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Python | Retain K consecutive elements
• Last Updated : 29 Nov, 2019

Sometimes while working with data, we can have a problem in which we need to select some of the elements that occur K times consecutively. This problem can occur in many domains. Let’s discuss certain ways in which this problem can be solved.

Method #1 : Using `groupby()` + list comprehension
This task can be performed using above functionalities. In this, we group all the numbers that are occurring K consecutively. We iterate the list using list comprehension.

 `# Python3 code to demonstrate working of``# Retain K consecutive elements``# using groupby() + list comprehension``from` `itertools ``import` `groupby`` ` `# initialize list ``test_list ``=` `[``1``, ``1``, ``4``, ``5``, ``5``, ``6``, ``7``, ``7``, ``8``]`` ` `# printing original list ``print``(``"The original list : "` `+` `str``(test_list))`` ` `# initialize K ``K ``=` `2`` ` `# Retain K consecutive elements``# using groupby() + list comprehension``res ``=` `[i ``for` `i, j ``in` `groupby(test_list) ``if` `len``(``list``(j)) ``=``=` `K]`` ` `# printing result``print``(``"The K consecutive elements are : "` `+` `str``(res))`
Output :
```The original list : [1, 1, 4, 5, 5, 6, 7, 7, 8]
The K consecutive elements are : [1, 5, 7]
```

Method #2 : Using list comprehension + `slice() + groupby()`
This task can also be performed using above functions. In this, we just perform grouping in similar way as above but the way we extract consecutive elements is by slice().

 `# Python3 code to demonstrate working of``# Retain K consecutive elements``# using groupby() + list comprehension + islice()``from` `itertools ``import` `groupby, islice`` ` `# initialize list ``test_list ``=` `[``1``, ``1``, ``4``, ``5``, ``5``, ``6``, ``7``, ``7``, ``8``]`` ` `# printing original list ``print``(``"The original list : "` `+` `str``(test_list))`` ` `# initialize K ``K ``=` `2`` ` `# Retain K consecutive elements``# using groupby() + list comprehension + islice()``res ``=` `[i ``for` `i, j ``in` `groupby(test_list) ``if` `len``(``list``(islice(j, ``0``, K))) ``=``=` `K]`` ` `# printing result``print``(``"The K consecutive elements are : "` `+` `str``(res))`
Output :
```The original list : [1, 1, 4, 5, 5, 6, 7, 7, 8]
The K consecutive elements are : [1, 5, 7]
```

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