`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

Note

Pythonicity

This example is rather contrived. One would rather use the built-in sum() function (documentation here), combined with the range() function (documentation here) to do the same.

sum(range(1,101))

`break` and `continue`¶

Fine grained loop control …

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

For example: roll dice, until it shows six eyes …

dice-infinite.py¶

import random

while True:
    eyes = random.randrange(1,7)
    if eyes == 6:
        print('hooray!')
        break

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

For example: roll dice six times. Win when it shows six eyes at least once, lose when not.

Non-pythonic (using a flag)

Pythonic (else on while)

dice-sixtimes-flag.py¶

import random

seen_sixeyes = False
n_tries = 0
while n_tries < 6:
    n_tries += 1
    eyes = random.randrange(1,7)
    if eyes == 6:
        seen_sixeyes = True
        break

if seen_sixeyes:
    print('hooray!')
else:
    print('lose!')

dice-sixtimes-else.py¶

import random

n_tries = 0
while n_tries < 6:
    n_tries += 1
    eyes = random.randrange(1,7)
    if eyes == 6:
        print('hooray!')
        break
else:
    print('lose!')

while Loops¶

Looping Constructs¶