# Python | Custom List slicing Sum

The problem of slicing a list has been dealt earlier, but sometimes we need to perform the slicing in variable lengths and its summation according to the input given in other list. This problem has its potential application in web development. Let’s discuss certain ways in which this can be done.

Method #1 : Using `itertools.islice() + sum()` + list comprehension
The list comprehension can be used to iterate through the list and the component issue is solved using the islice function. Summation is performed by sum().

 `# Python3 code to demonstrate ` `# Custom List slicing Sum ` `# using itertools.islice() + sum() + list comprehension ` `from` `itertools ``import` `islice ` ` `  `# initializing test list ` `test_list ``=` `[``1``, ``5``, ``3``, ``7``, ``8``, ``10``, ``11``, ``16``, ``9``, ``12``] ` ` `  `# initializing slice list  ` `slice_list ``=` `[``2``, ``1``, ``3``, ``4``] ` ` `  `# printing original list  ` `print``(``"The original list : "` `+` `str``(test_list)) ` ` `  `# printing slice list  ` `print``(``"The slice list : "` `+` `str``(slice_list)) ` ` `  `# using itertools.islice() + sum() + list comprehension ` `# Custom List slicing Sum ` `temp ``=` `iter``(test_list) ` `res ``=` `[``sum``(``list``(islice(temp, part))) ``for` `part ``in` `slice_list] ` ` `  `# print result ` `print``(``"The variable sliced sum list is : "` `+` `str``(res)) `

Output :

```The original list : [1, 5, 3, 7, 8, 10, 11, 16, 9, 12]
The slice list : [2, 1, 3, 4]
The variable sliced sum list is : [6, 3, 25, 48]
```

Method #2 : Using `zip() + accumulate() + sum()` + list slicing
Apart from using the list comprehension to perform the task of binding, this method uses zip function to hold sublist element together, accumulate function joins the elements, and slicing is used to construct the required slicing. Summation is performed by sum().

 `# Python3 code to demonstrate ` `# Custom List slicing Sum ` `# using zip() + accumulate() + sum() + list slicing ` `from` `itertools ``import` `accumulate ` ` `  `# initializing test list ` `test_list ``=` `[``1``, ``5``, ``3``, ``7``, ``8``, ``10``, ``11``, ``16``, ``9``, ``12``] ` ` `  `# initializing slice list  ` `slice_list ``=` `[``2``, ``1``, ``3``, ``4``] ` ` `  `# printing original list  ` `print``(``"The original list : "` `+` `str``(test_list)) ` ` `  `# printing slice list  ` `print``(``"The slice list : "` `+` `str``(slice_list)) ` ` `  `# using zip() + accumulate() + sum() + list slicing ` `# Custom List slicing Sum ` `res ``=` `[``sum``(test_list[i ``-` `j: i]) ``for` `i, j ``in` `zip``(accumulate(slice_list), slice_list)] ` ` `  `# print result ` `print``(``"The variable sliced sum list is : "` `+` `str``(res)) `

Output :

```The original list : [1, 5, 3, 7, 8, 10, 11, 16, 9, 12]
The slice list : [2, 1, 3, 4]
The variable sliced sum list is : [6, 3, 25, 48]
```

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