Good question. Use a while loop when you don't know in advance how many times you need to iterate. A for loop says "do this for each item." A while loop says "keep doing this until a condition becomes false."

Think about the difference between your spreadsheet work: processing every row in a table is a for loop. Waiting for a cell value to hit a threshold before stopping? That's a while loop.

count = 0
while count < 5:
    print(count)
    count += 1
# Prints: 0, 1, 2, 3, 4

The loop checks count < 5 before each iteration. The moment count reaches 5, the condition is False and the loop stops. If you forget count += 1, the condition never changes and you get an infinite loop — the program runs forever.

break exits the loop immediately — no more iterations, no checking the condition. continue skips the rest of the current iteration and jumps back to the top.

# break — stop the loop when we find what we want
numbers = [4, 7, 2, 9, 1, 8]
for n in numbers:
    if n == 9:
        print("Found 9!")
        break
    print(f"Checking {n}...")
# Prints: Checking 4... Checking 7... Checking 2... Found 9!

# continue — skip even numbers
for n in range(10):
    if n % 2 == 0:
        continue
    print(n)
# Prints: 1, 3, 5, 7, 9

Remember yesterday's zip() and enumerate()? break is especially powerful with those — imagine zipping through two columns of your spreadsheet and stopping the moment you find a mismatch.

A sentinel is a special value that signals "stop." It's a pattern for processing input until you see a specific marker. In real code, -1, 0, "quit", or None are common sentinels.

# Process scores until we see -1
scores = [88, 72, 95, 63, -1, 50]
total = 0
count = 0

for score in scores:
    if score == -1:
        break
    total += score
    count += 1

average = total / count if count > 0 else 0
print(f"Average of {count} scores: {average}")
# Average of 4 scores: 79.5

The -1 isn't a real score — it's the sentinel that tells our loop to stop. The 50 after it is never processed. This is like having a "END OF DATA" marker at the bottom of a spreadsheet column.

This is one of the most practical while loop patterns. You keep asking until the input is valid:

def get_valid_score():
    score = -1
    while score < 0 or score > 100:
        score = int(input("Enter a score (0-100): "))
    return score

The loop runs as long as score is outside the valid range. Once the user enters something between 0 and 100, the condition becomes False and the loop exits. See how yesterday's boolean logic (score < 0 or score > 100) drives the loop condition? Everything connects.

Here's another common pattern — a counter with early exit:

def find_first_failing(scores):
    """Return the index of the first failing score, or -1 if none."""
    i = 0
    while i < len(scores):
        if scores[i] < 60:
            return i
        i += 1
    return -1

result = find_first_failing([88, 72, 55, 90])
print(result)  # 2 (index of score 55)