Python – First Even Number in List

Last Updated : 06 May, 2023

Sometimes, while working with lists, we can have a problem in which we need to extract certain numbers occurring first time. This can also be even number. This kind of problem is common in day-day and competitive programming. Lets discuss certain ways in which this task can be performed.

Method #1 : Using loop This is brute force way in which this task can be performed. In this, we loop through the list and when 1st time even number occurs, we store and break the loop.

Python3

# Python3 code to demonstrate 
# First Even Number in List
# using loop
 
# Initializing list
test_list = [43, 9, 6, 72, 8, 11]
 
# printing original list
print("The original list is : " + str(test_list))
 
# First Even Number in List
# using loop
res = None
for ele in test_list:
    if not ele % 2 :
        res = ele
        break
     
# printing result 
print ("The first even element in list is : " + str(res))

Output :

The original list is : [43, 9, 6, 72, 8, 11]
The first even element in list is : 6

Time Complexity: O(n) where n is the number of elements in the list “test_list”.
Auxiliary Space: O(n) where n is the number of elements in the list “test_list”.

Method #2 : Using binary search (Sorted list) The binary search can also be employed on list when we have sorted list. In this, we keep looking for smaller even term when we find one and move to rear end in case we don’t find one.

Python3

# Python3 code to demonstrate 
# First Even Number in List
# using binary search
 
# Initializing list
test_list = [3, 7, 8, 9, 10, 15]
 
# printing original list
print("The original list is : " + str(test_list))
 
# First Even Number in List
# using binary search
res = len(test_list)
low = 0
high = len(test_list) - 1
while low <= high:
    mid = (low + high) // 2
    if test_list[mid] % 2:
        low = mid + 1
    else:
        res = test_list[mid]
        high = mid - 1
     
# printing result 
print ("The first even element in list is : " + str(res))

Output :

The original list is : [3, 7, 8, 9, 10, 15]
The first even element in list is : 8

Time Complexity: O(logn), where n is the length of the list test_list
Auxiliary Space: O(1) constant additional space is required

Using Recursion

Python3

# Python3 code to demonstrate
# First Even Number in List
# using recursion
def findfirsteven(lst,i):
  if lst[i]%2==0:
      return lst[i]
  else:
      return findfirsteven(lst,i+1)
  return -1
 
# Initializing list
test_list = [43, 9, 6, 72, 8, 11]
 
# printing original list
print("The original list is : " + str(test_list))
 
print ("The first even element in list is : " +str( findfirsteven(test_list,0))) 
 
#This code is contributed by Vinay Pinjala

Output

The original list is : [43, 9, 6, 72, 8, 11]
The first even element in list is : 6

Time Complexity:O(n)

Auxiliary Space: O(n)

Using filter function:

Python3

# Python3 code to demonstrate
# First Even Number in List
# using filter function
  
# Initializing list
test_list = [43, 9, 6, 72, 8, 11]
  
# printing original list
print("The original list is : " + str(test_list))
  
# First Even Number in List
# using filter function
res = list(filter(lambda x: (x % 2 == 0), test_list))[0]
  
# printing result
print ("The first even element in list is : " + str(res))

Output

The original list is : [43, 9, 6, 72, 8, 11]
The first even element in list is : 6

Time Complexity : O(n)

Auxiliary Space : O(n)

Method 5 : using a list comprehension.

Python3

# Initializing list
test_list = [43, 9, 6, 72, 8, 11]
  
# printing original list
print("The original list is : " + str(test_list))
  
# Finding the first even number in list
even_nums = [num for num in test_list if num % 2 == 0]
if even_nums:
    res = even_nums[0]
else:
    res = None
 
# Printing the result
if res:
    print("The first even element in list is :", res)
else:
    print("There are no even elements in the list.")

Output

The original list is : [43, 9, 6, 72, 8, 11]
The first even element in list is : 6

The time complexity of this method is O(n) as it involves iterating over the entire list.
The auxiliary space used by this method is O(n) as a new list even_nums is created to store the filtered elements.

Python program to print even numbers in a list

manjeet_04

Improve

Article Tags :

Practice Tags :

python

Python – First Even Number in List

Python3

Python3

Using Recursion

Python3

Python3

Python3

Similar Reads

Thank You!

What kind of Experience do you want to share?