# Python | Altering duplicate values from given list

• Last Updated : 29 Apr, 2019

Many times we deal with the lists having identical numbers as a sequence and we wish to just keep the 1st occurrence of element and substituting all the occurrences with the different number. Let’s discuss certain ways in which this can be done.

Method #1 : Using list comprehension + `enumerate()`

This task can be achieved using the list comprehension for traversal and checking for element occurrence and index checking can be done using enumerate function.

 `# Python3 code to demonstrate``# Altering duplicated``# using list comprehension + enumerate()`` ` `# initializing list``test_list ``=` `[``2``, ``2``, ``3``, ``3``, ``3``, ``3``, ``4``, ``4``, ``5``, ``5``, ``5``]`` ` `# printing original list``print``(``"The original list : "` `+` `str``(test_list))`` ` `# using list comprehension + enumerate()``# Altering duplicated``res ``=` `[``False` `if` `(ele ``in` `test_list[ :idx]) ``else` `ele ``             ``for` `idx, ele ``in` `enumerate``(test_list)]`` ` `# print result``print``(``"The altered duplicate list is : "` `+` `str``(res))`

Output :

The original list : [2, 2, 3, 3, 3, 3, 4, 4, 5, 5, 5]
The altered duplicate list is : [2, False, 3, False, False, False, 4, False, 5, False, False]

Method #2 : Using `itertools.groupby()` + list comprehension

This particular task can also be performed using a combination of above function, using `groupby` function to get the groups of different elements and list comprehension to alter duplicates.

 `# Python3 code to demonstrate``# Altering duplicated``# using itertools.groupby() + list comprehension``from` `itertools ``import` `groupby`` ` `# initializing list``test_list ``=` `[``2``, ``2``, ``3``, ``3``, ``3``, ``3``, ``4``, ``4``, ``5``, ``5``, ``5``]`` ` `# printing original list``print``(``"The original list : "` `+` `str``(test_list))`` ` `# using itertools.groupby() + list comprehension``# Altering duplicated``res ``=` `[val ``for` `key, grp ``in` `groupby(test_list) ``       ``for` `val ``in` `[key] ``+` `[``False``] ``*` `(``len``(``list``(grp))``-``1``)]`` ` `# print result``print``(``"The altered duplicate list is : "` `+` `str``(res))`

Output :

The original list : [2, 2, 3, 3, 3, 3, 4, 4, 5, 5, 5]
The altered duplicate list is : [2, False, 3, False, False, False, 4, False, 5, False, False]

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