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Python – First Even Number in List

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  • Last Updated : 05 Feb, 2023
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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

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

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)


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