Interactive while Loops

Note

Source: Adapted from the C# edition (while/whileinteractive.rst). The input_in_range example is a Python translation. The while True + break form and the sentinel-value pattern are original Python additions.

An interactive while loop prompts the user for input each time through the loop. The loop continues until the user supplies a valid or expected value.

Repeating Until Valid Input

A classic use: keep asking for a score until the user enters one in range.

score = int(input("Enter a score (0-100): "))
while score < 0 or score > 100:
    print(f"{score} is out of range!")
    score = int(input("Enter a score (0-100): "))
print(f"Score accepted: {score}")

Notice that input appears twice — once before the loop to get the initial value, and once inside to refresh the value. This is the read-before-loop pattern. The C# edition called this an unavoidable duplication; Python’s while True / break pattern avoids it:

while True:
    score = int(input("Enter a score (0-100): "))
    if 0 <= score <= 100:
        break
    print(f"{score} is out of range!")
print(f"Score accepted: {score}")

Both versions behave identically, but the while True form only writes the input call once.

Sentinel Values

A sentinel value is a special input that signals “stop the loop.” For example, entering -1 to end a list of scores:

total = 0
count = 0
score = int(input("Enter a score (-1 to stop): "))
while score != -1:
    total += score
    count += 1
    score = int(input("Enter a score (-1 to stop): "))

if count > 0:
    print(f"Average: {total / count:.1f}")
else:
    print("No scores entered.")

Equivalently with while True:

total = 0
count = 0
while True:
    score = int(input("Enter a score (-1 to stop): "))
    if score == -1:
        break
    total += score
    count += 1

if count > 0:
    print(f"Average: {total / count:.1f}")

Menu Loop

Interactive loops are often used to drive a menu:

def show_menu():
    print("\n1. Add")
    print("2. Subtract")
    print("3. Quit")

while True:
    show_menu()
    choice = input("Choose: ").strip()
    if choice == "1":
        a = float(input("a: "))
        b = float(input("b: "))
        print(f"Result: {a + b}")
    elif choice == "2":
        a = float(input("a: "))
        b = float(input("b: "))
        print(f"Result: {a - b}")
    elif choice == "3":
        print("Goodbye.")
        break
    else:
        print("Invalid choice.")

The loop exits only when the user chooses option 3.

Common Pitfalls

• Forgetting to update the input inside the loop: the loop never advances and runs forever.

• Using ``==`` to compare floats: rounding errors can prevent the condition from ever becoming False. Use a tolerance check instead: abs(x - target) < 0.0001.