# Finding index in nested list

Stack Overflow Asked on December 16, 2021

I am trying to create a function that will take as input a nested list and an item, and return a list of indices.
For example list = [0, 5, [6, 8, [7, 3, 6]], 9, 10] and item = 7 should return [2, 2, 0], since list[2][2][0] = 7

my code should work since I can print the desires output, but when i run it it returns None.

def find_item(list_input, item, current):
if list_input == item:
print(current) ## this does give the desires output, but the function should return not print
return current
else:
if isinstance(list_input, list):
for j in range(len(list_input)):
current.append(j)
find_item(list_input[j], item, current)
del current[-1]


what am I overlooking here?

## 2 Answers

It always bothers me when someone sticks a for loop in the middle of a recursive function! Here's a different way to go about this problem:

def find_item(list_input, item, index=0):
if list_input:
head, *tail = list_input

if head == item:
return [index]

if isinstance(head, list):
if result := find_item(head, item):
return [index] + result

return find_item(tail, item, index + 1)

return list_input

list_input  = [0, 5, [6, 8, [7, 3, 1]], 9, 10]

print(find_item(list_input, 7))


This solution doesn't require an explicit third argument. And returns an empty list upon failure to find the item, rather than None. Note the use of the new walrus operator :=

            if result := find_item(head, item):


If that's a problem, instead do:

            result = find_item(head, item)
if result:


Answered by cdlane on December 16, 2021

As @tzaman mentioned, you need to handle the return value of find_item recursive call. If the return value of the recursive call is a list, it will mean that searched item is found and we need to stop the recursion.

The following modification will return the earliest found index of the searched item. If no item is found, it will return None.

def find_item(list_input, item, current):
if list_input == item:
return current
else:
if isinstance(list_input, list):
for j in range(len(list_input)):
current.append(j)
search_result = find_item(list_input[j], item, current)
if isinstance(search_result, list):
return search_result
del current[-1]

list_input  = [0, 5, [6, 8, [7, 3, 6]], 9, 10]
item = 7
print(find_item(list_input, item, []))

list_input  = [0, 5, [6, 8, [7, 3, 6]], 9, 10]
item = 9
print(find_item(list_input, item, []))

list_input  = [0, 5, [6, 8, [7, 3, 6]], [30, 4], 9, 10]
item = 4
print(find_item(list_input, item, []))

list_input  = [0, 5, [6, 8, [7, 3, 6]], [30, 4], 9, 10]
item = 400
print(find_item(list_input, item, []))


Output:

[2, 2, 0]
[3]
[3, 1]
None


Answered by arsho on December 16, 2021

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