# Python – Incremental Sublist Sum

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 of summation of subsequent size i. Let’s discuss certain ways in which this can be done.

Method #1 : Using `islice() + sum()` + 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. The sum() is used to perform sum of created list.

 `# Python3 code to demonstrate ` `# Incremental Sublist Sum ` `# using islice() + sum() + 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() + sum() + dictionary comprehension ` `# Incremental Sublist Sum ` `temp ``=` `iter``(test_list) ` `res ``=` `{key: val ``for` `key, val ``in` `((i, ``sum``(``list``(islice(temp, i)))) ``for` `i ``in` `range``(``1``, ``len``(test_list))) ``if` `val} ` ` `  `# printing result ` `print``(``"The Incremental Sublist Sum is : "` `+` `str``(res)) `

Output :

```The original list : [4, 7, 8, 10, 12, 15, 13, 17, 14, 5]
The Incremental Sublist Sum is : {1: 4, 2: 15, 3: 37, 4: 49}
```

Method #2 : Using `itemgetter() + takewhile() + islice() + sum()`
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. The sum() is used to perform sum of created list.

 `# Python3 code to demonstrate ` `# Incremental Sublist Sum ` `# using itemgetter() + takewhile() + islice() + sum() ` `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() + sum() ` `# Incremental Sublist Sum ` `temp ``=` `iter``(test_list) ` `res ``=` `{key: val ``for` `key, val ``in` `takewhile(itemgetter(``1``), ((i, ``sum``(``list``(islice(temp, i)))) ``for` `i ``in` `range``(``1``, ``len``(test_list))))} ` ` `  `# printing result ` `print``(``"The Incremental Sublist Sum is : "` `+` `str``(res)) `

Output :

```The original list : [4, 7, 8, 10, 12, 15, 13, 17, 14, 5]
The Incremental Sublist Sum is : {1: 4, 2: 15, 3: 37, 4: 49}
```

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