Iterators
Overview
An iterator lets you go through items one by one.
- A
forloop is an easy way to iterate - Works with lists, strings, and more
A for loop is an example of iteration:
numbers = [1, 2, 3]
for num in numbers:
print(num)
Output:
1
2
3
Iterators vs. Iterables
Objects like lists and strings are iterables. They can be turned into iterators.
- An iterable has an
iter()method - A for loop turns iterables into iterators automatically
- An iterator has a
next()method that gives the next value
Example:
nums = [10, 20, 30]
iter_nums = iter(nums)
print(next(iter_nums))
print(next(iter_nums))
Output:
10
20
Not all iterables are actual list.
Using next() to Iterate
Instead of a for loop, you can manually step through an iterator using next().
iter()turns an iterable into an iteratornext()gets the next value- Stops with
StopIterationerror
Example:
letters = iter("abc")
print(next(letters))
print(next(letters))
print(next(letters))
Output:
a
b
c
Example: range()
The range() function can be used in a for loop as if it's a list:
for i in range(5):
print(i)
However, range() doesn't create an actual list. Instead, it generates values on demand using an iterator. If it created a full list, using a very large number (like 10¹⁰⁰) would likely exceed memory limits.
In the example below, we'll see how range() works with iterators:
- Iterator
word_rangeforrange(3)is created usingiter(). - Use a
forloop to iterate overrange(3), print each value. - Iterator
foobarforrange(10 ** 100)is also created.
Solution:
# Iterator for range(3)
word_range = iter(range(3))
print(next(word_range))
print(next(word_range))
print(next(word_range))
# Loop over range(3)
for x in range(3):
print(x)
# Iterator for range(10 ** 100)
foobar = iter(range(10 ** 100))
print(next(foobar))
print(next(foobar))
print(next(foobar))
print(next(foobar))
print(next(foobar))
The output shows how iterators fetch values dynamically rather than storing them all at once.
1
2
0
1
2
0
1
2
3
4
Unpack Iterators with *
The * operator prints all values at once.
- Works on iterators and iterables
- After use, the iterator is empty
Example:
nums = iter([1, 2, 3])
print(*nums)
Output:
1 2 3
Iterate Over Dictionaries
Dictionaries are also iterables, and you can loop through key-value pairs.
Use .items() to access both keys and values
info = {"name": "Alex", "age": 30}
for key, value in info.items():
print(key, value)
Output:
name Alex
age 30
Iterate Over Files
Files can be iterated line by line using iter() and next().
Each next() call reads the next line
with open("file.txt") as file:
lines = iter(file)
print(next(lines))
print(next(lines))
Iterators as Function Arguments
Some functions work with iterators and iterables.
range()creates a sequence of numberslist()converts it into a listsum()adds up the numbers
Example:
values = range(10, 21) # Create a range from 10 to 20
print(list(values))
values_list = list(values) # Convert range to a list
print(values_list)
values_sum = sum(values) # Sum the numbers
print(values_sum)
Output:
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
165
Using enumerate()
enumerate() adds an index to items in an iterable, creating pairs of index and value.
- Converts an iterable into an enumerate object
- Returns index-value pairs as tuples
- Default index starts at 0 but can be changed
Example:
mutants = ['charles xavier',
'bobby drake',
'kurt wagner',
'max eisenhardt',
'kitty pryde']
mutant_list = list(enumerate(mutants))
print(mutant_list)
Output:
[(0, 'charles xavier'), (1, 'bobby drake'), (2, 'kurt wagner'), (3, 'max eisenhardt'), (4, 'kitty pryde')]
Looping with enumerate()
You can loop through an enumerate object and unpack values.
for index, value in enumerate(iterable)unpacks pairs- Default index is 0, but can be changed with
start
Example:
mutants = ['charles xavier',
'bobby drake',
'kurt wagner',
'max eisenhardt',
'kitty pryde']
for index1, value1 in enumerate(mutants):
print(index1, value1)
Output:
0 charles xavier
1 bobby drake
2 kurt wagner
3 max eisenhardt
4 kitty pryde
Defining the start value of the index:
for index2, value2 in enumerate(mutants, start=1):
print(index2, value2)
Output:
1 charles xavier
2 bobby drake
3 kurt wagner
4 max eisenhardt
5 kitty pryde
Using zip()
zip() takes any number of iterables and returns a zip object that is an iterator of tuples.
- Combines multiple iterables into pairs or groups.
- Each tuple contains elements from corresponding positions in iterables
If you wanted to print the values of a zip object, you can convert it into a list and then print it. Printing just a zip object will not return the values unless you unpack it first.
Example:
mutants = ['charles xavier',
'bobby drake',
'kurt wagner',
'max eisenhardt',
'kitty pryde']
aliases = ['prof x',
'iceman',
'nightcrawler',
'magneto',
'shadowcat']
powers = [
'telepathy',
'thermokinesis',
'teleportation',
'magnetokinesis',
'intangibility']
# Create a list of tuples: mutant_data
mutant_data = list(zip(mutants, aliases, powers))
print(mutant_data)
Output:
[('charles xavier', 'prof x', 'telepathy'), ('bobby drake', 'iceman', 'thermokinesis'), ('kurt wagner', 'nightcrawler', 'teleportation'), ('max eisenhardt', 'magneto', 'magnetokinesis'), ('kitty pryde', 'shadowcat', 'intangibility')]
To create a zip object:
mutant_zip = zip(mutants, aliases, powers)
print(mutant_zip)
Output:
<zip object at 0x7f21f603ff40>
Looping with zip()
You can iterate over a zip object directly.
Example:
for value1, value2, value3 in mutant_zip:
print(value1, value2, value3)
Output:
charles xavier prof x telepathy
bobby drake iceman thermokinesis
kurt wagner nightcrawler teleportation
max eisenhardt magneto magnetokinesis
kitty pryde shadowcat intangibility
Printing with *
Use * (splat operator) to unpack zip results.
Example:
avengers = ["Iron Man", "Thor", "Hulk"]
real_names = ["Tony", "Thor", "Bruce"]
print(*zip(avengers, real_names))
Output:
('Iron Man', 'Tony') ('Thor', 'Thor') ('Hulk', 'Bruce')