Lists & loops
Iterate, transform, and combine sequences with readable loop patterns.
Extract Data by Type from a List in Python: Numbers and Strings
Loop through a mixed list to filter out numeric and string values into separate lists.
def extract_numbers(items):
"""Extract all numeric values from a mixed list."""
numbers = []
for item in items:
if isinstance(item, (int, float)) and not isinstance(item, bool):
numbers.append(item)
return numbers
def extract_strings(items):
"""Extract all string values from a…Find Duplicate Elements in a Python List
Identifies and returns duplicate elements from a Python list using sets for efficient membership tests.
def find_duplicates(lst):
seen = set()
duplicates = set()
for item in lst:
if item in seen:
duplicates.add(item)
else:
seen.add(item)
return list(duplicates)
if __name__ == "__main__":
sample = [1, 2, 3, 2, 4, 1, 5, 3]
print(find_duplicates(sample))How to Build a Frequency Map from a List in Python
This code builds a dictionary that maps each unique element in a list to its count using the Counter class from the collections module.
from collections import Counter
def build_frequency_map(values):
"""Return a dictionary mapping each unique value to its frequency."""
return dict(Counter(values))
if __name__ == "__main__":
data = ["apple", "banana", "apple", "cherry", "banana", "apple"]
freq_map = build_frequency_map(data)
prin…How to Build a Running Maximum List in Python
Compute a list where each element is the maximum of all numbers seen so far from an input list.
def running_maximum(numbers):
result = []
current_max = float('-inf')
for num in numbers:
if num > current_max:
current_max = num
result.append(current_max)
return result
if __name__ == "__main__":
numbers = [3, 1, 4, 1, 5, 9, 2, 6]
max_list = running_maximum(number…How to Compute Percentile Value from Sorted List in Python
Compute any percentile value from a sorted list using linear interpolation between ranks.
def percentile(sorted_data, percentile_value):
"""Return the value below which `percentile_value`% of data falls."""
if not sorted_data:
raise ValueError("Cannot compute percentile of empty list")
if not 0 <= percentile_value <= 100:
raise ValueError("Percentile must be between 0 and 100")
…How to Cycle Through a List Infinitely with itertools
This code uses itertools.cycle to create an infinite iterator over a list and returns the first n items from that cycle.
from itertools import cycle
def demonstrate_cycle(items, cycles=3):
"""
Cycle through a list infinitely using itertools.cycle.
Returns the first n items from the infinite cycle.
"""
cycled = cycle(items)
result = [next(cycled) for _ in range(len(items) * cycles)]
return result
if __name__…How to Filter Empty Strings in Python
Remove empty and whitespace-only strings from a list using a list comprehension with the strip() method.
def filter_empty_strings(strings):
"""
Filter out empty strings (including whitespace-only strings)
from a list of strings.
"""
return [s for s in strings if s.strip()]
if __name__ == "__main__":
sample_list = ["hello", "", "world", " ", "python", " ", "!"]
filtered = filter_empty_strin…How to Filter None Values from a Mixed List in Python
Filter None values from a mixed Python list using a list comprehension with the `is not None` condition.
mixed_list = [1, None, "hello", None, 3.14, None, [1, 2], None]
filtered_list = [item for item in mixed_list if item is not None]
print(f"Original list: {mixed_list}")
print(f"Filtered list: {filtered_list}")
print(f"Original length: {len(mixed_list)}, Filtered length: {len(filtered_list)}")How to Interleave Two Lists in Python Until One List Exhausted
Interleave elements from two lists pairwise using zip, stopping when either list runs out of items.
def interleave(a, b):
result = []
for x, y in zip(a, b):
result.extend([x, y])
return result
if __name__ == "__main__":
list1 = [1, 2, 3, 4, 5]
list2 = ["a", "b", "c"]
print(interleave(list1, list2))How to Parse Bullet Points in Python
Extract bullet point items from raw text by splitting lines and filtering those that start with '- ' or '* '.
def parse_bullet_points(text):
"""Extract bullet point items from raw text."""
lines = text.splitlines()
items = []
for line in lines:
stripped = line.strip()
if stripped.startswith("- ") or stripped.startswith("* "):
item = stripped[2:]
if item:
…How to Reverse a List in Place Without Using reverse() in Python
A two-pointer while loop swaps elements from both ends toward the center to reverse a list in place without creating a copy.
def reverse_list_in_place(lst):
left = 0
right = len(lst) - 1
while left < right:
lst[left], lst[right] = lst[right], lst[left]
left += 1
right -= 1
if __name__ == "__main__":
my_list = [1, 2, 3, 4, 5]
print("Original:", my_list)
reverse_list_in_place(my_list)
prin…How to Truncate a List to Max Length in Python (Keep Head)
This code returns a new list containing only the first max_length items from the original list, using Python's slice syntax.
from typing import List
def truncate_head(lst: List[object], max_length: int) -> List[object]:
"""Return a new list with at most max_length items from the head."""
if max_length < 0:
raise ValueError("max_length must be non-negative")
return lst[:max_length]
if __name__ == "__main__":
# Examp…Round Robin Merge Multiple Lists in Python
Merge multiple lists by taking one element from each in turn, stopping when all lists are exhausted.
from itertools import cycle
def round_robin_merge(*lists):
"""Merge multiple lists by taking one element from each in turn."""
result = []
max_len = max(len(lst) for lst in lists)
for i in range(max_len):
for lst in lists:
if i < len(lst):
result.append(lst[i])…Truncate List Keeping Last N Elements in Python
Return a new list containing only the last N elements from a sequence, handling edge cases like zero or oversized counts.
def truncate(seq, keep_last_n):
"""Return a new list keeping only the last n elements."""
if keep_last_n <= 0:
return []
return list(seq)[-keep_last_n:]
if __name__ == "__main__":
data = [10, 20, 30, 40, 50, 60]
print(truncate(data, 3))
print(truncate(data, 0))
print(truncate(data…Browse by section
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