
# “for” and “while”, “break” and “else:”¶

In Brisk introduction to Python, we saw the use of break in for and while loops.

for and while loops that use break, can be followed by else: clauses. The else: clause executes only when there was no break during the loop.

In the next fragment, we are doing an inefficient search for prime numbers from 2 through 30. In this basic for loop, we use the is_prime variable as a flag to indicate whether we have found the current number to be prime:

>>> primes = []
>>> for x in range(2, 30):
...     # Assume x is prime until shown otherwise
...     is_prime = True
...     for p in primes:
...         # x exactly divisible by prime -> x not prime
...         if (x % p) == 0:
...             is_prime = False
...             break
...     if is_prime:
...         primes.append(x)
...
>>> print("Primes in 2 through 30", primes)
Primes in 2 through 30 [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]


Using a flag variable like is_prime is a common pattern, so Python allows us to do the same thing with an extra else: clause:

>>> primes = []
>>> for x in range(2, 30):
...     for p in primes:
...         # x exactly divisible by prime -> x not prime
...         if (x % p) == 0:
...             break
...     else:
...         # else: block executes if no 'break" in previous loop
...         primes.append(x)
...
>>> print("Primes in 2 through 30", primes)
Primes in 2 through 30 [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]