Lists & loops
Iterate, transform, and combine sequences with readable loop patterns.
Enumerate a Python List with a Custom Start Index
Iterate over a list with an index that starts at a custom value (like 5) using Python's built-in enumerate() function with the start parameter.
fruits = ["apple", "banana", "cherry", "date"]
for index, fruit in enumerate(fruits, start=5):
print(f"{index}: {fruit}")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 Build a Text Processor with Lists and Loops in Python
A beginner-friendly Python script that analyzes text by counting sentences, words, and word lengths using lists and for loops, then prints the results.
def process_text(text):
"""Simple text processor for beginners using lists and loops."""
sentences = text.replace('!', '.').replace('?', '.').split('.')
words = text.split()
word_counts = []
for sentence in sentences:
sentence_word_count = len(sentence.split())
word_counts.appe…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 Average of a List of Numbers in Python
Compute the arithmetic mean of a numeric list using Python's built-in sum() and len() functions, returning 0.0 for an empty list.
def calculate_average(numbers):
if not numbers:
return 0.0
return sum(numbers) / len(numbers)
if __name__ == "__main__":
sample_numbers = [10, 20, 30, 40, 50]
result = calculate_average(sample_numbers)
print(f"Average: {result}")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 Zip Two Lists into Pairs in Python
Combine two lists element-wise into a list of tuples using Python's built-in zip() function.
def zip_lists_into_pairs(list1, list2):
pairs = list(zip(list1, list2))
return pairs
if __name__ == "__main__":
fruits = ["apple", "banana", "cherry"]
quantities = [3, 5, 2]
result = zip_lists_into_pairs(fruits, quantities)
print(result)How to summarize and transform lists in Python
Compute count, sum, min, max, and average for a list and multiply each element by a factor using simple loops and built-in functions.
def summarize(data):
"""Return a summary of a list: count, sum, min, max, average."""
count = len(data)
total = sum(data)
minimum = min(data)
maximum = max(data)
average = total / count if count else 0
return count, total, minimum, maximum, average
def multiply_elements(data, factor=2):
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Lists & loops — Python code examples
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