Open In App
Related Articles

# 2D Vector In C++ With User Defined Size

A 2D vector is a vector of the vector. Like 2D arrays, we can declare and assign values to a 2D vector!

Assuming you are familiar with a normal vector in C++, with the help of an example we demonstrate how a 2D vector differs from a normal vector below:

## C++

 `/* Vectors belong to a C++ library ``   ``called STL so we need to import ``   ``it first! */``#include  ``using` `namespace` `std;``int` `main()``{``    ``/*``    ``In the case of a normal vector we initialize it as:``     ` `    ``1. vector variable_name``     ` `    ``Now in the case of a 2D vector all we do is create``    ``a vector of datatype vector.``     ` `    ``We simply replace "datatype" with "vector":``     ` `    ``1. vector> variable_name``     ` `    ``That's literally it! We just created a 2D vector!``    ``On line 23 below we declare an actual 2D vector``    ``named "vect".``    ``*/``     ` `    ``vector> vect;`` ` `    ``return` `0;``}`

In a 2D vector, every element is a vector.

Time Complexity: O(1)

Auxiliary Space: O(1)

## C++

 `/* C++ code to demonstrate a 2D vector ``   ``with elements(vectors) inside it. */``#include ``#include ``using` `namespace` `std;`` ` `int` `main()``{``    ``/*``    ``Below we initialize a 2D vector ``    ``named "vect" on line 12 and then``    ``we declare the values on ``    ``line 14, 15 and 16 respectively.``    ``*/``     ` `    ``vector> vect``    ``{``        ``{1, 2, 3},``        ``{4, 5, 6},``        ``{7, 8, 9}``    ``};``     ` `    ``/*``    ``Now we print the values that ``    ``we just declared on lines``    ``14, 15 and 16 using a simple ``    ``nested for loop.``    ``*/``     ` `    ``for` `(``int` `i = 0; i < vect.size(); i++)``    ``{``        ``for` `(``int` `j = 0; j < vect[i].size(); j++)``        ``{``            ``cout << vect[i][j] << ``" "``;``        ``}    ``        ``cout << endl;``    ``}`` ` `    ``return` `0;``}`

Output

```1 2 3
4 5 6
7 8 9 ```

Time Complexity: O(N*N)

Auxiliary Space: O(N*N)

Another approach to access the vector elements:

## C++

 `/* C++ code to demonstrate a 2D vector ``   ``with elements(vectors) inside it. */``#include ``#include ``using` `namespace` `std;`` ` `int` `main()``{``    ``/*``    ``Below we initialize a 2D vector ``    ``named "vect" on line 12 and then``    ``we declare the values on ``    ``line 14, 15 and 16 respectively.``    ``*/``     ` `    ``vector> vect``    ``{``        ``{1, 2, 3},``        ``{4, 5, 6},``        ``{7, 8, 9}``    ``};``     ` `    ``/*``    ``Now we print the values that ``    ``we just declared on lines``    ``14, 15 and 16 using a simple ``    ``nested for loop with the help of iterator.``    ``*/``     ` `    ``/*``    ``vector> vect``    ``We can divide this declaration to two parts, which will``    ``help us to understand the below concepts.``     ` `    ``1. vect is a 2D vector consisting multiple elements of type vector. ``    ``2. vector is a 1D vector consisting of multiple int data.``     ` `    ``So we can use iterator provided by STL instead of ``    ``i,j variable used in for loop. It can reduce the error which can ``    ``happen wrt to i, j operations(i++, j++)     ``     ` `    ``In the below code we are using iterator to access the vector elements.``    ``1. We are getting vect1D vectors of type vector from the 2D vector vect.``    ``2. We are getting int elements to x from the vector vect 1D vector.``     ` `    ``*/``     ` `    ``for` `(vector<``int``> vect1D : vect)``    ``{``        ``for` `(``int` `x : vect1D)``        ``{``            ``cout << x << ``" "``;``        ``}    ``        ``cout << endl;``    ``}`` ` `    ``return` `0;``}`

Output

```1 2 3
4 5 6
7 8 9 ```

Time Complexity: O(N*N)

Auxiliary Space: O(N*N)

Like Java’s jagged arrays, each element of a 2D vector can contain a different number of values.

## C++

 `/*``C++ program to demonstrate a 2D vector where``each of its elements is of different size.``*/``#include ``#include ``using` `namespace` `std;``int` `main()``{``    ``/*``    ``We initialize a 2D vector ``    ``named "vect" on line 16 with``    ``different number of values ``    ``in each element.``    ``*/``     ` `    ``vector> vect``    ``{``        ``/* Element one with 2 values in it. */``        ``{1, 2}, ``       ` `        ``/* Element two with 3 values in it. */``        ``{4, 5, 6}, ``       ` `         ``/* Element three with 4 values in it. */``        ``{7, 8, 9, 10} ``    ``};`` ` `    ``/*``    ``Now we print the vector that we ``    ``just defined using a simple``    ``nested for loop.``    ``*/``     ` `    ``for` `(``int` `i = 0; i < vect.size(); i++) ``    ``{``        ``for` `(``int` `j = 0; j < vect[i].size(); j++)``        ``{``            ``cout << vect[i][j] << ``" "``;``        ``}    ``        ``cout << endl;``    ``}``    ``return` `0;``}    `

Output

```1 2
4 5 6
7 8 9 10 ```

Exercise Problem: Define the 2D vector with different sizes of columns.
Examples:

```Input : Number of rows : 5
Number of columns in rows :
2 3 4 5 1
Output : 1 2
1 2 3
1 2 3 4
1 2 3 4 5
1

Input : Number of rows : 3
Number of columns in rows :
3 2 1

Output : 1 2 3
1 2
1```

2D vectors are often treated as a matrix with “rows” and “columns” inside it. Under the hood they are actually elements of the 2D vector.
We first declare an integer variable named “row” and then an array named “column” which is going to hold the value of the size of each row.

After that we proceed to initialize the memory of every row by the size of column.

## C++

 `/* ``C++ program to create a 2D vector where ``every row has a certain number of values``as defined by the user.(On line 13)``*/`   ` ` `#include ``#include ``using` `namespace` `std;``int` `main()``{``     ` `    ``/* Here we tell how many rows ``    ``the 2D vector is going to have. */``    ``int` `row = 5;``   ` `    ``/* We define the number of values ``    ``each row is supposed to have. */``    ``int` `column[] = {5, 3, 4, 2, 1}; `` ` `    ``/*``    ``We now create a vector of vector with size``    ``equal to row.``    ``*/``     ` `    ``vector> vec(row);``    ``/*``    ``On line 21 we created a 2D vector and assigned``    ``it a capacity of "row"(in this case 5) units.``    ``*/``     ` `    ``/*``    ``Now we will proceed to create the structure of``    ``our 2D vector by assigning the value of rows and``    ``columns through a nested for loop.``    ``*/`` ` `    ``for``(``int` `i = 0; i < row; i++)``    ``{   ``        ``/* Declaring the size of the column. */``        ``int` `col = column[i]; `` ` `        ``/*``        ``On the 43rd line we declare the ``        ``i-th row to the size of the column.``        ``We create a normal vector of capacity "col" which``        ``in every iteration of the for loop will define the``        ``values inside of each row.``        ``*/``        ``vec[i] = vector<``int``>(col);``        ``for``(``int` `j = 0; j < col; j++)``        ``{``            ``vec[i][j] = j + 1;``        ``}    ``    ``}``     ` `    ``/*``    ``We now finally use a simple nested for loop``    ``to print the 2D vector that we just created above.``    ``*/`` ` `    ``for``(``int` `i = 0; i < row; i++)``    ``{``        ``for` `(``int` `j = 0; j < vec[i].size(); j++)``        ``{``            ``cout << vec[i][j] << ``" "``;``        ``}    ``        ``cout << endl;``    ``}``    ``return` `0;``}`

Output

```1 2 3 4 5
1 2 3
1 2 3 4
1 2
1 ```

Another Approach
Suppose we want to initialize a 2D vector of “n” rows and “m” columns, with a value 0.

## C++

 `// CPP program``#include ``#include ``using` `namespace` `std;``int` `main()``{``    ``int` `n = 3;``    ``int` `m = 4;`` ` `    ``/*``    ``We create a 2D vector containing "n"``    ``elements each having the value "vector (m, 0)".``    ``"vector (m, 0)" means a vector having "m"``    ``elements each of value "0".``    ``Here these elements are vectors.``    ``*/``    ``vector> vec( n , vector<``int``> (m, 0)); `` ` `    ``for``(``int` `i = 0; i < n; i++)``    ``{``        ``for``(``int` `j = 0; j < m; j++)``        ``{``            ``cout << vec[i][j] << ``" "``;``        ``}``        ``cout<< endl;``    ``}``     ` `    ``return` `0;``}`

Output

```0 0 0 0
0 0 0 0
0 0 0 0 ```

Time Complexity: O(N*M)

Auxiliary Space: O(N*M)

Yet Another Approach:
Suppose we want to create a 2D vector of “n” rows and “m” columns and input values.

## C++

 `// CPP program``#include ``#include  ``using` `namespace` `std;``int` `main()``{``    ``int` `n = 4;``    ``int` `m = 5;`` ` `    ``/*``    ``Create a vector containing "n"``    ``vectors each of size "m".``    ``*/` `    ``vector> vec( n , vector<``int``> (m)); `` ` `    ``for``(``int` `i = 0; i < n; i++)``    ``{``        ``for``(``int` `j = 0; j < m; j++)``        ``{``            ``vec[i][j] = j + i + 1;``        ``}``    ``}`` ` `    ``for``(``int` `i = 0; i < n; i++)``    ``{``        ``for``(``int` `j = 0; j < m; j++)``        ``{``            ``cout << vec[i][j] << ``" "``;``        ``}``        ``cout << endl;``    ``}``     ` `   ``return` `0;``}`

Output

```1 2 3 4 5
2 3 4 5 6
3 4 5 6 7
4 5 6 7 8 ```

Time Complexity: O(N*M)

Auxiliary Space: O(N*M)

We hope you that you leave this article with a better understanding of 2D vectors and are now confident enough to apply them on your own.

This article is contributed by Amit Verma. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.