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+--- Thread: I am getting the wrong answer, and not sure why (/thread-30781.html)
I am getting the wrong answer, and not sure why - riskeay - Nov-05-2020
This is my code:
def triangles():
N = [1]
while True:
yield N
N.insert(0,0)
N.append(0)
N = [N[i]+N[i+1] for i in range(len(N)-1)]
n = 0
results = []
for t in triangles():
results.append(t)
n = n + 1
if n == 10:
break
for t in results:
print(t)The answer I want to get:[1],
[1, 1],
[1, 2, 1],
[1, 3, 3, 1],
[1, 4, 6, 4, 1],
[1, 5, 10, 10, 5, 1],
[1, 6, 15, 20, 15, 6, 1],
[1, 7, 21, 35, 35, 21, 7, 1],
[1, 8, 28, 56, 70, 56, 28, 8, 1],
[1, 9, 36, 84, 126, 126, 84, 36, 9, 1]
The output of that code:
[0, 1, 0]
[0, 1, 1, 0]
[0, 1, 2, 1, 0]
[0, 1, 3, 3, 1, 0]
[0, 1, 4, 6, 4, 1, 0]
[0, 1, 5, 10, 10, 5, 1, 0]
[0, 1, 6, 15, 20, 15, 6, 1, 0]
[0, 1, 7, 21, 35, 35, 21, 7, 1, 0]
[0, 1, 8, 28, 56, 70, 56, 28, 8, 1, 0]
[1, 9, 36, 84, 126, 126, 84, 36, 9, 1]
I try to debug that code,The value of 'results' changes from [[1]] into [[0,1]] when the N.insert(0,0) is ran, why?
[Image: 2c45be315c6034a8bbdb67c1dc13495409237612.jpg]
RE: I am getting the wrong answer, and not sure why - riskeay - Nov-05-2020
The next step
[Image: bf35b119ebc4b74508736756d8fc1e178b82156f.jpg]
RE: I am getting the wrong answer, and not sure why - bowlofred - Nov-05-2020
N is a list and is therefore mutable.
In triangles() you hand back an object in line 4 (yield N), but then in line 5 you modify that object. If the caller didn't make a copy yet, their copy of the object is changed as well.
A simple change here would be to return a copy of the list so that the caller's object can't be modified later by the function.
yield list(N)
RE: I am getting the wrong answer, and not sure why - deanhystad - Nov-05-2020
This is a fun problem.
Triangles has a list N which it yields. A new N is generated each time triangles is called. HOWEVER, before the new N is generated you insert and append 0 to the old N.
This code prints the desired output because it prints the result of triangles() before they get "corrupted".
def triangles():
N = [1]
while True:
yield N
N.insert(0,0)
N.append(0)
N = [N[i]+N[i+1] for i in range(len(N)-1)]
for i, t in enumerate(triangles()):
print(t)
if i >= 5:
breakAn easy fix to your problem is to create to create a new N at the start of each iteration so you don't mess up the previous.def triangles():
N = [1]
while True:
yield N
N = (0, *N, 0)
N = [N[i]+N[i+1] for i in range(len(N)-1)]
# Or this one that uses zip and temporary lists.
def triangles():
N = [1]
while True:
yield N
N = [p + n for p, n in zip([0]+N, N+[0])]