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+--- Thread: python nested list assignment weird behavior (/thread-7584.html)
python nested list assignment weird behavior - eyalk1 - Jan-16-2018
mat was initialized like this:
mat = [[0]*26]*5
The problem is that after an inner loop iteration all the rows in mat get changed the same but i clearly stated that only the ith row should change.
the problematic lines are the ones after the # comment
please help me :(
def fill_letters_matrix(mat, word_list):
for i in range(len(mat)):
for j, item in enumerate(word_list):
if j != len(word_list)-1:
if len(item)-i-1 < 0:
continue
#the jibrish in the second brackets works,so no need to dive in...
mat[i][ord(item[len(item)-i-1])-ord("A")] += 1
else:
mat[i][ord(item[len(item) - i-1]) - ord("A")] -= 1
RE: python nested list assignment weird behavior - buran - Jan-16-2018
Please, read this https://nedbatchelder.com/text/names1.html
the way you initialize it is a problematic, because actually every row is a reference to the same 26-element list (row).
>>> mat = [[0]*6]*5 >>> print mat [[0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0]] >>> mat[1][1]=1 >>> mat [[0, 1, 0, 0, 0, 0], [0, 1, 0, 0, 0, 0], [0, 1, 0, 0, 0, 0], [0, 1, 0, 0, 0, 0], [0, 1, 0, 0, 0, 0]] >>> mat = [[0]*6 for _ in range(5)] >>> mat [[0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0]] >>> mat[1][1] = 1 >>> mat [[0, 0, 0, 0, 0, 0], [0, 1, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0]] >>>
RE: python nested list assignment weird behavior - wavic - Jan-16-2018
In [1]: mat = [[0]*26]*5 In [2]: id(mat[0]) Out[2]: 140082606853896 In [3]: id(mat[1]) Out[3]: 140082606853896As you can see the list elements are one object.
Use list comprehension instead.
mat = [0 for _ in range(26)] for range(5)]]See this video: https://www.youtube.com/watch?v=F6u5rhUQ6dU