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How can we get the previous and next item of a list in python? My code isn't working as it was supposed to. It always returns me 0th index of list. I dont want to use loop in this method.

Here's my code:

def provide_activetab_index(self,index):
        if index > 0:
            return index -1
        if index < len(self.activetabs) - 1:
            return index + 1

When i call this method it returns 0 when the self.current is 0.

    def next(self):
        index = self.provide_activetab_index(self.current+1)
        self.display(self.frames[index])

    def previous(self):
        index = self.provide_activetab_index(self.current-1)
        self.display(self.frames[index])
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3 Answers 3

Reset to default
1 
def next(self):
    index = self.provide_activetab_index(self.current,1)
    self.display(self.frames[index])

def previous(self):
    index = self.provide_activetab_index(self.current,-1)
    self.display(self.frames[index])

def provide_activetab_index(self,index,new):
    if (index >= 0 and new == 1 and index < len(self.activetabs)-1) or (new == -1 and index > 0 and index < len(self.activetabs)):
        return index + new
    return index

EDIT

def provide_activetab_index(self,index,new):
    l = len(self.activetabs)-1
    if index >= l and new == 1:
        return 0 # Return fist element if no last element
    if index <= 0 and new == -1:
        return l # Return last element if no previous element
    else:
        return index + new

To update self.current everytime

def next(self):
    self.current = self.provide_activetab_index(self.current,1)
    self.display(self.frames[self.current])
def previous(self):
        self.current = self.provide_activetab_index(self.current,-1)
        self.display(self.frames[self.current])
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10 Comments

It stucked after the last value. Can't reach to the 0th index after the last index of list.
I have edited the answer, What you wish to do after last index
TypeError: provide_activetab_index() takes 2 positional arguments but 3 were given
Please check index = self.provide_activetab_index(self.current,1), I have passed 2 arguments
I hope my future answers from you sir. thank you again.
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1

Hi i couldn't understand what you want exactly because there are few inputs that i don't know like self.current or self.activetabs so it is hard to modify your algorithm but there is data structure called "doubly_linked_list" you can implement that easily and than go to next or prev item with itemname.next or itemname.prev.Here is code

class Node:

    def __init__(self, data, next_n=None, prev_n=None):
        self.data = data
        self.next = next_n
        self.prev = prev_n

class Doubly_linked_list:
        def __init__(self):
           self.size = 0
           self.root = None  # points last member added
           self.last = None  # points first member added first member added is accepted as last member end next is None

       def add(self, data):
            if self.size == 0:
                new_node = Node(data)
                self.root = new_node
                self.last = new_node

            else:
                new_node = Node(data)
                new_node.next = self.root
                self.root.prev = new_node
                self.root = new_node
            self.size += 1

        def remove(self, data):
            this_node = self.root
            while this_node is not None:
                if this_node.data == data:
                    if this_node.next is not None:
                        if this_node.prev is not None:
                            this_node.prev.next = this_node.next
                            this_node.next.prev = this_node.prev
                        else:
                            this_node.next.prev = None
                            self.root = this_node.next
                    else:
                        this_node.prev.next = None
                        self.last = this_node.prev
                    self.size -= 1
                    return True
                else:
                    this_node = this_node.next
            return False

        def find_all(self, data):
            this_node = self.root
            while this_node is not None:
                if this_node.data == data:
                    return data
                elif this_node.next is None:
                    return False
                else:
                    this_node = this_node.next

        def print_all(self):
            if self.root is None:
                return None
            this_node = self.root
            print(this_node, end="-->")
            while this_node.next != self.root:
                this_node = this_node.next
                print(this_node, end="-->")
            print()

if you insist on finding nodes with indexes you can create a list object inside doubly_linked_list class inside init function self.list = [] and add all the nodes also to self.list and reach them by indexes.

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1 Comment

That's so amazing you did a great job.
0

With your code

def provide_activetab_index(self,index):
    if index > 0:
        return index -1
    if index < len(self.activetabs) - 1:
        return index + 1

when you pass index greater that 0 it will always return index-1.
As you mentioned self.current is 0 and you call it with

def next(self):
    index = self.provide_activetab_index(self.current+1)
    self.display(self.frames[index])

you actually calling

index = self.provide_activetab_index(1)

so this returns you 1-1 = 0

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5 Comments

I tried to explain while it is not working. There is no fixed code in my answer
Oh, i need solution. I can't figure out sorry. :(
Hi kanwarprogrammer, SO is not a place where others will just give you working code but rather a place where you get help to solve the problem yourself.
since you dont change the value of self.current it will always be 0 and when you run your next function then it will always return 0. you need to increment the value of self.current
en.wikipedia.org/wiki/…. your explanation is appreciated but this place can also be used to get solutions of problems.

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