iterating.md

Looping and Iterating

Python offers various ways to loop and iterate over elements, making it highly efficient and readable for traversing through data structures like lists, tuples, dictionaries, and sets. This document explores the different methods and nuances associated with looping and iterating in Python.

The 'while' Loop

In Python, while loops are used for repeated execution as long as an expression is true.

count = 0
while count < 5:
    print(count)
    count += 1

The 'for' Loop

The for loop in Python is used for iterating over a sequence (like a list, tuple, set, or string) or other iterable objects. The basic syntax is as follows:

for element in sequence:
    # Do something with element

Iterating Over Lists

myList = [1, 2, 3, 4, 5]
for number in myList:
    print(number)

Iterating Over Strings

for char in "Hello":
    print(char)

Nested Loops

Python allows using one loop inside another loop.

myListA = [1, 2, 3, 4, 5]
myListB = [1, 2, 3]
for i in myListA:  # Outer loop
    for j in myListB:  # Inner loop
        print(i, end=" ")
    print()

Loop Control Statements

Loop control statements change the execution from its normal sequence.

break Statement

Terminates the loop and transfers execution to the statement immediately following the loop.

for number in myList:
    if number == 3:
        break
    print(number)

continue Statement

Causes the loop to skip the remainder of its body and immediately retest its condition.

for number in myList:
    if number == 3:
        continue
    print(number)

pass Statement

The pass statement is a null operation; nothing happens when it executes. It's used as a placeholder.

for number in myList:
    if number == 3:
        pass
    print(number)

The range() Function

In Python, the range() function returns a range object, which is an iterator that yields a sequence of numbers. This range object does not store all its values in memory; it generates the numbers on demand during iteration. However, this means you cannot directly access an element by its index as you would with a list. To access a specific item by its index, you need to convert the range object into a list.

# Using range
for num in range(5):
    print(num)  # This works fine for iteration

# Directly accessing an item from range - This will cause an error
# print(range(5)[2])

# Converting to list and accessing an item
numbers_list = list(range(5))
print(numbers_list[2])  # This will print 2

Specifying Start, Stop, and Step

for i in range(2, 10, 2):  # Starts at 2, stops before 10, steps by 2
    print(i)

Iterating With Index

Using enumerate()

Sometimes it's useful to have access to the indices of the values when iterating.

for index, value in enumerate(myList):
    print(index, value)

Iterating Over Multiple Lists

Using zip()

zip() is used to iterate over two or more lists in parallel.

list1 = [1, 2, 3]
list2 = ['a', 'b', 'c']
for number, letter in zip(list1, list2):
    print(number, letter)