Vectors in cpp

How do create a two dimensional vector ?? Let us find out today with this code nugget,

#include <iostream>
#include <vector>

#define rowCount 3
#define columnCount 4

int main()
{
  std::vector<int> singleRow(columnCount, 5); 
  std::vector<std::vector<int>> matrix(rowCount, singleRow); 

  for(auto i : matrix) 
    for(auto i : singleRow)
        std::cout<<i<<" ";
  return 0;
}

Taking baby steps, With reference to above program, a vector called singleRow is initialized with 4 elements each holding the value 5, so far so good,

std::vector<int> singleRow(columnCount, 5);

Digging deep into above line of code, this constructor from vector takes two parameters,

Parameter one takes the no of elements in vector
Parameter two takes the value to be initialized

Going by this description, this is the result of above piece of code

Now coming to next step,

Q :What is a 2D vector ?
A : It is a vector of vector

Not so helpful right ? Imagine a 2D vector as a table, how to store a table in memory ?
This is how we do it,

std::vector<std::vector<int>> matrix(rowCount, singleRow);

With the definition of vector initialization we learnt, dissect above piece of code,

Parameter one provides no of elements for the vector named matrix,
Parameter two provides, the value to initialize the vector with, in this case, you guessed it right, another vector named singleRow. Look closely at the type of matrix, it is a vector which holds a vector
```
std::vector<std::vector<int>> matrix;
```
End result will look like this,

And before we wind up, remember, in memory vector content will be stored sequentially just like a series of boxes, the table structure is just a visual model for us to relate.

You are not believin me ?? Observe the output this piece of code produces,

  for(auto i : matrix) 
    for(auto i : singleRow)
        std::cout<<i<<" ";

A beautiful, single file of integers, That's all for this nugget, have a great day ☕

2D Vectors in cpp visually explained

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