Unexpected behavior in nested for loop in python

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

• stanri: Thanks for this, I understand now. Holy crap, python is powerful.

Answer 2

With

coefficient_array=[[0]*Coefficients_per_channel]*Number_of_channels

you do a duplication of references to the same object:

coefficient_array[0] is coefficient_array[1]

evaluates to True.

Instead, build your array with

[[coeff*channel for coeff in range(Coefficients_per_channel)] for channel in range(Number_of_channels)]

Answer 3

Try this instead:

coefficient_array=[0]*Number_of_channels 
print coefficient_array
for channel in range(Number_of_channels):
 coefficient_array[channel] = [0] * Coefficients_per_channel
 for coeff in range(Coefficients_per_channel):
 coefficient_array[channel][coeff]=coeff*channel
 print (channel, coeff)
 print coefficient_array[channel][coeff]
print coefficient_array

• Bittrance: How about replacing the inner loop with coefficient_array[channel] = range(0, Coefficients_per_channel * channel, channel)?