Question

Very simple code nested example:

All the code does is create a list of lists that is initialized to zero.
It iterates through the list rows and columns and each position is given a value.
For some reason the last row of the 2D list is duplicated for each row when the final vector is printed.

Number_of_channels=2
Coefficients_per_channel=3

coefficient_array=[[0]*Coefficients_per_channel]*Number_of_channels 
print coefficient_array

for channel in range(Number_of_channels):
    for coeff in range(Coefficients_per_channel):
        coefficient_array[channel][coeff]=coeff*channel
        print coefficient_array[channel][coeff]

print coefficient_array

Output:

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

I actually expect:

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

Anyone have any idea how come this is happening?

Answer 1

You only duplicate the outer list, but the values of that list are left untouched. Thus, all (both) outer lists contain references to the same inner, mutable list.

>>> example = [[1, 2, 3]]
>>> example *= 2
>>> example
[[1, 2, 3], [1, 2, 3]]
>>> example[0][0] = 5
[[5, 2, 3], [5, 2, 3]]
>>> example[0] is example[1]
True

Better create the inner lists in a loop:

coefficient_array=[[0]*Coefficients_per_channel for i in xrange(Number_of_channels)]

or, illustrated with the python prompt again:

>>> example = [[i, i, i] for i in xrange(2)]
>>> example
[[0, 0, 0], [1, 1, 1]]
>>> example[0][0] = 5
>>> example
[[5, 0, 0], [1, 1, 1]]
>>> example[0] is example[1]
False