The while Loop in Python

By the end you should be able to: Write Python while loops with correct initialisation, condition and update, use sentinel-controlled input, and choose between while and for appropriately.

while repeats as long as a condition holds. Use it when you do not know in advance how many iterations you need.

i = 1                   # 1. initialise
while i <= 5:           # 2. condition
    print(i)
    i += 1              # 3. update

Same three parts as Module 2, same failure if you drop the update — an infinite loop.

Choosing between for and while

Use	When
`for`	The number of iterations is known: a fixed count, or every item of a sequence
`while`	The stopping point is discovered during execution

If you find yourself writing while with a counter you increment yourself, a for is almost always clearer:

i = 0                       # works, but
while i < len(items):       # this is a for loop in disguise
    print(items[i])
    i += 1

for item in items:          # say what you mean
    print(item)

Sentinel-controlled input

total = 0
x = float(input("Enter a number (0 to stop): "))   # priming read

while x != 0:
    total += x
    x = float(input("Enter a number (0 to stop): "))

print("Sum is", total)

The priming read before the loop and the second read at the end of the body are both essential — exactly as in Module 2. Reading only at the top of the body would add the sentinel to the total.

The `while True` / `break` idiom

Python has no repeat-until. When you need at-least-once behaviour, this is the standard replacement:

while True:
    marks = int(input("Enter a mark (0-100): "))
    if 0 <= marks <= 100:
        break
    print("Out of range, try again.")

This is the direct translation of Module 2's REPEAT … UNTIL marks >= 0 AND marks <= 100. Note the condition is not negated here — break fires on the success case, whereas a while condition describes the continue case. Getting that backwards is a common slip.

Infinite loops, in Python terms

while i < 10:           # i never changes
    print(i)

while x != 0.1:         # float equality may never hold exactly
    x -= 0.01

The second is a real trap and connects to the float discussion in the selection topic: prefer < or > over == and != with floats.

If you do hang a program, Ctrl+C stops it.

while … else

Like for … else, the else runs only if the loop ended without break:

while attempts < 3:
    ...
else:
    print("Ran out of attempts")