24 min read

while Loops

Repeat while a condition is true — running totals with a sentinel, PIN attempts, and avoiding infinite loops.

The while Loop

A while loop repeats its block as long as its condition is True. Use it when you do not know in advance how many repeats are needed.

Counting with whilePython

1count = 12while count <= 5:3    print("Count is", count)4    count += 15print("Done")

Expected output

Count is 1
Count is 2
Count is 3
Count is 4
Count is 5
Done

The same three parts as a flowchart loop: start value (count = 1), condition (count <= 5) and change (count += 1).

Adding Prices Until 0

The sentinel loop from the flowchart course

Read this flowchart as text

Terminator: Start (then → total = 0)
Process: total = 0 (then → INPUT price)
Input/output: INPUT price (then → price > 0?)
Decision: price > 0? (Yes → total = total + price; No → OUTPUT total)
Process: total = total + price (then → INPUT price)
Input/output: OUTPUT total (then → Stop)
Input/output: INPUT price (then → price > 0?)
Terminator: Stop

till.pyPython

1total = 02price = float(input("Price (0 to finish): "))3 4while price > 0:5    total += price6    price = float(input("Price (0 to finish): "))7 8print(f"Total: LKR {total:,.2f}")

Inputs (one per line — used by input() in order)

Expected output

Price (0 to finish): 250
Price (0 to finish): 480
Price (0 to finish): 120
Price (0 to finish): 0
Total: LKR 850.00

Notice the input line appears twice : once before the loop (to get the first price), and again at the end of the loop body (to get the next one). This is the standard pattern for a sentinel loop.

A Loop with Two Conditions

Up to three PIN attempts

Read this flowchart as text

Terminator: Start (then → attempts = 0 pin = 0)
Process: attempts = 0 pin = 0 (then → pin != 4321 and attempts < 3?)
Decision: pin != 4321 and attempts < 3? (Yes → INPUT pin; No → pin == 4321?)
Input/output: INPUT pin (then → attempts = attempts + 1)
Decision: pin == 4321? (Yes → OUTPUT "Welcome"; No → OUTPUT "Card locked")
Process: attempts = attempts + 1 (then → pin != 4321 and attempts < 3?)
Input/output: OUTPUT "Welcome" (then → Stop)
Input/output: OUTPUT "Card locked" (then → Stop)
Terminator: Stop

pin.pyPython

1attempts = 02pin = 03 4while pin != 4321 and attempts < 3:5    pin = int(input("Enter PIN: "))6    attempts += 17 8if pin == 4321:9    print("Welcome")10else:11    print("Card locked")

Inputs (one per line — used by input() in order)

Expected output

Enter PIN: 1111
Enter PIN: 4321
Welcome

for or while?

$for$	$while$
$You know how many times (or have a range to go through)$	$You repeat until something happens$
$The counter changes automatically$	$You must change the condition yourself$
$Print 10 labels; a times table$	$Add prices until 0; ask until the PIN is right$

Infinite Loops

An infinite loop — press Run and wait for it to be stoppedPython

1count = 12while count <= 5:3    print(count)

Practice

In till.py, why is there an input() line inside the loop as well as before it?

More lessons in Python Programming · Next: Functions · Previous: for Loops and range()

The while Loop

A while loop repeats its block as long as its condition is True. Use it when you do not know in advance how many repeats are needed.

Counting with whilePython

1count = 12while count <= 5:3    print("Count is", count)4    count += 15print("Done")

Expected output

Count is 1
Count is 2
Count is 3
Count is 4
Count is 5
Done

The same three parts as a flowchart loop: start value (count = 1), condition (count <= 5) and change (count += 1).

Adding Prices Until 0

The sentinel loop from the flowchart course

Read this flowchart as text

Terminator: Start (then → total = 0)
Process: total = 0 (then → INPUT price)
Input/output: INPUT price (then → price > 0?)
Decision: price > 0? (Yes → total = total + price; No → OUTPUT total)
Process: total = total + price (then → INPUT price)
Input/output: OUTPUT total (then → Stop)
Input/output: INPUT price (then → price > 0?)
Terminator: Stop

till.pyPython

1total = 02price = float(input("Price (0 to finish): "))3 4while price > 0:5    total += price6    price = float(input("Price (0 to finish): "))7 8print(f"Total: LKR {total:,.2f}")

Inputs (one per line — used by input() in order)

Expected output

Price (0 to finish): 250
Price (0 to finish): 480
Price (0 to finish): 120
Price (0 to finish): 0
Total: LKR 850.00

Notice the input line appears twice : once before the loop (to get the first price), and again at the end of the loop body (to get the next one). This is the standard pattern for a sentinel loop.

A Loop with Two Conditions

Up to three PIN attempts

Read this flowchart as text

Terminator: Start (then → attempts = 0 pin = 0)
Process: attempts = 0 pin = 0 (then → pin != 4321 and attempts < 3?)
Decision: pin != 4321 and attempts < 3? (Yes → INPUT pin; No → pin == 4321?)
Input/output: INPUT pin (then → attempts = attempts + 1)
Decision: pin == 4321? (Yes → OUTPUT "Welcome"; No → OUTPUT "Card locked")
Process: attempts = attempts + 1 (then → pin != 4321 and attempts < 3?)
Input/output: OUTPUT "Welcome" (then → Stop)
Input/output: OUTPUT "Card locked" (then → Stop)
Terminator: Stop

pin.pyPython

1attempts = 02pin = 03 4while pin != 4321 and attempts < 3:5    pin = int(input("Enter PIN: "))6    attempts += 17 8if pin == 4321:9    print("Welcome")10else:11    print("Card locked")

Inputs (one per line — used by input() in order)

Expected output

Enter PIN: 1111
Enter PIN: 4321
Welcome

for or while?

$for$	$while$
$You know how many times (or have a range to go through)$	$You repeat until something happens$
$The counter changes automatically$	$You must change the condition yourself$
$Print 10 labels; a times table$	$Add prices until 0; ask until the PIN is right$

Infinite Loops

An infinite loop — press Run and wait for it to be stoppedPython

1count = 12while count <= 5:3    print(count)

Practice

In till.py, why is there an input() line inside the loop as well as before it?