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Python | Summation of first N matching condition
• Last Updated : 27 Dec, 2019

Summation of elements in list can be performed using many inbuilt function. Normal summation functions have many applications in various domains. This article discusses to sum just the first N occurrences of elements matching particular condition.

Method #1 : Naive Method
We can sum the elements that are matching condition, after N occurrences of elements have been done and we can stop the operation. Code below demonstrates the same.

 `# Python 3 code to demonstrate ``# Summation of first N matching condition``# using Naive Method `` ` `# initializing list``test_list ``=` `[``3``, ``5``, ``1``, ``6``, ``7``, ``9``, ``8``, ``5``]`` ` `# printing original list``print` `(``"The original list is : "` `+` `str``(test_list))`` ` `# using Naive Method ``# Summation of first N matching condition``# sums first 3 odd occurrences``counter ``=` `1``res ``=` `0``for` `i ``in` `test_list:``    ``if` `counter <``=` `3` `and` `(i ``%` `2` `!``=` `0``):``        ``res ``=` `res ``+` `i``        ``counter ``=` `counter ``+` `1`` ` `# printing result``print` `(``"The filtered list is : "` `+` `str``(res))`
Output :
```The original list is : [3, 5, 1, 6, 7, 9, 8, 5]
The filtered list is : 9
```

Method #2 : Using `sum() + list comprehension`
This is different and elegant way to perform this particular task. It filters out all the numbers that are less than equal N and sums according to condition. This is one liner and preferred method to achieve this task.

 `# Python 3 code to demonstrate ``# Summation of first N matching condition``# using sum() + list comprehension`` ` `# initializing list``test_list ``=` `[``3``, ``5``, ``1``, ``6``, ``7``, ``9``, ``8``, ``5``]`` ` `# printing original list``print` `(``"The original list is : "` `+` `str``(test_list))`` ` `# using sum() + list comprehension``# to sum first N elements matching condition ``# sum first 3 odd occurrences``res ``=` `sum``([i ``for` `i ``in` `test_list ``if` `i ``%` `2` `!``=` `0``][:``3``])`` ` `# printing result``print` `(``"The filtered list is : "` `+` `str``(res))`
Output :
```The original list is : [3, 5, 1, 6, 7, 9, 8, 5]
The filtered list is : 9
```

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