# Python | Subgroups of i’th index size in list

• Last Updated : 18 Mar, 2019

Sometimes we need to group elements and grouping techniques and requirements vary accordingly. One such way to group the elements is by the i’th size in list which stores the dictionary of index keys with values as list elements of subsequent size i.

Input : [4, 7, 8, 10, 12, 15, 13, 17, 14, 5]
Output: {1: [4], 2: [7, 8], 3: [10, 12, 15], 4: [13, 17, 14, 5]}

Let’s discuss certain ways in which this can be done.

Method #1 : Using `islice()` + dictionary comprehension
The slice method can be used to group the chunks of list required to be made as values of the dictionaries which are then assigned to their destined index key using the dictionary comprehension.

 `# Python3 code to demonstrate``# Subgrouping of i'th index size in list``# using islice() + dictionary comprehension``from` `itertools ``import` `islice`` ` `# initializing list``test_list ``=` `[``4``, ``7``, ``8``, ``10``, ``12``, ``15``, ``13``, ``17``, ``14``, ``5``]`` ` `# printing original list ``print``(``"The original list : "` `+` `str``(test_list))`` ` `# using islice() + dictionary comprehension``# Subgrouping of i'th index size in list``temp ``=` `iter``(test_list)``res ``=` `{key: val ``for` `key, val ``in` `((i, ``list``(islice(temp, i)))``                ``for` `i ``in` `range``(``1``, ``len``(test_list))) ``if` `val}`` ` `# printing result``print``(``"The grouped dictionary is : "` `+` `str``(res))`
Output :

The original list : [4, 7, 8, 10, 12, 15, 13, 17, 14, 5]
The grouped dictionary is : {1: [4], 2: [7, 8], 3: [10, 12, 15], 4: [13, 17, 14, 5]}

Method #2 : Using `itemgetter() + takewhile() + islice()`
In order to increase the computation speed, we introduce new functions to perform this particular task, takewhile and itemgetter functions which performs the task of grouping the sliced values.

 `# Python3 code to demonstrate``# Subgrouping of i'th index size in list``# using itemgetter() + takewhile() + islice()``from` `itertools ``import` `islice, takewhile``from` `operator ``import` `itemgetter`` ` `# initializing list``test_list ``=` `[``4``, ``7``, ``8``, ``10``, ``12``, ``15``, ``13``, ``17``, ``14``, ``5``]`` ` `# printing original list ``print``(``"The original list : "` `+` `str``(test_list))`` ` `# using itemgetter() + takewhile() + islice()``# Subgrouping of i'th index size in list``temp ``=` `iter``(test_list)``res ``=`  `{key: val ``for` `key, val ``in` `        ``takewhile(itemgetter(``1``), ((i, ``list``(islice(temp, i)))``        ``for` `i ``in` `range``(``1``, ``len``(test_list))))}`` ` `# printing result``print``(``"The grouped dictionary is : "` `+` `str``(res))`
Output :

The original list : [4, 7, 8, 10, 12, 15, 13, 17, 14, 5]
The grouped dictionary is : {1: [4], 2: [7, 8], 3: [10, 12, 15], 4: [13, 17, 14, 5]}

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