Python – Extract element from list succeeded by K

Given a list, extract the elements which are having K as the next element.

Input : test_list = [2, 3, 5, 7, 8, 5, 3, 5], K = 3 
Output : [2, 5] 
Explanation : Elements before 3 are 2, 5.

Input : test_list = [2, 3, 5, 7, 8, 5, 3, 8], K = 8 
Output : [7, 3] 
Explanation : Elements before 8 are 7, 3. 

Method #1: Using loop

In this, we iterate the list and look for each K, and extract the element preceding it.

Python3

# Python3 code to demonstrate working of
# Extract elements succeeded by K
# Using loop

# initializing list
test_list = [2, 3, 5, 7, 8, 5, 3, 5]

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

# initializing K
K = 5

# Using loop to extract elements succeeded by K
res = []
for idx in range(len(test_list) - 1):

    # checking for succession
    if test_list[idx + 1] == K:
        res.append(test_list[idx])

# printing result
print("Extracted elements list : " + str(res))

Output

The original list is : [2, 3, 5, 7, 8, 5, 3, 5]
Extracted elements list : [3, 8, 3]

Method #2: Using list comprehension

Another way to solve this question, in this, we use list comprehension as shorthand to solve the problem of getting elements.

Python3

# Python3 code to demonstrate working of
# Extract elements succeeded by K
# Using list comprehension

# initializing list
test_list = [2, 3, 5, 7, 8, 5, 3, 5]

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

# initializing K
K = 5

# List comprehension used as shorthand
res = [test_list[idx]
       for idx in range(len(test_list) - 1) if test_list[idx + 1] == K]

# printing result
print("Extracted elements list : " + str(res))

Output

The original list is : [2, 3, 5, 7, 8, 5, 3, 5]
Extracted elements list : [3, 8, 3]