range() / enumerate() / zip()

# Generates a sequence of consecutive integers.
range(stop)
range(start, stop)
range(start, stop, step)

# Returns each element of an iterable paired with its index.
enumerate(iterable, start=0)

# Combines multiple iterables element by element.
zip(iterable1, iterable2, ...)

# Returns an iterable in reverse order.
reversed(sequence)

# Use range() to repeat a for loop.
for i in range(5):
    print(i, end=" ")  # 0 1 2 3 4

# Specify a start, stop, and step.
for i in range(2, 10, 2):
    print(i, end=" ")  # 2 4 6 8

# Iterate in reverse using range().
for i in range(5, 0, -1):
    print(i, end=" ")  # 5 4 3 2 1

# Convert range() to a list.
nums = list(range(1, 6))
print(nums)  # [1, 2, 3, 4, 5]

# Use enumerate() to get both the index and value at once.
fruits = ["apple", "orange", "grape"]
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")
# 0: apple
# 1: orange
# 2: grape

# Use start=1 to begin the index at 1.
for i, fruit in enumerate(fruits, start=1):
    print(f"#{i}: {fruit}")
# #1: apple

# Use zip() to process multiple lists in parallel.
names = ["Alice", "Bob", "Carol"]
scores = [85, 92, 78]
for name, score in zip(names, scores):
    print(f"{name}: {score} points")
# Alice: 85 points

# Use zip() to create a dictionary.
person = dict(zip(["name", "age", "city"], ["Alice", 20, "Tokyo"]))
print(person)  # {'name': 'Alice', 'age': 20, 'city': 'Tokyo'}

# Use reversed() to iterate over a list in reverse.
for item in reversed(fruits):
    print(item, end=" ")  # grape orange apple

range() returns a lazy-evaluated object that is highly memory-efficient. Even range(1000000) does not load a million integers into memory at once — it generates values on demand. The stop value is not included in the result (e.g., range(5) yields 0 through 4; 5 is not included).

enumerate() eliminates the need to manage indexes manually. Writing for i, fruit in enumerate(fruits): is considered more Pythonic than for i in range(len(fruits)):.

zip() stops when the shortest iterable is exhausted. If you need to process all elements from iterables of different lengths, use itertools.zip_longest() instead. zip() is also commonly used to loop over multiple lists together, and is often paired with sorted() for sorting operations.