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# Python | Selective indices Summation

• Last Updated : 17 Feb, 2020

Accessing an element from its index is easier task in python, just using the [] operator in a list does the trick. But in certain situations we are presented with tasks when we have more than once indices and we need to get all the elements corresponding to those indices and then perform the summation. Lets discuss certain ways to achieve this task.

Method #1 : Using List comprehension + `sum()`
This task is easy to perform with a loop, and hence shorthand for it is the first method to start with this task. Iterating over the index list to get the corresponding elements from list into new list is brute method to perform this task. The task of summation is performed using sum().

 `# Python3 code to demonstrate ``# Selective indices Summation``# using list comprehension + sum()`` ` `# initializing lists ``test_list ``=` `[``9``, ``4``, ``5``, ``8``, ``10``, ``14``] ``index_list ``=` `[``1``, ``3``, ``4``] `` ` `# printing original lists ``print` `(``"Original list : "` `+` `str``(test_list)) ``print` `(``"Original index list : "` `+` `str``(index_list)) `` ` `# using list comprehension + sum() to ``# Selective indices Summation``res_list ``=` `sum``([test_list[i] ``for` `i ``in` `index_list]) ``     ` `# printing result ``print` `(``"Resultant list : "` `+` `str``(res_list)) `
Output :
```Original list : [9, 4, 5, 8, 10, 14]
Original index list : [1, 3, 4]
Resultant list : 22
```

Method #2 : Using `map() + __getitem__ + sum()`
Yet another method to achieve this particular task is to map one list with other and get items of indexes and get corresponding matched elements from the search list. This is quite quick way to perform this task. The task of summation is performed using sum().

 `# Python3 code to demonstrate ``# Selective indices Summation``# using map() + __getitem__ + sum()`` ` `# initializing lists ``test_list ``=` `[``9``, ``4``, ``5``, ``8``, ``10``, ``14``] ``index_list ``=` `[``1``, ``3``, ``4``] `` ` `# printing original lists ``print` `(``"Original list : "` `+` `str``(test_list)) ``print` `(``"Original index list : "` `+` `str``(index_list)) `` ` `# using map() + __getitem__ + sum() to ``# Selective indices Summation``res_list ``=` `sum``(``list``(``map``(test_list.__getitem__, index_list)))``     ` `# printing result ``print` `(``"Resultant list : "` `+` `str``(res_list)) `
Output :
```Original list : [9, 4, 5, 8, 10, 14]
Original index list : [1, 3, 4]
Resultant list : 22
```

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