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python-3.2

Python 2D Array Issue

I am trying to set a specific value of a 2D array to True. Here is the snippet of code:

b [ [False] * 3] * 3
b[2][1] = True

Unfortunately, this is setting the entire row to True (so b[0][1] would then be changed to True). Any ideas on what is going on?

EDIT:

Just tried this code and it worked:

 b = []
 for i in range(3):
     b.append([False, False, False])
 b[1][2] = True

Why would it work in that case and not the former?

Solution

  • [[False]*3]*3 creates three references to the same [False,False,False] object, thus when you change one item, your other rows/cols change as well.

    Your second version avoids this by createing a new [False, False, False] object for every row/col.