Python Loops and Iterations

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1. What are Loops?

• Loops are used to repeatedly execute a block of code.
• They are essential for automating repetitive tasks and iterating over sequences like lists, tuples, strings, etc.
• Python supports two types of loops:
  1. for loops
  2. while loops

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2. for Loop

• The for loop is used to iterate over a sequence (e.g., list, tuple, string, range) or other iterable objects.
• Syntax:
  for item in sequence:
      # Code to execute
  

names = ["Raj", "Ram", "Anand", "Bala"]
for name in names:
    print(f"Hello, {name}!")

Hello, Raj!
Hello, Ram!
Hello, Anand!
Hello, Bala!

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Using range() in for Loops

• The range() function generates a sequence of numbers.
• It is commonly used to iterate a specific number of times.

for i in range(5):  # Iterates from 0 to 4
    print(i)

0
1
2
3
4

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3. while Loop

• The while loop repeats a block of code as long as a condition is True.
• Syntax:
  while condition:
      # Code to execute
  

count = 0
while count < 5:
    print(f"Count: {count}")
    count += 1

Count: 0
Count: 1
Count: 2
Count: 3
Count: 4

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4. Loop Control Statements

• These statements alter the flow of loops:
  1. break: Exits the loop immediately.
  2. continue: Skips the rest of the code in the current iteration and moves to the next iteration.
  3. pass: A placeholder for future code (does nothing).

• break:
  for i in range(10):
      if i == 5:
          break  # Exit the loop when i is 5
      print(i)
  
  Output:
  0
  1
  2
  3
  4
  
• continue:
  for i in range(5):
      if i == 2:
          continue  # Skip the rest of the code for i = 2
      print(i)
  
  Output:
  0
  1
  3
  4
  
• pass:
  for i in range(3):
      if i == 1:
          pass  # Placeholder for future code
      print(i)
  
  Output:
  0
  1
  2
  

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5. Nested Loops

• A loop inside another loop is called a nested loop.
• Commonly used for working with multi-dimensional data structures.

for i in range(3):  # Outer loop
    for j in range(2):  # Inner loop
        print(f"i={i}, j={j}")

i=0, j=0
i=0, j=1
i=1, j=0
i=1, j=1
i=2, j=0
i=2, j=1

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6. Iterating Over Sequences

• Loops can iterate over various sequences like lists, tuples, strings, dictionaries, etc.

• List:
  numbers = [10, 20, 30, 40]
  for num in numbers:
      print(num)
  
• String:
  name = "Karthik"
  for char in name:
      print(char)
  
• Dictionary:
  person = {"name": "Kumar", "age": 25, "city": "Chennai"}
  for key, value in person.items():
      print(f"{key}: {value}")
  

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7. Additional Examples

• for Loop:
  names = ["Siva", "Ramesh", "Suresh", "Sathish"]
  for name in names:
      print(f"Welcome, {name}!")
  
• while Loop:
  count = 1
  while count <= 3:
      print(f"Attempt {count}")
      count += 1
  
• Nested Loops:
  for i in range(2):  # Outer loop
      for j in range(3):  # Inner loop
          print(f"i={i}, j={j}")
  
• Loop Control:
  for i in range(10):
      if i == 7:
          break  # Exit the loop when i is 7
      print(i)
  

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8. Best Practices

• Use meaningful variable names for iterators (e.g., name instead of x).
• Avoid infinite loops in while loops by ensuring the condition eventually becomes False.
• Use break and continue sparingly to keep the code readable.
• Use list comprehensions for simple loops that create lists.

squares = [x ** 2 for x in range(5)]  # List comprehension
print(squares)  # Output: [0, 1, 4, 9, 16]