What is a vector <vector<int>>

[EwCjE3v7]

What is a

  vector <vector<int>> ivv;

I know what a vector <int> iv; but what is vector <vector<int>>

Thanks

[N36fSL3A]

It's a 2D vector. Just like a 2D array.

[Catfish666]

It's a 2D vector. Just like a 2D array.

Just a small observation: a plain 2D array looks like a matrix, because all rows have equal lengths.

However with a vector of vectors, you need to make sure personally that this is so:

int a[3][4];

[[xxxx]
 [xxxx]
 [xxxx]]

vector<vector<int> > vvi;

[
]

vector<vector<int> > vvi(3);

[[]
 []
 []]

vector<vector<int> > vvi(3, vector<int>(4));

[[xxxx]
 [xxxx]
 [xxxx]]

[EwCjE3v7]

Sorry for my late reply. I still don`t understand as i donno 2d arrays or even arrays :-/

[N36fSL3A]

They're not really required to be learned. They are just arrays of arrays.

http://www.cplusplus.com/doc/tutorial/arrays/

[kevinkjt2000]

A vector of vectors also can have rows of unequal sizes.

[Stormboy]

You need to learn arrays first and then multi-dimensional arrays. An array is just like a collection of stuff of the same data type.

Here is a crash-course for you:

You declare an array like this:
data_type arrayname[ARRAY_SIZE];

An example:
int numbers[5]; //This creates an array of type int to store 5 integers

Indexing of the array starts at 0 and ends at ARRAY_SIZE-1.

If I have an array of size 5, the first element would be at index 0, the second would be at index 1 and the last element at index 4 (5-1).

You access an element in the array with array_name[index];.

An array's size cannot be changed once declared.

A 2D array is basically an array of arrays. You declare it like this type array_name[siz_x][siz_y];;

It looks like this:

int myarray[3][3];

/*
If you fill the above with zeroes it'd look like:
[[0,0,0],
[0,0,0],
[0,0,0]]
*/

An example program using array:

#include <iostream>

int main()
{
    int userInputs[5];
    
    int sum = 0;
    
    std::cout << "Enter a number: ";
    
    std::cin >>  userInputs[0];
    
    for(int i = 1; i<5; i++)
    {
        std::cout << "Enter another number: ";
        std::cin >> userInputs[i];
    }
    
    for(int j = 0; j<5; j++)
    {
        sum += userInputs[j];
    }
    
    std::cout << "The sum of those numbers are: " << sum << std::endl;
    
    return 0;
}

[EwCjE3v7]

Thanks guys, I'll learn arrays once they come in my book even tho I think I know most because of Stormboy. Thank you all great help