Why is the output different for following part of code?

Question

The output for G and G1 list is different.

numCourses = 2
prerequisites = [[1,0]]

G = [[] for i in range(numCourses)]

G1 = [[]]*numCourses

for i,j in prerequisites:
    G[j].append(i)

for i,j in prerequisites:
    G1[j].append(i)

I get following output G = [[1],[]] and G1 = [[1],[1]]

Please also read: stackoverflow.com/help/minimal-reproducible-example — DSC
– DSC, Commented Jun 9, 2019 at 16:05

blue note · Accepted Answer · 2019-06-09 16:03:31Z

1

It's a classic python gotcha

G = [[] for i in range(numCourses)]

creates numCourses new, empty list, while

G1 = [[]]*numCourses

first creates the (inner) empty list, and then a list consisting of numCourses copies of it. That's why when you change one list in G1 all of them change.

answered Jun 9, 2019 at 16:03

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