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iter() and next() Functions Complexity

The iter() function creates an iterator object from an iterable, and next() retrieves the next item from an iterator.

Complexity Reference

Operation	Time	Space	Notes
iter(iterable)	O(1)	O(1)	Create iterator from iterable
iter(callable, sentinel)	O(1)	O(1)	Create callable iterator
next(iterator)	O(1)*	O(1)	Get next item
next(iterator, default)	O(1)*	O(1)	Get next or default

*O(1) for built-in types; may vary for custom iterators (e.g., filter iterator is O(k) where k items are skipped)

Creating Iterators

Iterating Over Sequences

# Manual iteration using iter() and next() - O(1) per item
lst = [10, 20, 30]
iterator = iter(lst)

while True:
    try:
        item = next(iterator)  # O(1) per call
        print(item)
    except StopIteration:
        break

# Equivalent to for loop
for item in lst:  # for loop internally uses iter() and next()
    print(item)

Callable Iterator Pattern

Creating Iterator from Callable

# Create iterator from callable with sentinel - O(1)
def read_line():
    """Simulate reading from a file"""
    # Would read actual file in real scenario
    pass

# Create iterator that calls function until sentinel reached
iterator = iter(read_line, "EOF")  # O(1)

# Get items until sentinel - O(1) per next() call
item = next(iterator)  # O(1)

Real-world Example: File Reading

# Read file without loading all in memory - O(1) per line
with open('largefile.txt') as f:
    # Create iterator from lambda until empty string
    iterator = iter(lambda: f.read(1024), '')  # O(1) to create

    for chunk in iterator:  # O(n) to iterate, O(1) per chunk
        process(chunk)  # O(1)

Using next()

Basic next() Usage

# Get next item - O(1)
lst = [10, 20, 30]
it = iter(lst)

item1 = next(it)  # 10 - O(1)
item2 = next(it)  # 20 - O(1)
item3 = next(it)  # 30 - O(1)

# next() after exhaustion raises StopIteration
try:
    item4 = next(it)  # Raises StopIteration
except StopIteration:
    print("Iterator exhausted")

Using Default Value

# Use default if iterator exhausted - O(1)
it = iter([1, 2, 3])
next(it)  # 1
next(it)  # 2
next(it)  # 3
item = next(it, "END")  # "END" - returns default instead of raising

# Useful for safe iteration
def safe_next(iterator, default=None):
    return next(iterator, default)  # O(1)

Custom Iterators

Implementing Iterator Protocol

# Custom iterator class - O(1) per operation
class CountUp:
    def __init__(self, max):
        self.max = max
        self.current = 0

    def __iter__(self):
        return self  # O(1) - return iterator itself

    def __next__(self):
        if self.current < self.max:
            self.current += 1
            return self.current  # O(1)
        else:
            raise StopIteration

# Usage
counter = CountUp(3)
it = iter(counter)  # O(1)

item1 = next(it)  # 1 - O(1)
item2 = next(it)  # 2 - O(1)
item3 = next(it)  # 3 - O(1)

Custom Iterable with Iterator

# Iterable that creates fresh iterator - O(1)
class Range:
    def __init__(self, start, stop):
        self.start = start
        self.stop = stop

    def __iter__(self):
        return RangeIterator(self.start, self.stop)  # O(1)

class RangeIterator:
    def __init__(self, start, stop):
        self.current = start
        self.stop = stop

    def __iter__(self):
        return self  # O(1)

    def __next__(self):
        if self.current < self.stop:
            value = self.current
            self.current += 1
            return value  # O(1)
        else:
            raise StopIteration

# Usage
r = Range(0, 3)
it1 = iter(r)  # O(1) - fresh iterator
it2 = iter(r)  # O(1) - another fresh iterator

Generator Functions

Generators use iter() and next()

# Generator function returns iterator - O(1)
def count_up(max):
    i = 0
    while i < max:
        yield i  # Yields value, pauses execution
        i += 1

# Create iterator from generator - O(1)
gen = count_up(3)  # O(1) - returns generator object

# Iterate using next() - O(1) per call
item1 = next(gen)  # 0 - O(1), resumes at yield
item2 = next(gen)  # 1 - O(1)
item3 = next(gen)  # 2 - O(1)

# For loop equivalent
for item in count_up(3):  # Uses iter() and next() internally
    print(item)

Generator Expression

# Generator expression returns iterator - O(1)
gen = (x**2 for x in range(5))  # O(1) - lazy evaluation

# Iterate using next() - O(1) per item
item1 = next(gen)  # 0 - O(1)
item2 = next(gen)  # 1 - O(1)
item3 = next(gen)  # 4 - O(1)

# For loop
for item in (x**2 for x in range(5)):  # O(1) per iteration
    print(item)

Common Patterns

Converting to List (Forces Evaluation)

# Create iterator - O(1)
it = iter([1, 2, 3, 4, 5])

# Convert to list - forces all iterations O(n)
lst = list(it)  # O(5) - calls next() 5 times

# Iterator is now exhausted
try:
    next(it)  # Raises StopIteration
except StopIteration:
    print("Exhausted")

Chaining Multiple Iterators

# Create multiple iterators - O(1) each
it1 = iter([1, 2, 3])
it2 = iter([4, 5, 6])
it3 = iter([7, 8, 9])

# Iterate through each - O(n) total
for it in [it1, it2, it3]:
    while True:
        try:
            item = next(it)  # O(1) per call
            print(item)
        except StopIteration:
            break

Consuming First Item Separately

# Use iter() explicitly when you need to advance past some items manually
data = ["header", "row1", "row2", "row3"]
it = iter(data)

header = next(it)       # O(1) - consume first item separately
for row in it:          # O(n) - continues from second item
    print(f"Row: {row}")

Zip with Iterators

# Create iterators - O(1) each
it1 = iter([1, 2, 3])
it2 = iter(['a', 'b', 'c'])

# zip pairs items from iterators - O(1) to create
paired = zip(it1, it2)

# Iterate - O(1) per pair
for num, letter in paired:  # O(n) total
    print(num, letter)

# Equivalent manual iteration
it1 = iter([1, 2, 3])
it2 = iter(['a', 'b', 'c'])
while True:
    try:
        item1 = next(it1)  # O(1)
        item2 = next(it2)  # O(1)
        print(item1, item2)
    except StopIteration:
        break

Performance Considerations

Iterator vs List Memory

# Iterator - constant memory O(1)
def infinite_count():
    i = 0
    while True:
        yield i
        i += 1

gen = infinite_count()  # O(1) memory
item1 = next(gen)  # O(1)
item2 = next(gen)  # O(1)

# List - linear memory O(n)
lst = list(range(1000000))  # O(n) memory!

Lazy Evaluation Benefits

import itertools

# Create iterator (lazy) - O(1)
def get_data():
    for i in range(1000000):
        yield i

# Don't load all in memory
for item in get_data():  # O(1) per item, O(1) total memory
    if item > 100:
        break  # Can stop early without processing all

# Avoid with list
data = list(range(1000000))  # O(n) memory - loaded all upfront!
for item in data:
    if item > 100:
        break

Efficient Filtering

# Generator with filter (lazy) - O(1) per item
def filtered_data():
    for i in range(1000000):
        if i % 2 == 0:
            yield i

# Process only needed items
for item in filtered_data():  # O(1) per item, stop early
    print(item)
    if item > 100:
        break

# Vs materializing to list
data = [i for i in range(1000000) if i % 2 == 0]  # O(n) memory
for item in data:
    print(item)
    if item > 100:
        break

Built-in Iterator Functions

map() Returns Iterator

# map creates iterator (lazy) - O(1)
def square(x):
    return x ** 2

it = map(square, [1, 2, 3, 4, 5])  # O(1)

# Iterate - O(1) per item
item1 = next(it)  # 1 - O(1)
item2 = next(it)  # 4 - O(1)
item3 = next(it)  # 9 - O(1)

filter() Returns Iterator

# filter creates iterator (lazy) - O(1)
def is_even(x):
    return x % 2 == 0

it = filter(is_even, [1, 2, 3, 4, 5])  # O(1)

# Iterate - O(1) per item
for item in it:  # O(n) total, O(1) per item
    print(item)

enumerate() Returns Iterator

# enumerate creates iterator - O(1)
it = enumerate(['a', 'b', 'c'])  # O(1)

# Iterate - O(1) per item
item1 = next(it)  # (0, 'a') - O(1)
item2 = next(it)  # (1, 'b') - O(1)
item3 = next(it)  # (2, 'c') - O(1)

Version Notes

• Python 2.x: range() returns list, not iterator (use xrange() for iterator)
• Python 3.x: range() returns iterator-like object, map() and filter() return iterators
• All versions: Iterator protocol with __iter__() and __next__() (Python 3) or next() (Python 2)

Related Functions

• zip() - Iterate multiple sequences
• map() - Transform iterator items
• filter() - Filter iterator items
• enumerate() - Get index and value
• range() - Iterator-like sequence

Best Practices

✅ Do:

• Use iter() and next() for custom iterators
• Use generators for memory-efficient iteration
• Use default value in next() to avoid exceptions
• Use for loops (they use iter() and next() internally)

❌ Avoid:

• Calling next() without handling StopIteration
• Creating iterators you won't use (lazy evaluation wasted)
• Assuming you can iterate an iterator twice (exhausts on first pass)
• Using next() in tight loops without performance profiling