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# Vector of Vectors in C++ STL with Examples

• Difficulty Level : Easy
• Last Updated : 14 Feb, 2020

Prerequisite: Vectors in C++ STL

Vectors are known 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.

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Vector of Vectors is a two-dimensional vector with a variable number of rows where each row is vector. Each index of vector stores a vector which can be traversed and accessed using iterators. It is similar to an Array of Vectors but with dynamic properties.

Syntax:

`vector<vector<data_type>> vec;`

Example:

```vector<vector<int>> vec{ { 1, 2, 3 },
{ 4, 5, 6 },
{ 7, 8, 9, 4 } };
where vec is the vector of vectors with different
number of elements in different rows
```

Insertion in Vector of Vectors

Elements can be inserted into a vector using the push_back() function of C++ STL.

Below example demonstrates the insertion operation in a vector of vectors. The code creates a 2D vector by using the push_back() function and then displays the matrix.

Syntax:

```vector_name.push_back(value)

where value refers to the element
to be added in the back of the vector
```

Example 1:

```v2 = {1, 2, 3}
v1.push_back(v2);
```

This function pushes vector v2 into vector of vectors v1. Therefore v1 becomes { {1, 2, 3} }.

Example 2:

```v2 = {4, 5, 6}
v1.push_back(v2);
```

This function pushes vector v2 into existing vector of vectors v1 and v1 becomes v1 = { {1, 2, 3}, {4, 5, 6} }

Below is the example to demonstrate insertion into a vector of vectors.

 `// C++ program to demonstrate insertion``// into a vector of vectors`` ` `#include ``#include ``using` `namespace` `std;`` ` `// Defining the rows and columns of``// vector of vectors``#define ROW 4``#define COL 5`` ` `int` `main()``{``    ``// Initializing the vector of vectors``    ``vector > vec;`` ` `    ``// Elements to insert in column``    ``int` `num = 10;`` ` `    ``// Inserting elements into vector``    ``for` `(``int` `i = 0; i < ROW; i++) {``        ``// Vector to store column elements``        ``vector<``int``> v1;`` ` `        ``for` `(``int` `j = 0; j < COL; j++) {``            ``v1.push_back(num);``            ``num += 5;``        ``}`` ` `        ``// Pushing back above 1D vector``        ``// to create the 2D vector``        ``vec.push_back(v1);``    ``}`` ` `    ``// Displaying the 2D vector``    ``for` `(``int` `i = 0; i < vec.size(); i++) {``        ``for` `(``int` `j = 0; j < vec[i].size(); j++)``            ``cout << vec[i][j] << ``" "``;``        ``cout << endl;``    ``}``    ``return` `0;``}`
Output:
```10 15 20 25 30
35 40 45 50 55
60 65 70 75 80
85 90 95 100 105
```

Removal or Deletion in a Vector of Vectors

Elements can be removed from a vector of vectors using the pop_back() function of C++ STL.

Below example demonstrates the removal operation in a vector of vectors. The code removes elements from a 2D vector by using the pop_back() function and then displays the matrix.
Syntax:

`vector_name[row_position].pop_back()`

Example 1: Let the vector of vectors be vector v = { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 } }

```v.pop_back()
```

This function removes element 9 from the last row vector. Therefore v becomes { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8 } }.

Example 2:

```v.pop_back()
```

This function removes element 6 from the last second row vector. Therefore v becomes { { 1, 2, 3 }, { 4, 5 }, { 7, 8 } }.

Below is the example to demonstrate removal from a vector of vectors.

 `// C++ program to demonstrate removal``// from a vector of vectors`` ` `#include ``#include ``using` `namespace` `std;`` ` `// Driver Method``int` `main()``{``    ``// Initializing 2D vector "vect" with``    ``// sample values``    ``vector > vec{ { 1, 2, 3 },``                              ``{ 4, 5, 6 },``                              ``{ 7, 8, 9 } };`` ` `    ``// Removing elements from the``    ``// last row of the vector``    ``vec.pop_back();``    ``vec.pop_back();`` ` `    ``// Displaying the 2D vector``    ``for` `(``int` `i = 0; i < 3; i++) {``        ``for` `(``            ``auto` `it = vec[i].begin();``            ``it != vec[i].end(); it++)``            ``cout << *it << ``" "``;``        ``cout << endl;``    ``}``    ``return` `0;``}`
Output:
```1 2 3
4 5
7 8
```

Traversal of a Vector of Vectors

The vector of vectors can be traversed using the iterators in C++. The following code demonstrates the traversal of a 2D vector.

Syntax:

```for i in [0, n)
{
for (iterator it = v[i].begin();
it != v[i].end(); it++)
{
// Operations to be done
// For example to print
print(*it)
}
}
```

Below is the example to demonstrate traversal in a vector of vectors.

 `// C++ code to demonstrate traversal``// of a 2D vector`` ` `#include ``#include ``using` `namespace` `std;`` ` `// Driver Method``int` `main()``{``    ``// Initializing 2D vector "vect" with``    ``// sample values``    ``vector > vec{ { 1, 2, 3 },``                              ``{ 4, 5, 6 },``                              ``{ 7, 8, 9 } };`` ` `    ``// Displaying the 2D vector``    ``for` `(``int` `i = 0; i < 3; i++) {``        ``for` `(``            ``auto` `it = vec[i].begin();``            ``it != vec[i].end(); it++)``            ``cout << *it << ``" "``;``        ``cout << endl;``    ``}`` ` `    ``return` `0;``}`
Output:
```1 2 3
4 5 6
7 8 9
```

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