10 -2
   

3. Set:

   // C++ program to illustrate the iteration
   // in set without using iterator
   #include <bits/stdc++.h>
   using namespace std;
   
   // Driver Code
   int main()
   {
   
       // Declare the set
       set<int> S;
   
       // Inserting values in set
       S.insert(3);
       S.insert(-1);
       S.insert(3);
       S.insert(4);
   
       // Iterate using value in set
       for (auto it : S) {
   
           // Print the elements
           cout << it << ' ';
       }
       return 0;
   }
   Output:

   -1 3 4
   

4. Deque:

   // C++ program to illustrate the iteration
   // in deque without using iterator
   #include <bits/stdc++.h>
   using namespace std;
   
   // Driver Code
   int main()
   {
   
       // Declare the deque
       deque<int> dq;
   
       // Inserting values in deque
       dq.push_front(1);
       dq.push_front(2);
       dq.push_front(3);
   
       dq.push_back(4);
       dq.push_back(5);
       // Iterate using value in set
       for (auto it : dq) {
   
           // Print the elements
           cout << it << ' ';
       }
       return 0;
   }
   Output:

   3 2 1 4 5