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vector::at() and vector::swap() in C++ STL

  • Difficulty Level : Easy
  • Last Updated : 22 Sep, 2021
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Vectors are same as dynamic arrays with the ability to resize itself automatically when an element is inserted or deleted, with their storage being handled automatically by the container.

vector::at()

at() function is used reference the element present at the position given as the parameter to the function. 
Syntax: 

vectorname.at(position)
Parameters: 
Position of the element to be fetched.
Returns: 
Direct reference to the element at the given position.

Examples:  

Input: myvector = 1, 2, 3
         myvector.at(2);
Output: 3

Input: myvector = 3, 4, 1, 7, 3
         myvector.at(3);
Output: 7

Errors and Exceptions  

  1. If the position is not present in the vector, it throws out_of_range.
  2. It has a strong no exception throw guarantee otherwise.

C++




// CPP program to illustrate
// Implementation of at() function
#include <iostream>
#include <vector>
using namespace std;
 
int main()
{
    vector<int> myvector;
    myvector.push_back(3);
    myvector.push_back(4);
    myvector.push_back(1);
    myvector.push_back(7);
    myvector.push_back(3);
    cout << myvector.at(3);
    return 0;
}
Output



7

Applications: 
Given a vector of integers, print all the integers present at even positions.  

Input: 1, 2, 3, 4, 5, 6, 7, 8, 9
Output: 1 3 5 7 9
Explanation - 1, 3, 5, 7 and 9 are at position 0, 2, 4, 6 and 8 which are even

Algorithm  

  1. Run a loop till the size of the vector.
  2. Check if the position is divisible by 2, if yes, print the element at that position.

C++




// CPP program to illustrate
// Application of at() function
#include <iostream>
#include <vector>
using namespace std;
 
int main()
{
    vector<int> myvector;
    myvector.push_back(1);
    myvector.push_back(2);
    myvector.push_back(3);
    myvector.push_back(4);
    myvector.push_back(5);
    myvector.push_back(6);
    myvector.push_back(7);
    myvector.push_back(8);
    myvector.push_back(9);
    // vector becomes 1, 2, 3, 4, 5, 6, 7, 8, 9
 
    for (int i = 0; i < myvector.size(); i += 2) {
 
        cout << myvector.at(i);
        cout << " ";
    }
 
    return 0;
}
Output
1 3 5 7 9 
vector::swap()

This function is used to swap the contents of one vector with another vector of same type and sizes of vectors may differ.

Syntax: 

vectorname1.swap(vectorname2)
Parameters:
The name of the vector with which
the contents have to be swapped.
Result: 
All the elements of the 2 vectors are swapped.

Examples:  

Input: myvector1 = {1, 2, 3, 4}
         myvector2 = {3, 5, 7, 9}
         myvector1.swap(myvector2);
Output: myvector1 = {3, 5, 7, 9}
         myvector2 = {1, 2, 3, 4}

Input: myvector1 = {1, 3, 5, 7}
         myvector2 = {2, 4, 6, 8}
         myvector1.swap(myvector2);
Output: myvector1 = {2, 4, 6, 8}
         myvector2 = {1, 3, 5, 7}

Errors and Exceptions  

  1. It throws an error if the vector is not of the same type.
  2. It has a basic no exception throw guarantee otherwise.

C++




// CPP program to illustrate
// Implementation of swap() function
#include <iostream>
#include <vector>
using namespace std;
 
int main()
{
    // vector container declaration
    vector<int> myvector1{ 1, 2, 3, 4 };
    vector<int> myvector2{ 3, 5, 7, 9 };
 
    // using swap() function to swap
    // elements of vector
    myvector1.swap(myvector2);
 
    // printing the first vector
    cout << "myvector1 = ";
    for (auto it = myvector1.begin();
         it < myvector1.end(); ++it)
        cout << *it << " ";
 
    // printing the second vector
    cout << endl
         << "myvector2 = ";
    for (auto it = myvector2.begin();
         it < myvector2.end(); ++it)
        cout << *it << " ";
    return 0;
}
Output
myvector1 = 3 5 7 9 
myvector2 = 1 2 3 4 

If the size of the vectors differ:

C++




// CPP program
#include <iostream>
#include <vector>
using namespace std;
 
int main()
{
    vector<int> vec1{ 100, 100, 100 };
    vector<int> vec2{ 200, 200, 200, 200, 200 };
 
    vec1.swap(vec2);
 
    cout << "The vec1 contains:";
    for (int i = 0; i < vec1.size(); i++)
        cout << ' ' << vec1[i];
    cout << '\n';
 
    cout << "The vec2 contains:";
    for (int i = 0; i < vec2.size(); i++)
        cout << ' ' << vec2[i];
    cout << '\n';
 
    return 0;
}
Output
The vec1 contains: 200 200 200 200 200
The vec2 contains: 100 100 100
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