Iterating Through Data Structures in Python: A Practical Guide

Updated: at 02:12 AM

Iterating through data structures is a fundamental concept in Python programming. The ability to loop over elements in a data structure allows you to perform operations and manipulate data efficiently. In this comprehensive guide, we will explore practical techniques for iterating through Python’s core data structures - lists, tuples, dictionaries, sets and custom objects.

Table of Contents

Open Table of Contents

Lists

Lists are Python’s most versatile ordered collection object. Let’s look at different ways to iterate through lists:

Basic For Loop

The basic way is using a for loop combined with the range() and len() functions to iterate through the list index by index:

nums = [1, 2, 3, 4, 5]

for i in range(len(nums)):
  print(nums[i])

# Outputs:
# 1
# 2
# 3
# 4
# 5

Python Docs: for Statements

Enumerate

We can use the enumerate() function to iterate through the list keeping track of both the index and value:

nums = [1, 2, 3, 4, 5]

for i, num in enumerate(nums):
  print(i, num)

# Outputs:
# 0 1
# 1 2
# 2 3
# 3 4
# 4 5

Real Python: enumerate() Function

Zip

To loop over two or more lists simultaneously, zip() can be used:

names = ['John', 'Jane', 'Joe']
ages = [25, 20, 18]

for name, age in zip(names, ages):
  print(name, age)

# Outputs:
# John 25
# Jane 20
# Joe 18

Python Docs: zip()

List Comprehension

List comprehension provides a concise way to create new iterables by iterating through a list:

nums = [1, 2, 3, 4, 5]

squared_nums = [x**2 for x in nums]
print(squared_nums)

# Outputs:
# [1, 4, 9, 16, 25]

Real Python: List Comprehensions

While Loop

A while loop can be used to iterate through a list until a condition is met:

names = ['John', 'Jane', 'Joe']

i = 0
while i < len(names):
  print(names[i])
  i += 1

# Outputs:
# John
# Jane
# Joe

Python Docs: while Statements

Tuples

Tuples are immutable ordered sequences that work similarly to lists. Here are ways to iterate over tuples:

For Loop

Standard for loop iteration:

colors = ('red', 'green', 'blue')

for color in colors:
  print(color)

# Outputs:
# red
# green
# blue

Enumerate

enumerate() can also be used:

digits = (0, 1, 2, 3, 4)

for i, digit in enumerate(digits):
  print(i, digit)

# Outputs:
# 0 0
# 1 1
# 2 2
# 3 3
# 4 4

Zip

Zipping tuples works the same as lists:

names = ('John', 'Jane', 'Joe')
ages = (25, 20, 18)

for name, age in zip(names, ages):
  print(name, age)

# Outputs:
# John 25
# Jane 20
# Joe 18

Tuple Unpacking

Iterating through tuples can be simplified by unpacking elements directly in the for loop:

measurements = [(100, 20), (30, 15), (25, 10)]

for length, width in measurements:
  print(length, width)

# Outputs:
# 100 20
# 30 15
# 25 10

Real Python: Tuple Unpacking

Dictionaries

Dictionaries are unordered mappings of unique keys to values. Here are ways to iterate over dictionaries:

Iterating Keys

Use .keys() to iterate over just the dictionary keys:

student_scores = {'Jane': 95, 'Bob': 87, 'Alex': 74}

for student in student_scores.keys():
  print(student)

# Outputs:
# Jane
# Bob
# Alex

Python Docs: dict.keys()

Iterating Values

Use .values() to iterate over just dictionary values:

fruit_colors = {'apple': 'red', 'banana': 'yellow', 'grape': 'purple'}

for color in fruit_colors.values():
  print(color)

# Outputs:
# red
# yellow
# purple

Python Docs: dict.values()

Iterating Keys and Values

Use .items() to iterate through keys and values together:

product_info = {'apple': 1.99, 'orange': 2.35, 'peach': 4.50}

for product, price in product_info.items():
  print(product, price)

# Outputs:
# apple 1.99
# orange 2.35
# peach 4.5

Python Docs: dict.items()

Dictionary Comprehension

Dictionary comprehension provides concise iteration to transform or filter dictionaries:

ages = {'Sue': 25, 'Bob': 30, 'Ann': 27}

over_30 = {name:age for name, age in ages.items() if age > 30}
print(over_30)

# Outputs:
# {'Bob': 30}

Python Docs: Dict Comprehensions

Sets

Sets are unordered collections of unique elements. Here are ways to iterate through sets:

For Loop

Basic for loop:

vowels = {'a', 'e', 'i', 'o', 'u'}

for vowel in vowels:
  print(vowel)

# Outputs:
# u
# a
# o
# i
# e

Sets have no indices, so elements are printed in an arbitrary order.

Set Comprehension

We can use set comprehension to iterate through a set and perform some operation:

nums = {1, 3, 5, 7, 9}

squared = {x**2 for x in nums}
print(squared)

# Outputs:
# {1, 9, 25, 49, 81}

Custom Objects

To iterate through custom classes, define an __iter__() method that yields item values:

class AnimalNames:

  def __init__(self):
    self.names = ['Lion', 'Zebra', 'Cat']

  def __iter__(self):
    for name in self.names:
      yield name

animal_names = AnimalNames()
for name in animal_names:
  print(name)

# Outputs:
# Lion
# Zebra
# Cat

Python Docs: Iterator Types

Conclusion

Iterating through data structures provides a powerful way to manipulate and transform data in Python. The key takeaways include:

With these techniques, you can write concise yet flexible Python code to effectively process data in your programs.