Python | Even Front digits Test in List

Sometimes we may face a problem in which we need to find a list if it contains numbers that are even. This particular utility has an application in day-day programming. Let’s discuss certain ways in which this task can be achieved.

Method #1: Using list comprehension + map() We can approach this problem by converting the elements to the strings and then testing the starting element of the string if they are even we can return true and then convert to set and test for the size of result to be one. The conversion is done by map, set function converts to set and list comprehension checks for first element of string. 

Python3

# Python3 code to demonstrate
# Even Front digits Test in List
# using list comprehension + map()

# initializing list
test_list = [25, 6, 828829, 432]

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

# using list comprehension + map()
# Even Front digits Test in List
res = len(set((int(sub[0]) % 2) for sub in map(str, test_list))) == 1

# print result
print("Does each element start with even digit ? " + str(res))

Output : 

The original list : [25, 6, 828829, 432]
Does each element start with even digit ? True

Time Complexity: O(n*n) where n is the number of elements in the test_list. The list comprehension + map() is used to perform the task and it takes O(n*nlogn) time.
Auxiliary Space: O(1) constant additional space is needed.

Method #2: Using all() + list comprehension

This is yet another approach in which this problem can be solved. In this we use all function to check for all elements and return a Boolean result and list comprehension does the part of the conversion of string by str function and checking for all elements with the first digit of first element to be even. 

Python3

# Python3 code to demonstrate
# Even Front digits Test in List
# using all() + list comprehension

# initializing list
test_list = [25, 6, 828829, 432]

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

# using all() + list comprehension
# Even Front digits Test in List
res = all(not int(str(i)[0]) % 2 for i in test_list)

# print result
print("Does each element start with even digit ? " + str(res))

Output : 

The original list : [25, 6, 828829, 432]
Does each element start with even digit ? True

Time complexity: O(n*k), where n is the number of elements in the list and k is the number of digits in the largest element.
Auxiliary space: O(1). We only use a constant amount of extra space to store the Boolean result of the all() function. 

Method #3: Using for loop 

Python3

# Python3 code to demonstrate
# Even Front digits Test in List

# initializing list
test_list = [25, 6, 828829, 432]

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

# Even Front digits Test in List
res=False
c=0
for i in test_list:
    a=str(i)
    if(int(a[0])%2==0):
        c+=1
if(c==len(test_list)):
    res=True
# print result
print("Does each element start with even digit ? " + str(res))

Output

The original list : [25, 6, 828829, 432]
Does each element start with even digit ? True

Time Complexity: O(N)
Auxiliary Space: O(1)

Method 4: Using filter() + lambda function

Python3

# Python3 code to demonstrate
# Even Front digits Test in List
# using filter() + lambda function

# initializing list
test_list = [25, 6, 828829, 432]

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

# using filter() + lambda function
# Even Front digits Test in List
res = len(list(filter(lambda x: int(str(x)[0]) % 2 == 0, test_list))) == len(test_list)

# print result
print("Does each element start with even digit ? " + str(res))

Output

The original list : [25, 6, 828829, 432]
Does each element start with even digit ? True

Time Complexity: O(N)
Auxiliary Space: O(1)

Approach #5: Using a regular expression and match() method:

• Importing the “re” module 
• Defining the regular expression pattern
• Applying the regular expression in the list to check if all elements start with even digits
• Printing the result

Python3

import re

# initializing list
test_list = [25, 6, 828829, 432]

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

# using regex
# Even Front digits Test in List
regex_pattern = r'^[02468]+'
res = all(re.match(regex_pattern, str(num)) for num in test_list)

# print result
print("Does each element start with even digit ? " + str(res))

Output

The original list : [25, 6, 828829, 432]
Does each element start with even digit ? True

Time Complexity: O(N) as we are traversing over the list.
Auxiliary Space: O(1) as we are not using any extra memory.

Method 5: Using pandas module

• Import pandas module.
• Convert the list to a pandas DataFrame.
• Use the str accessor to access the first digit of each element and check if it is even or not.
• Use the all() function to check if all the results are True or not.
• Print the final result.

Python3

# importing pandas module
import pandas as pd

# initializing list
test_list = [25, 6, 828829, 432]

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

# using pandas module
# Even Front digits Test in List
df = pd.DataFrame(test_list, columns=['numbers'])
res = df['numbers'].astype(str).str[0].isin(['0', '2', '4', '6', '8']).all()

# print result
print("Does each element start with even digit ? " + str(res))

OUTPUT:

The original list : [25, 6, 828829, 432]
Does each element start with even digit ? True

Time Complexity: O(n)
Auxiliary Space: O(n)