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

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);
Outpu: 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.
 `// CPP program to illustrate` `// Implementation of at() function` `#include ` `#include ` `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.
 `// CPP program to illustrate` `// Application of at() function` `#include ` `#include ` `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.
 `// CPP program to illustrate` `// Implementation of swap() function` `#include ` `#include ` `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:

 `// CPP program` `#include ` `#include ` `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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