May-15-2019, 01:38 PM
Suppose that two numbers are given: the number of rows of n and the number of columns m. You must create a list of size n×m, filled with, say, zeros.
The obvious solution appears to be wrong:
a = [[0] * m] * n
This can be easily seen if you set the value of a[0][0] to 5, and then print the value of a[1][0] — it will also be equal to 5. The reason is, [0] * m returns just a reference to a list of m zeros, but not a list. The subsequent repeating of this element creates a list of n items that all reference to the same list (just as well as the operation b = a for lists does not create the new list), so all rows in the resulting list are actually the same string.
The obvious solution appears to be wrong:
a = [[0] * m] * n
This can be easily seen if you set the value of a[0][0] to 5, and then print the value of a[1][0] — it will also be equal to 5. The reason is, [0] * m returns just a reference to a list of m zeros, but not a list. The subsequent repeating of this element creates a list of n items that all reference to the same list (just as well as the operation b = a for lists does not create the new list), so all rows in the resulting list are actually the same string.