Lists & loops
Iterate, transform, and combine sequences with readable loop patterns.
Check if List is Sorted Ascending in Python
Verify that a list is sorted in ascending order using the all() function and a generator expression.
def is_sorted_ascending(lst):
return all(lst[i] <= lst[i + 1] for i in range(len(lst) - 1))
if __name__ == "__main__":
test_lists = [
[1, 2, 3, 4, 5],
[1, 3, 2, 4, 5],
[5, 4, 3, 2, 1],
[1, 1, 2, 2, 3],
[10],
[]
]
for lst in test_lists:
print(f"{l…Find All Occurrences of an Item in a Python List
Loop through a list with enumerate() to collect the index of every match for a target value.
def find_all(data, target):
"""Return indices of every occurrence of target in a list."""
indices = []
for index, item in enumerate(data):
if item == target:
indices.append(index)
return indices
if __name__ == "__main__":
sample = [10, 20, 30, 20, 40, 20, 50]
target_value …Find Local Minima (Valleys) in a Numeric List in Python
This code finds indices of all local minima (valleys) in a numeric list, including edge cases, using a simple loop that compares each element with its neighbors.
def find_local_minima(numbers):
"""Find indices of local minima (valleys) in a numeric list.
A value is a local minimum if it's less than or equal to its neighbors.
Edge elements are considered minima if they're less than or equal to their single neighbor.
"""
if not numbers:
return []…Find Maximum Value in a List of Numbers in Python
Iterate through a list with a for loop to manually find and return the maximum numeric value.
def find_max(numbers):
"""Return the maximum value in a list of numbers."""
if not numbers:
return None
max_value = numbers[0]
for num in numbers[1:]:
if num > max_value:
max_value = num
return max_value
if __name__ == "__main__":
sample_list = [3, 7, 2, 15, 9, 11]
…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 Calculate a Cumulative Sum in Python
Build a new list where each element equals the running total of all numbers up to that index in the original list.
numbers = [1, 2, 3, 4, 5]
cumulative_sum = []
running_total = 0
for num in numbers:
running_total += num
cumulative_sum.append(running_total)
print(cumulative_sum)How to Calculate the Sum of List Elements in Python
Iterates over a list with a for loop, accumulates each number into a total variable, and returns the sum of all elements.
def sum_list_elements(numbers):
"""Return the sum of all elements in a list."""
total = 0
for num in numbers:
total += num
return total
if __name__ == "__main__":
sample_list = [1, 2, 3, 4, 5]
result = sum_list_elements(sample_list)
print(f"The sum of {sample_list} is {result}")How to Check if a List is Sorted in Descending Order in Python
This code defines a function that returns True if a given list is sorted in descending order, using a generator expression with all() to compare each adjacent pair.
def is_descending(lst):
"""Return True if list is sorted in descending order."""
return all(lst[i] >= lst[i + 1] for i in range(len(lst) - 1))
if __name__ == "__main__":
test_cases = [
[5, 4, 3, 2, 1],
[3, 3, 2, 1],
[1, 2, 3],
[10, 8, 9],
[]
]
for case in …How to Convert Data Types in Python Lists
Convert a mixed list of values to integers, floats, or strings based on their content, with graceful fallback for unparseable strings.
def convert_data(data):
"""Convert a mixed list of values to strings, ints, and floats."""
result = []
for item in data:
if isinstance(item, (int, float)):
result.append(str(item))
elif isinstance(item, str):
try:
if '.' in item:
r…How to Find Local Maxima in a Python List
Return the indices of all local maxima in a numeric list, where a peak is an element greater than both its immediate neighbors.
def find_peaks(numbers):
"""
Return the indices of local maxima in a numeric list.
A local maximum is an element greater than both its neighbors.
"""
if len(numbers) < 3:
return []
peaks = []
for i in range(1, len(numbers) - 1):
if numbers[i] > numbers[i - 1] and number…How to Find the Third Smallest Element in a Python List
Find the third smallest distinct value in a Python list by sorting unique elements and returning the third index.
def find_third_smallest(numbers):
if len(numbers) < 3:
return None
unique_sorted = sorted(set(numbers))
if len(unique_sorted) < 3:
return None
return unique_sorted[2]
if __name__ == "__main__":
sample = [5, 2, 8, 2, 9, 1, 7, 3]
result = find_third_smallest(sampl…How to Sort a List of Dictionaries by a Key in Python
Sort a list of dictionaries by a specified key field, optionally in descending order, using Python's built-in sorted() function.
def sort_dicts_by_key(data, key, reverse=False):
return sorted(data, key=lambda item: item.get(key), reverse=reverse)
if __name__ == "__main__":
people = [
{"name": "Alice", "age": 30},
{"name": "Bob", "age": 25},
{"name": "Charlie", "age": 35},
]
sorted_by_age = sort_dicts_b…How to Validate List Data in Python
A beginner-friendly validation helper that checks if data is a list, enforces minimum length, and optionally verifies item types with clear error messages.
def validate_data(data, expected_types=None, min_length=1):
"""Validate that data is a non-empty list and optionally check item types."""
if not isinstance(data, list):
return False, f"Expected a list, got {type(data).__name__}"
if len(data) < min_length:
return False, f"List must have…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])…Browse by section
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Lists & loops — Python code examples
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