Python – Extract element from a list succeeded by K

When it is required to extract elements from a list that are succeeded by K (elements that come before K), we can use simple iteration with the append method or list comprehension.

Method 1: Using Loop

Iterate through the list and check if the next element equals K ?

numbers = [45, 65, 32, 78, 99, 10, 21, 2]
print("The list is:")
print(numbers)

K = 99
print("The value of K is:")
print(K)

result = []
for i in range(len(numbers) - 1):
    if numbers[i + 1] == K:
        result.append(numbers[i])

print("Elements succeeded by K:")
print(result)

The list is:
[45, 65, 32, 78, 99, 10, 21, 2]
The value of K is:
99
Elements succeeded by K:
[78]

Method 2: Using List Comprehension

A more concise approach using list comprehension ?

numbers = [45, 65, 32, 78, 99, 10, 21, 2, 99, 50]
K = 99

result = [numbers[i] for i in range(len(numbers) - 1) if numbers[i + 1] == K]
print(f"Elements succeeded by {K}: {result}")

Elements succeeded by 99: [78, 2]

Method 3: Using zip()

Using zip() to pair adjacent elements ?

numbers = [1, 5, 3, 5, 7, 5, 9]
K = 5

result = [current for current, next_elem in zip(numbers, numbers[1:]) if next_elem == K]
print(f"Elements succeeded by {K}: {result}")

Elements succeeded by 5: [1, 3, 7]

Comparison

Conclusion

Use the loop method for clarity and readability. List comprehension provides a concise solution, while zip() offers a functional approach for pairing adjacent elements.