I was trying to code Bubble Sort using recursion, when I ran into this problem.
def bubble_sort_recur(a):
print(a)
if len(a)==1:
return
for i in range(len(a)-1):
if a[i] > a[i+1] :
a[i]=a[i]^a[i+1] # swap using XOR
a[i+1]=a[i]^a[i+1]
a[i]=a[i]^a[i+1]
print(a) # shows the changes made in the current call
bubble_sort_recur(a[:-1])
b=[1,6,1,66,3,32,21,33,1] # initial list
bubble_sort_recur(b) # function call
I think that python uses pass by Object reference for objects(lists). The output of the recursion also was expected, and as follows:
[1, 6, 1, 66, 3, 32, 21, 33, 1]
[1, 1, 6, 3, 32, 21, 33, 1, 66] # 66 bubbles up
[1, 1, 6, 3, 32, 21, 33, 1]
[1, 1, 3, 6, 21, 32, 1, 33]
[1, 1, 3, 6, 21, 32, 1]
[1, 1, 3, 6, 21, 1, 32]
[1, 1, 3, 6, 21, 1]
[1, 1, 3, 6, 1, 21]
[1, 1, 3, 6, 1]
[1, 1, 3, 1, 6]
[1, 1, 3, 1]
[1, 1, 1, 3]
[1, 1, 1]
[1, 1, 1]
[1, 1]
[1, 1]
[1]
One can see that the third '1' bubbles down to the 2nd index as expected.
But when I do print(b) I get [1, 1, 6, 3, 32, 21, 33, 1, 66] which is neither the sorted list nor the original list(b=[1,6,1,66,3,32,21,33,1]).
Could you please explain the reason for getting such a mixed behaviour?
Something I tried to explore the working is as follows:
def func(a):
a[0]='only one altered'
def func2(b):
a=['this','is', 'completely', 'new']
a=[1,'initial',2, 'array']
func(a) # this changes the list
print(a)
func2(a) # this does not change the list 'a' , in any way.
print(a)
>> ['only one altered', 'initial', 2, 'array']
['only one altered', 'initial', 2, 'array']
I notice that , the actual list gets altered when I change the value of the list inside of the function by using the index. Otherwise the list remains unaffected. Any help appreciated.
Your function does not return a value and relies on the list being mutable for it to be changed in place. However, you then call recursion with a slice of the the current list, which makes a copy (and therefore a new list), so the list b is no longer being referenced.
You can verify this by adding a print(b) line before recursion.
def bubble_sort_recur(a):
print(a)
if len(a)==1:
return
for i in range(len(a)-1):
if a[i] > a[i+1] :
a[i]=a[i]^a[i+1] # swap using XOR
a[i+1]=a[i]^a[i+1]
a[i]=a[i]^a[i+1]
print(a) # shows the changes made in the current call
print(b)
print()
bubble_sort_recur(a[:-1])
b=[1,6,1,66,3,32,21,33,1] # initial list
bubble_sort_recur(b) # function call
Output
[1, 6, 1, 66, 3, 32, 21, 33, 1]
[1, 1, 6, 3, 32, 21, 33, 1, 66]
[1, 1, 6, 3, 32, 21, 33, 1, 66] # first call changed b
[1, 1, 6, 3, 32, 21, 33, 1]
[1, 1, 3, 6, 21, 32, 1, 33]
[1, 1, 6, 3, 32, 21, 33, 1, 66] # none of the recursions changed b
[1, 1, 3, 6, 21, 32, 1]
[1, 1, 3, 6, 21, 1, 32]
[1, 1, 6, 3, 32, 21, 33, 1, 66]
[1, 1, 3, 6, 21, 1]
[1, 1, 3, 6, 1, 21]
[1, 1, 6, 3, 32, 21, 33, 1, 66]
[1, 1, 3, 6, 1]
[1, 1, 3, 1, 6]
[1, 1, 6, 3, 32, 21, 33, 1, 66]
[1, 1, 3, 1]
[1, 1, 1, 3]
[1, 1, 6, 3, 32, 21, 33, 1, 66]
[1, 1, 1]
[1, 1, 1]
[1, 1, 6, 3, 32, 21, 33, 1, 66]
[1, 1]
[1, 1]
[1, 1, 6, 3, 32, 21, 33, 1, 66]
[1]
One solution would be to include a return call.
def bubble_sort_recur(a):
if len(a) > 1:
for i in range(len(a)-1):
if a[i] > a[i+1] :
a[i], a[i+1] = a[i+1], a[i] # more pythonic syntax
# or
# a[i:i+2] = a[i+1:i-1:-1]
# or your original code
# a[i]=a[i]^a[i+1] # swap using XOR
# a[i+1]=a[i]^a[i+1]
# a[i]=a[i]^a[i+1]
print(a)
a[:-1] = bubble_sort_recur(a[:-1]) # catch the result of recursion
return a
b = [1,6,1,66,3,32,21,33,1] # initial list
b = bubble_sort_recur(b) # function call
[1, 1, 6, 3, 32, 21, 33, 1, 66]
[1, 1, 3, 6, 21, 32, 1, 33]
[1, 1, 3, 6, 21, 1, 32]
[1, 1, 3, 6, 1, 21]
[1, 1, 3, 1, 6]
[1, 1, 1, 3]
[1, 1, 1]
[1, 1]
>>> print(b)
[1, 1, 1, 3, 6, 21, 32, 33, 66]