Why the 'list' is resulting empty after getting appended with a deleted list?-CodePudding

Here is my code, Could anyone please help me why the 'long_band' resulted in empty list?

Expected output: [[9,10],[0,1,2,3,4,5,6,7],[18],[12]]

Code begins here:

arr = [1, 9, 3, 0, 18, 5, 2, 4, 10, 7, 12, 6]
len1 = len(arr)
long_band = list()
chain = list()

def band(i):
    chain.append(i)
    j = i 1
    if j in arr:
        band(j)
    else:
        #print(chain)
        #print(long_band)
        long_band.append([chain])
        #print(long_band)
        del chain[:]


def isHead(n):
    x = n-1
    if x in arr:
        None
    else:
        band(n)


for i in range(len1):
    isHead(arr[i])


print(long_band)

CodePudding user response：

arr = [1, 9, 3, 0, 18, 5, 2, 4, 10, 7, 12, 6]
len1 = len(arr)
long_band = list()
chain = list()

def band(i):
    chain.append(i)
    j = i 1
    if j in arr:
        band(j)
    else:
        long_band.append(chain[:])
        del chain[:]


def isHead(n):
    x = n-1
    if x in arr:
        None
    else:
        band(n)


def FindMaxLen(long_band):
    maxLength = max(len(x) for x in long_band)
    print(maxLength)


for i in range(len1):
    isHead(arr[i])


print(long_band)
FindMaxLen(long_band)

CodePudding user response：

The immediate problem is this:

long_band.append([chain])
del chain[:]

You're appending the chain object to the list, and then immediately clear it. This also clears the object in the list, because there's only one object. Appending it to a list doesn't create a copy of it.

The bigger problem is that your algorithm is way overcomplicated. Here's a much simpler approach:

>>> arr = [1, 9, 3, 0, 18, 5, 2, 4, 10, 7, 12, 6]
>>> arr.sort()
>>> res = [arr[0:1]]
>>> for i, j in zip(arr, arr[1:]):
...   if i != j - 1:
...     res.append([])
...   res[-1].append(j)
... 
>>> res
[[0, 1, 2, 3, 4, 5, 6, 7], [9, 10], [12], [18]]