quebrar, continuar e mais em loops

Inside a loop (both for and while) you can alter the normal flow with three constructs:

• break — exits the loop immediately.
• continue — skips the rest of the body and jumps to the next iteration.
• else (on the loop) — executed when the loop terminates without a break.

break

for n in [3, 7, 11, 4, 9]:
    if n % 2 == 0:
        primo_pari = n
        break
primo_pari  # 4

break stops the loop as soon as possible. Anything after break in the current iteration's body is NOT executed.

continue

risultati = []
for n in range(1, 11):
    if n % 2 == 0:
        continue   # skip evens
    risultati.append(n * n)
risultati  # [1, 9, 25, 49, 81]

Think of continue as "go to the next iteration". It is used to filter items without adding one more indentation level.

else on the loop (Pythonic idiom)

A Python peculiarity: loops can also have an else, executed only if the loop terminated naturally (without break). It is perfect for searches:

target = 7
for n in [2, 4, 6, 8]:
    if n == target:
        trovato = True
        break
else:
    trovato = False
trovato  # False

Read it as: "for each n in the list; if you take the break branch, skip the else; otherwise execute the else".

Nested loops: break exits only the innermost one

for x in range(3):
    for y in range(3):
        if x == y == 1:
            break
        print(x, y)

break exits only the for y, not the for x. To exit both you must use a flag or a function + return.

The unusual loop...else construct

In Python, both for and while loops can feature an optional else clause. The else block is executed only if the loop completes normally (i.e. without encountering a break statement). This is very helpful for search algorithms (e.g. "search for an element: if found, break; else, report that it was not found").

Try it yourself

Review exercise

Additional challenge