CODEWITHSUNDEEP
pythonlist
rish0912
rish0912

Reputation: 1

Python : Deep Reverse List

I have a nested list, and I need to reverse every element in the list.

Below is the example :

L = [[0, 1, 2], [1, 2, 3]]

Expected Output :

L = [[3, 2, 1], [2, 1, 0]]

I tried with the below piece of code, it works individually but when I am putting this code within function, then the list is not getting updated.

L = [list(reversed(row)) for row in L]

L.reverse()

This code works, and the List "L" is getting updated. But when I put this code in function

def deep_rev(L):

       L = [list(reversed(row)) for row in L]
       L.reverse()

L = [[0, 1, 2], [1, 2, 3]]

deep_rev(L)

print(L)

This is returning the same list = [[0,1,2],[1,2,3]]

Can anyone please help me in this regard, why in function this is not working?

Upvotes: 0

Views: 984

Answers (6)

Rodrigo Santamaría
Rodrigo Santamaría

Reputation: 11

This is the solution I find out using recursion:

l=[[0, 1, 2], [1, 2, 3]]
def treereverse(l):
    l.reverse()
    for x in l:
        if isinstance(x,list):
            treereverse(x)
    return l
print(treereverse(l))

Upvotes: 1

Yossarian42
Yossarian42

Reputation: 2050

deep_rev(L) modifies one local list, its scope is inside this function. Either make one deep copy of the list or return the reversed list.

Upvotes: 0

PythonProgrammi
PythonProgrammi

Reputation: 23443

def deep_rev(l):

    l = [list(reversed(row)) for row in l]
    l.reverse()
    return l

l = [[0, 1, 2], [1, 2, 3]]

l = deep_rev(l)

print(l)

output

[[3, 2, 1], [2, 1, 0]]

Upvotes: 0

Blckknght
Blckknght

Reputation: 104702

Your current code creates a new list, rather than modifying the exiting list in place. You can make it work, just get rid of the list comprehension and do in-place reversals for the inner lists too:

def deep_rev(L):
   for inner in L:
       inner.reverse()
   L.reverse()

If you want to support more than two levels of nesting, you could recursively call deep_rev on the inner lists, rather than just reversing them as I did above. You'd need to check if the values were lists or not, so you'd have a base case.

Upvotes: 1

tobias_k
tobias_k

Reputation: 82889

With L = ... you are assigning a new value to the parameter L within the function, without modifying the original list. Instead, you can use L[:] = ... to replace all the elements in the original list with the values from the new list.

def deep_rev(L):
    L[:] = [list(reversed(row)) for row in reversed(L)]

L = [[0, 1, 2], [1, 2, 3]]
deep_rev(L)
print(L) # [[3, 2, 1], [2, 1, 0]]

Upvotes: 0

Rohit-Pandey
Rohit-Pandey

Reputation: 2159

This Code works for your problem,

L = [[0, 1, 2], [1, 2, 3]]
b=list(reversed(L))
q=[]
for i in b:
    q.append(list(reversed(i)))
print q

Output:-

 [[3, 2, 1], [2, 1, 0]]

Upvotes: 0

Related Questions