Lecture on For Loops in Python

Overview

• A for loop executes a block of code a fixed number of times.
• Commonly used to iterate over ranges, strings, sequences, or anything iterable.
• For loops are ideal when you know the exact number of iterations needed.

Basic Syntax

• Structure: for variable in iterable:
  • variable: Counter or iterator, can be named anything (e.g., x, counter).
  • iterable: Can be a range, string, etc.
• Indent the code block under the for loop to specify the code to repeat.

Example: Counting to Ten

• Code: for x in range(1, 11): print(x)
• Explanation:
  • range(1, 11): Starts at 1, ends before 11, effectively counting to 10.
  • Prints numbers 1 through 10.

Counting Backwards

• Use the reversed() function with range.
• Code: for x in reversed(range(1, 11)): print(x) print("Happy New Year")
• Explanation:
  • Counts from 10 to 1.
  • Prints "Happy New Year" after loop, indicating exiting the loop.

Counting with Steps

• Use an additional parameter for step size in range().
• Code: for x in range(1, 11, 2): print(x)
• Explanation:
  • Uses a step of 2, printing 1, 3, 5, 7, 9.

Iterating Over Strings

• For loops can iterate over characters in a string.
• Example: credit_card = "1234-5678" for x in credit_card: print(x)
• Explanation:
  • Prints each character of the credit_card string, including dashes.

Useful Keywords: continue and break

Continue

• Skips the current iteration of the loop.
• Example: for x in range(1, 21): if x == 13: continue print(x)
• Explanation:
  • Skips number 13 while printing numbers 1 to 20.

Break

• Exits the loop entirely.
• Example: for x in range(1, 21): if x == 13: break print(x)
• Explanation:
  • Stops the loop at number 12, not printing 13 or any numbers after.

Comparison with While Loops

• While loops are better suited for scenarios where the number of iterations isn't known beforehand (potentially infinite tasks).

Conclusion

• For loops are versatile and can handle iterations over any iterable.
• Useful for tasks with a known repeat count or iterating over collections like strings.