3.4 break and continue

break and continue both change the normal flow of a loop, but they mean different things.

• break: end the current loop immediately.
• continue: skip the rest of the current iteration and start the next one.

break

Use break when the loop has already achieved its goal, or when continuing the loop no longer makes sense.

The program below checks whether an integer is prime. As soon as it finds one divisor of n, it knows n is not prime, so it can stop checking.

import math

n = int(input("Enter an integer: "))

is_prime = n > 1

for i in range(2, int(math.sqrt(n)) + 1):
    if n % i == 0:
        is_prime = False
        break

if is_prime:
    print(n, "is a prime number")
else:
    print(n, "is not a prime number")

One possible run:

Enter an integer: 17
17 is a prime number

There is no need to check all the way to n - 1. If a number has a factor larger than its square root, it must also have a paired factor smaller than its square root, so checking through sqrt(n) is enough.

continue

continue does not end the whole loop. It skips the remaining code in the current iteration and moves directly to the next iteration.

The program below reads 10 integers and computes the sum of squares of non-negative integers. When it sees a negative number, it skips that iteration.

n = 10
print("Enter %d integers:" % n)

sum_square = 0

for i in range(n):
    num = int(input())

    if num < 0:
        continue

    sum_square += num * num

print("Sum of squares of non-negative integers:", sum_square)

One possible run:

Enter 10 integers:
5
7
-2
0
4
-4
-9
3
9
5
Sum of squares of non-negative integers: 205

When num < 0, continue skips sum_square += num * num, so negative numbers do not contribute to the result. Note that 0 is not negative; its square is 0, so adding it does not change the total.

Choosing between them

Use break when you want the whole loop to stop. Use continue when you only want to skip the rest of the current iteration and let the next iteration begin normally.

for x in range(1, 6):
    if x == 3:
        continue
    print(x)

Output:

1
2
4
5

The value 3 is skipped, but the loop does not end.

Comparison experiment

After reading what break and continue mean, use the component below to compare them step by step. When the loop reaches x == 3, continue only skips the current iteration, while break ends the whole loop early.