Pattern: Flag

Sometimes we don't need to count things or accumulate a result. We just need to know: did something happen or not? Yes or no. This is the flag pattern

A flag is a boolean variable that starts as True or False and flips when a certain condition is met

numbers = [2, 4, 6, 8, 10]
all_even = True

for n in numbers:
    if n % 2 != 0:
        all_even = False

if all_even:
    print("All numbers are even")
else:
    print("Not all numbers are even")

Output All numbers are even

We started with the assumption all_even = True (optimistic, we believe they're all even). If we find even one number that's not even, we set the flag to False. At the end, we check the flag

numbers = [2, 4, 7, 8, 10]
all_even = True

for n in numbers:
    if n % 2 != 0:
        all_even = False

if all_even:
    print("All numbers are even")
else:
    print("Not all numbers are even")

Output Not all numbers are even (because of the 7)

The opposite works too: starting with False and switching to True

names = ["Tommy", "Lance", "Cortez", "Phil"]
target = "Cortez"
found = False

for name in names:
    if name == target:
        found = True

if found:
    print(f"{target} is in the list!")
else:
    print(f"{target} is not in the list")

Output Cortez is in the list!

We start pessimistic (found = False) and only switch to True if we actually find the target

We can combine the flag with break for efficiency. Once we found what we need, why keep looking?

names = ["Tommy", "Lance", "Cortez", "Phil"]
target = "Lance"
found = False

for name in names:
    if name == target:
        found = True
        break

if found:
    print(f"Found {target}!")
else:
    print(f"{target} not found")

A practical example: checking if a password is strong (has at least one uppercase letter, one lowercase letter, and one digit)

password = "Tommy123"
has_upper = False
has_lower = False
has_digit = False

for char in password:
    if char.isupper():
        has_upper = True
    elif char.islower():
        has_lower = True
    elif char.isdigit():
        has_digit = True

if has_upper and has_lower and has_digit:
    print("Strong password")
else:
    print("Weak password")

Output Strong password. Three flags, one for each requirement

Mission: Access Code Check

A station access code is valid only if it meets all three rules: it has at least one uppercase letter, at least one digit, and is at least 6 characters long.

Read a code, then use flags (has_upper and has_digit, each starting False and flipped to True when you find one) plus a length check. Print each result, then the verdict:

Input:

• the access code

Output: four lines: the three checks, then Access code valid or Access code invalid.

Example

If the user types

Orbit42

the program should print

Has uppercase: True
Has digit: True
Long enough: True
Access code valid

If the user types

orbit

the program should print

Has uppercase: False
Has digit: False
Long enough: False
Access code invalid