Question

I am a beginner in Python. The dynamic programming solution to the coin change problem requires an initialized table that basically could, for example, look like this:

[
[1,0,0,0],
[1,0,0,0],
[1,0,0,0]
]

Which table I specified this way:

table=[[1]+[0]*3]*3

Now, if I write

table[1][2]=1
for x in table: print x

I get a print of:

[1, 0, 2, 0]
[1, 0, 2, 0]
[1, 0, 2, 0]

And I have no damn clue why the whole column gets changed instead of the specified number.

Answer 1

The error is in the code you didn’t show. Presumably, you initialised your list like

a = [1, 0, 0, 0]
table = [a, a, a]

or

table = [[1, 0, 0, 0]] * 3

or something equivalent, which leads to a list containing three times the same list object. If you modify this single object, it changes everywhere it is referenced, because it is only a single object.

If you need further details, show us the code creating the table.

A correct way of initializing table is

table = [[1, 0, 0, 0] for i in range(3)]

or simply

table = [[1, 0, 0, 0], [1, 0, 0, 0], [1, 0, 0, 0]]