Vector

Introduction

• A vector is a collection of objects of a single type, each of which has an associated integer index.
• Vector declaration: vector<type> variable_name (optional_number_of_elements);
• Vectors can be resized during the execution of the program to accommodate extra elements as needed.

Vectors are good at:

• Accessing individual elements by their position index (constant time).
• Iterating over the elements in any order (linear time).
• Add and remove elements from its end (constant amortized time).

Examples

Example 1: Ask the user to give marks for 5 students.

#include <iostream> 
#include <vector> 
 
using namespace std;
 
    int main() 
    {
        vector<double> student_marks (5);
 
        for (int i = 0; i < 5; i++) 
        { 
            cout << "Enter marks for student #" << i+1 << ": "; 
            cin >> student_marks[i]; 
        }
 
        return 0; 
    }

Example 2: Ask the user to define number of students. Use of resizable vectors.

     #include <iostream> 
    #include <vector> 
    using namespace std;
 
    int main() 
    {
        vector<double> student_marks;     // no size specified: vector contains zero elements
 
        int num_students;
 
        cout << "Number of students in this group: ";
        cin >> num_students;
 
        student_marks.resize (num_students);
 
        for (int i = 0; i < num_students; i++)  // valid subscripts for a vector with num_students elements are 0 to num_students-1
        { 
            cout << "Enter marks for student #" << i+1 << ": "; 
            cin >> student_marks[i]; 
        }
 
        return 0; 
    }

Example 3: Using the pushback() method.

    #include <iostream> 
    #include <vector> 
    using namespace std;
 
    int main() 
    {
        vector<double> student_marks; 
        int number; 
        char answer;
 
        cout << "Do you want to enter numbers (y/n)? "; 
        cin >> answer;
 
        while (answer == 'y') 
        { 
            cout << "Enter value: "; 
            cin >> number;
 
                // Now that we have the number from the user, 
                // append it at the end of the vector
 
            student_marks.push_back(number);
 
            cout << "Do you want to enter more numbers (y/n)? "; 
            cin >> answer; 
        }
 
                // Now print all the values;  notice that we didn't need to 
                // count how many elements were entered:  we can always use 
                // the size() method to ask student_marks how many are there!
 
        for (int i = 0; i < student_marks.size(); i++) 
        { 
            cout << "Student #" << i+1 << '\t' << student_marks[i]; 
        } 
                // '\t' is the code for the TAB character
 
        return 0; 
    }

Accessing elements of a vector with Iterators

Example 4: The following fragment of code illustrates the use of iterators to print all the elements of a vector.

// values has been declared as:  vector<int> values;
    for (vector<int>::iterator i = values.begin(); i != values.end(); ++i)
    {
        cout << *i << endl;
    }

Examples

Example 5: General example that demonstrates most methods.

#include <iostream>
#include <vector>
using namespace std;
 
void print_vector(vector<int> sm);
 
int main()
{
    int i = 0;
    int input = 0;
    vector<int> student_marks; // vector that holds student marks
    vector<int>::iterator iter;
 
    cout << "Please enter marks for students (enter 0 to end):\n";
 
    //push_back(): Adds a new element at the end of the vector
    do
    {
        cout << "Enter mark for student #" << i+1 << ": ";
        cin >> input;
        if(input != 0)
            student_marks.push_back(input);
        i++;
    }while(input);
    print_vector(student_marks);
 
    //capacity(): Returns the size of the allocated storage space
    cout << "Capacity of student_marks: " << student_marks.capacity() << endl;
 
    //size() Returns the number of elements in the vector container
    cout << "Size of student_marks: " << student_marks.size() << endl;
 
    //insert() The vector is extended by inserting new elements before the element at position
    cout << "insert mark 20 at position 2" << endl;
    student_marks.insert(student_marks.begin()+2,20);
    print_vector(student_marks);
 
    //front(): Returns a reference to the first element
    cout << "Marks of first student: " << student_marks.front() << endl;
 
    // at(): Returns a reference to the element at position n in the vector
    cout << "Marks of student #2: " << student_marks.at(2) << endl;
 
    // back(): returns a reference to the last element in the vector
    cout << "Marks of last student:  " << student_marks.back() << endl;
 
    //pop_back(): Removes the last element
    cout << "Removing the last element" << endl;
    student_marks.pop_back();
    print_vector(student_marks);
 
    // erase(): Removes from the vector container
    // erase the 3rd element
    cout << "Erasing the 3rd element" << endl;
    student_marks.erase (student_marks.begin()+2);
    print_vector(student_marks);
 
    //  assign() Assigns new content to the vector object, 
    //dropping all the elements contained in the vector before the call
    //and replacing them by those specified by the parameters
    cout << "Assign value 2 to all students" << endl;
    student_marks.assign(10, 2);
    print_vector(student_marks);
}
 
void print_vector(vector<int> sm)
{
 
    // begin(): Returns an iterator referring to the first element in the vector container.
    // end(): Returns an iterator referring to the past-the-end element in the vector
    for(vector<int>::iterator iter = sm.begin(); iter != sm.end(); ++iter)
    {
        cout << *iter << " " ;
    }
    cout << endl;
 
}