Lecture-16 Iteration while Python 24-25

COMP101 Introduction to Programming 2024-25 Lecture-16: Iteration with while in Python

Overview of While Loop

Definition: A non-deterministic loop where the end condition is unknown until evaluated.
Control Mechanism: The loop starts with a test condition, tested for True/False values.
Execution Flow:
- Start loop execution while the condition is True.
- Exit the loop when the condition turns False.
- Test condition must eventually change to ensure loop termination.

Structure of While Loop

Syntax:
```
while CONDITION:
    <STATEMENTS>
```

Example:

i = 0
while (i < 5):
    i = i + 1
    print(i)
print("loop ended")

Key Points:
- Use a sentry variable initialized close to the loop’s entry point (0 for numbers, space for strings).
- Modify the sentry variable within the loop to prevent infinite loops.
- The first line aligned with the 'while' indicates the loop's termination.

Common Errors in While Loops

Infinite Loops

Example:

name = input("Enter your name: ")
onScreen = input("Want to see your name on screen? Y/N: ")
onScreen = onScreen.upper()
while onScreen == "Y":
    print(name)
    print()  # helps with readability
print("Loop end")

Issue: If the sentry variable isn't updated within the loop, it leads to an infinite loop.
Resolution:
- Re-prompt for user input within the loop to avoid continuous output.

Logic Errors

Example:

lives_left = 7
while lives_left != 0:
    print("Hero is hurt in battle - lose 3 lives")
    lives_left = lives_left - 3
    print("Hero has", lives_left, "lives left")
print("Hero has no lives remaining")
print("Loop end")

Issue: Subtracting 3 repeatedly from an initial value of 7 doesn’t provide a terminating condition.
Resolution: Change the initialization (set to multiple of 3) or adjust the loop condition for greater than checks.

Correcting Logic Errors in Output

Initial Error:

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

Issue: Loop outputs 0 to 6 instead of 1 to 5.
Correction:
- Initialize the variable to 1, really start counting from there.
```
i = 1
while i <= 5:
    print(i)
    i += 1
```
- New Output: Correct range of 1 to 5.

Construction of While Loops in Menu Systems

Requirement

Access a program via a main menu.
Single entry and exit via the main menu.

Steps to Implement Menu

Output Menu:

print("Main Menu")
print("A: Option-A code")
print("B: Option-B code")
print("C: Option-C code")
print("X: Exit")

User Input:
- Capture user option and handle invalid inputs gracefully.

Selection Logic:

if option == "A":
    print("Option-A under development")
elif option == "B":
    print("Option-B under development")
elif option == "C":
    print("Option-C under development")
else:
    print("Program ended")

Iteration Construct:
```
while option not in "X":
    [menu code]
```
- Continues until user opts to exit.

Program Usability Enhancements

Cosmetic changes
- Use whitespace for readability.
- Clear output to the user upon interactions.
- Example: Keep outputs held on screen using input() before returning to the menu.

Testing While Loops

Checklist:
- Ensure the loop body executes at least once.
- Verify the test condition will eventually be met.
- Review statements order in loop body and initialization of control variables.

Summary of while Loop Characteristics

Basic structure is: while (test = T): executes the loop body.
Can support nested constructs like: while-else, break, continue.
Reminder: The test must be refreshed in the loop to allow for re-evaluation at the next iteration.