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GO Classes DPP | GATE DA | Python Programming | List Multiplication & Aliasing

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1 1 vote

Consider the following Python code:

grid = [[0] * 2] * 3

grid[0][1] = 5
grid[2] = [7, 8]
grid[1][0] = 9

print(grid)

What is the output of the code above?

  1. [[0, 5], [9, 5], [7, 8]]
  2. [[9, 5], [9, 5], [7, 8]]
  3. [[9, 5], [9, 5], [9, 5]]
  4. [[0, 0], [0, 0], [7, 8]]

1 Answer

1 1 vote

The expression $\texttt{[[0] * 2] * 3}$ creates $3$ references to the same inner list.

So after $\texttt{grid[0][1] = 5}$, the shared inner list becomes:

[0, 5]

At this point, all $3$ rows appear as $\texttt{[0, 5]}$ because they refer to the same list.

Then $\texttt{grid[2] = [7, 8]}$ replaces only the third row with a new separate list.

Now $\texttt{grid[0]}$ and $\texttt{grid[1]}$ still share the same list, but $\texttt{grid[2]}$ is separate.

So $\texttt{grid[1][0] = 9}$ changes both the first and second rows.

Therefore, the output is:

[[9, 5], [9, 5], [7, 8]]

Correct Option: B

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