Python – Sum elements matching condition

Checking a number/element by a condition is a common problem one faces and is done in almost every program. Sometimes we also require to get the totals sum that match the particular condition to have a distinguish which to not match for further utilization. Lets discuss certain ways in which this task can be achieved.

Method #1 : Using loop
This is brute force method to perform this particular task. In this, we iterate list, find elements that match a particular condition and take sum.

Python3

# Python 3 code to demonstrate
# Sum elements matching condition
# using loop

# initializing list
test_list = [3, 5, 1, 6, 7, 9]

# printing original list
print ("The original list is : " + str(test_list))

# using loop
# Sum elements matching condition
# checks for odd
res = 0
for ele in test_list:
    if ele % 2 != 0:
        res = res + ele

# printing result
print ("The sum of odd elements: " + str(res))

Output : 

The original list is : [3, 5, 1, 6, 7, 9]
The sum of odd elements: 25

Time Complexity: O(n)
Auxiliary Space: O(n), where n is length of list

Method #2 : Using sum() + generator expression
This method uses the trick of adding element to the sum whenever the generator expression returns true. By the time list gets exhausted, summation of numbers matching a condition is returned.

Python3

# Python 3 code to demonstrate
# Sum elements matching condition
# using sum() + generator expression

# initializing list
test_list = [3, 5, 1, 6, 7, 9]

# printing original list
print ("The original list is : " + str(test_list))

# using sum() + generator expression
# Sum elements matching condition
# checks for odd
res = sum(i for i in test_list if i % 2 != 0)

# printing result
print ("The sum of odd elements: " + str(res))

Output : 

The original list is : [3, 5, 1, 6, 7, 9]
The sum of odd elements: 25

Time Complexity: O(n) where n is the number of elements in the string list. The sum() + generator expression is used to perform the task and it takes O(n) time.
Auxiliary Space: O(1) constant additional space is required.

Method #3: Using filter()+list()+sum()+ lambda functions

Python3

# Python 3 code to demonstrate
# Sum elements matching condition

# initializing list
test_list = [3, 5, 1, 6, 7, 9]

# printing original list
print("The original list is : " + str(test_list))

res = sum(list(filter(lambda x: x % 2 != 0, test_list)))
# printing result
print("The sum of odd elements: " + str(res))

Output

The original list is : [3, 5, 1, 6, 7, 9]
The sum of odd elements: 25

Time Complexity: O(N)

Auxiliary Space: O(N)