How to remove items from a list while comparing it to the previous element?

Question

I am trying to remove items from a list using a for loop in Python, if they are equal to the previous element in the list.

When I try to use del, I get a list index out of range error, which through research I have learned that it is because using del alters the length of my list.

Other advice has said to use list comprehension instead, but I'm unsure how to compare the previous element to the current element, while doing so.

This is my current code:

for x in range(1, len(items)):
    if items[x] == items[x-1]:
        del items[x]
        del items[x - 1]
return result

Answer 1

Try this:

import pandas as pd


items = [1, 4, 4, 5, 5, 5, 6, 4, 5, 7]
remove_us = []

for x in range(1, len(items)):
    if items[x] == items[x - 1]:
        # save the index
        remove_us.append(x)

# create a Series out of your items
items_temp = pd.Series(items)
items_filtered = items_temp.drop(axis=1, index=remove_us).values
# With .values you get a numpy.ndarray
# If, for some reason, you need a list you can do:
# items_filtered = list(items_filtered)

result:

array([1, 4, 5, 6, 4, 5, 7])

---

The following is an approach, in case you want to remove dublicates in general:

import pandas as pd


items_temp = pd.Series(items)
items_filtered = items_temp[~items_temp.duplicated(keep=False).values

Answer 2

I tried to address your problem changing the mechanics only where necessary:

def my_func(list):

    # copy input list for data-safety reasons
    result = list.copy()

    # empty list and 1-element list require special treatment
    if len(list)==0 or len(list)==1:
        return result

    # init current_index
    current_index = 0

    while(current_index < len(result)-1):

        # validation-check for testing purposes, comment or delete for final version
        print("at current_index: {}".format(current_index))
        print("analyzed list is: {}".format(result))
        print("analyzed list is of length: {}".format(len(result)))
        print("comparing: {} with {}".format(result[current_index], result[current_index+1]))

        if result[current_index] == result[current_index+1]:
            result.pop(current_index)
            # don't augment current_index
            current_index += 0

        else:
            # augment current_index
            current_index += 1

        print()

    return result

Tests:

>>> my_func([])
[]

my_func(['a'])
['a']

>>> my_func(['a','a'])
at current_index: 0
analyzed list is: ['a', 'a']
analyzed list is of length: 2
comparing: a with a

['a']

>>> my_func(['a','a','a'])
at current_index: 0
analyzed list is: ['a', 'a', 'a']
analyzed list is of length: 3
comparing: a with a

at current_index: 0
analyzed list is: ['a', 'a']
analyzed list is of length: 2
comparing: a with a

['a']

>>> my_func(['a','b','b','c','b','c','c'])
at current_index: 0
analyzed list is: ['a', 'b', 'b', 'c', 'b', 'c', 'c']
analyzed list is of length: 7
comparing: a with b

at current_index: 1
analyzed list is: ['a', 'b', 'b', 'c', 'b', 'c', 'c']
analyzed list is of length: 7
comparing: b with b

at current_index: 1
analyzed list is: ['a', 'b', 'c', 'b', 'c', 'c']
analyzed list is of length: 6
comparing: b with c

at current_index: 2
analyzed list is: ['a', 'b', 'c', 'b', 'c', 'c']
analyzed list is of length: 6
comparing: c with b

at current_index: 3
analyzed list is: ['a', 'b', 'c', 'b', 'c', 'c']
analyzed list is of length: 6
comparing: b with c

at current_index: 4
analyzed list is: ['a', 'b', 'c', 'b', 'c', 'c']
analyzed list is of length: 6
comparing: c with c

['a', 'b', 'c', 'b', 'c']