while Loops

Looping Constructs

Program flow is rarely linear …

  • Branches ⟶ if/elif/else

  • Repeated execution ⟶ loops

  • Python has only two looping constructs

  • while

    • Handcrafted loop condition

    • ⟶ very verbose coding

    • Most general looping construct

  • for

    • iteration over something sequencish

    • Iteration … generators … yield … outright genius!

    • ⟶ later

while Loops

General form of a ``while`` loop …

while condition:
    statements

  • condition is a boolean expression

  • statements is an indented block of … well

    … statements

  • Block is executed while condition holds

Example: sum of numbers 1..100

sum = 0
i = 1
while i <= 100:
    sum += i
    i += 1

break and continue

Fine grained loop control …

  • break ends the loop

  • continue ends the current loop and continues with the next - evaluating the condition

while True:
    line = sys.stdin.readline()
    for c in line: print(c, ord(c))
    if len(line) == 0:
        # eof seen
        break
    if line.strip() == '':
        # ignore empty lines
        continue

    # ... do something ...

Esoteric Feature: while/else

Loops can have an “else” clause

  • Entered when loop terminates naturally

  • … not terminated by a break

  • Natural while loop termination: loop condition evaluates to False

i = 0
while i < 100:
    i += 1
    number = random.randrange(0,1001)
    if number == 42:
        break
else:
    print('no answer found')