+ 3

Why the List is [[7],[7],[7]],not[[7],[0],[0]]?

this is a code questions which I originally meet when I challenge other in sololearn(I add some code wanting to figure out the list)I wonder that why the List is [[7],[7],[7]],not[[7],[0],[0]]?thanks~ arr=[[]]*3 print(arr)#output is [[],[],[]] arr[0].append(7) print (arr)#output is [[7],[7],[7]] #why not [[7]],[],[]] try: print(arr[1][0]) except IndexError: print("0")

20th Feb 2018, 11:46 AM
呂羿賢
呂羿賢 - avatar
2 odpowiedzi
+ 2
When you create a two dimensional array in python like the following arr=[[]]*3 All the three rows refer to single row in memory Row 0-->[ X, X, X] Row 1 --| Row 2--| So if we assign a value to any row, it will be affected in all other rows. To avoid this , you can create a matrix like the following. columns = 3 rows = 3 mat = [ [0 for i in range(columns)] for j in range(rows)] mat[0][0] = 1 # Will not affect other rows print(mat([1][0]) # Prints Zero
20th Feb 2018, 2:46 PM
Ravi Chandra Enaganti
Ravi Chandra Enaganti - avatar
0
thank you very much!!!
21st Feb 2018, 12:00 AM
呂羿賢
呂羿賢 - avatar