Python for Loops: Iteration and Traversal

• Python for loops are used to iterate over a sequence or other iterable objects
• Iterating over a sequence is called traversal
• The for loop executes a block of code repeatedly for each item in the sequence

Basic Syntax

• for element in sequence: starts a for loop, where element is a variable that takes on the value of each item in the sequence during each iteration
  • sequence is the iterable you are looping over
  • The code block within the for loop (indented) is executed for each item in the sequence

Example with a List

• fruits = ["apple", "banana", "cherry"] defines a list called fruits
• for fruit in fruits: starts a for loop that iterates through each item in the fruits list
  • print(fruit) prints the current fruit during each iteration
• The output will be each fruit printed on a new line: apple, banana, cherry

Example with a String

• for char in "Python": starts a for loop that iterates through each character in the string "Python"
  • print(char) prints the current character during each iteration
• The output will be each character printed on a new line: P, y, t, h, o, n

The range() Function

• The range() function is used to generate a sequence of numbers
• range(start, stop, step) creates a sequence of numbers from start (inclusive) to stop (exclusive), incrementing by step
  • If start is omitted, it defaults to 0
  • If step is omitted, it defaults to 1
• range(5) generates numbers 0, 1, 2, 3, 4
• range(2, 7) generates numbers 2, 3, 4, 5, 6
• range(0, 10, 2) generates numbers 0, 2, 4, 6, 8

Using range() in a for Loop

• for i in range(5): starts a for loop that iterates through the numbers generated by range(5) (0 to 4)
  • print(i) prints the current number i during each iteration
• for i in range(1, 11): iterates from 1 to 10

break Statement

• The break statement prematurely exits a for loop
• When break is encountered, the loop terminates immediately, and the program continues with the next statement after the loop
• Example:
  • numbers = [1, 2, 3, 4, 5]
  • for num in numbers:
    • if num == 3:
      • break
    • print(num)
• The output : 1, 2

continue Statement

• The continue statement skips the rest of the current iteration and proceeds to the next iteration of the loop
• Example:
  • numbers = [1, 2, 3, 4, 5]
  • for num in numbers:
    • if num == 3:
      • continue
    • print(num)
• The output: 1, 2, 4, 5 (3 is skipped)

else Clause in for Loops

• A for loop can have an optional else clause

• The else block executes after the loop completes normally, i.e., without encountering a break statement

• If the loop terminates because of a break statement, the else block is not executed

• Example:

  • numbers = [1, 2, 3, 4, 5]
  • for num in numbers:
    • print(num)
  • else:
    • print("Loop completed successfully")

• The output is the numbers from 1 to 5 each on a new line, followed by "Loop completed successfully" on a new line

• Example with break:

  • numbers = [1, 2, 3, 4, 5]
  • for num in numbers:
    • if num == 3:
      • break
    • print(num)

• else:

  • print("Loop completed successfully")

• The output is the numbers 1 and 2 each on a new line; the else clause will not be executed

Nested for Loops

• Nested for loops are for loops inside other for loops
• The inner loop executes completely for each iteration of the outer loop
• Example:
  • adj = ["red", "big", "tasty"]
  • fruits = ["apple", "banana", "cherry"]
  • for a in adj:
  • for f in fruits:
    • print(a, f)
• The outer loop iterates through adj, and, for each adjective, the inner loop iterates through fruits

Looping Through Dictionaries

• By default, iterating through a dictionary iterates through its keys
• my_dict = {"name": "Alice", "age": 30, "city": "New York"}
• for key in my_dict:
  • print(key) prints the keys: name, age, city

Looping Through Dictionary Values

• Use .values() to iterate through the values of a dictionary
• for value in my_dict.values():
  • print(value) prints the values: Alice, 30, New York

Looping Through Dictionary Items (Key-Value Pairs)

• Use .items() to iterate through keys and values as tuples
• for key, value in my_dict.items():
  • print(key, value) prints each key-value pair: name Alice, age 30, city New York

Using enumerate()

• The enumerate() function adds a counter to an iterable and returns it as an enumerate object
• It can retrieve both the index and the value of each item in a sequence during iteration
• Example:
  • fruits = ["apple", "banana", "cherry"]
  • for index, fruit in enumerate(fruits):
    • print(index, fruit)
• The output: 0 apple, 1 banana, 2 cherry
• enumerate() also accepts a start argument to specify the starting index value
• Example:
  • fruits = ["apple", "banana", "cherry"]
  • for index, fruit in enumerate(fruits, start=1):
    • print(index, fruit)
• The output: 1 apple, 2 banana, 3 cherry

List Comprehensions (Concise for Loops)

• List comprehensions provide a concise way to create lists
• They replace for loops in creating a new list based on an existing iterable
• Syntax: new_list = [expression for item in iterable if condition]
  • expression is the value to be included in the new list
  • item is the variable representing each item in the iterable
  • iterable is the sequence you are looping over
  • condition (optional) is a filter that determines whether an item should be included
• Example:
  • numbers = [1, 2, 3, 4, 5]
  • squares = [num ** 2 for num in numbers]
  • print(squares)
• Output: [1, 4, 9, 16, 25]

List Comprehensions with Conditionals

• Example:
  • numbers = [1, 2, 3, 4, 5]
  • even_squares = [num ** 2 for num in numbers if num % 2 == 0]
  • print(even_squares)
• Output: [4, 16]

Practical Considerations

• For large datasets, consider the performance implications of using for loops
• In some cases, vectorization techniques (e.g., using NumPy) can provide significant performance improvements
• Avoid modifying the sequence being iterated over within the loop, as this can lead to unexpected behavior