# Between Python and c++ 2d array initialization. What is this ? and why?

A robot is located at the top-left corner of a m x n grid (marked 'Start' in the diagram below).

The robot can only move either down or right at any point in time. The robot is trying to reach the bottom-right corner of the grid (marked 'Finish' in the diagram below).

How many possible unique paths are there?

I came up with the solution and did my code in C++, my native programming language.

```class Solution {
public:
int uniquePaths(int m, int n) {
vector<vector<int>> matrix(m, vector<int>(n, 0));
for (int i = 0; i < m; i++) {
matrix[i] = 1;
}
for (int j = 0; j < n; j++) {
matrix[j] = 1;
}
for(int i = 1; i < m; i++) {
for(int j = 1; j < n; j++) {
matrix[i][j] = matrix[i][j-1] + matrix[i-1][j];

}
}
return matrix[m-1][n-1];
}
};
```

The 2d vector being initialized as `arr[m][n]`

As I am learning Python I decided to solve it in Python as well. I faced an absurd problem. Here,

``````arr = [[0 for x in range(n)] for y in range(m)]
``````

I initialised the 2d array as

``````arr = [[0 for x in range(m)] for y in range(n)]
``````

and, I was getting Error-List index out of range. I really got screwed up figuring out this needle from the hay, but anyway I got it. Can you please explain why these clashes between rows and columns even though everything is the same?

``````class Solution:
def uniquePaths(self, m: int, n: int) -> int:
if m == 0 or n == 0:
return 0
arr = [[0 for x in range(n)] for y in range(m)]
for i in range(m):
arr[i] = 1
for j in range(n):
arr[j] = 1
for i in range(1,m):
for j in range(1,n):
arr[i][j] = arr[i][j-1] + arr[i-1][j]
return arr[m-1][n-1]
``````

Here is the question link for reference: Unique paths-leetcode

``````vector<vector<int>> matrix(m, vector<int>(n, 0));
``````

you're creating a vector of `m` vectors each containing `n` elements initialised to 0. That's like a 2d matrix of `m` rows x `n` columns.

The equivalent code in Python is:

``````arr = [[0 for x in range(n)] for y in range(m)]
``````

which is a list comprehension. This syntax looks back to front for people coming from a C++ background, but the format for list comprehension is:

``````[ expression for item in list if conditional ]
``````

So in your case for every index in the range `0` to `m` you create an inner list containing `n` zeroes, i.e. a `m` x `n` matrix.

This code:

``````arr = [[0 for x in range(m)] for y in range(n)]
``````

is not equivalent to your C++ code. It's producing a list of `n` lists, each containing `m` zeroes, i.e. a `n` x `m` matrix. Hence the errors you got.