break and continue statements

break statement

Sometimes you don't know it's time to end a loop until you get half way through the body. In that case you can use the break statement to jump out of the loop.

For example, suppose you want to take input from the user until they type done. You could write:

while True: 
    line = input('> ') 
    if line == 'done': 
        break 
    print('You wrote:', line)

print('Done!') 

The loop condition is True, which is always true, so the loop runs until it hits the break statement.

Each time through, it prompts the user with an angle bracket. If the user types done, the break statement exits the loop. Otherwise the program echoes whatever the user types and goes back to the top of the loop. Here's a sample run:

> not done
You wrote: not done 
> done 
Done!

A more general flow diagram using a break statement is shown below. In the middle of the body we have a if statement with a testA condition. If testA is true, the break statement is called and the program exits the loop immediately without executing statements M. Otherwise, the program skips over the break and continue the loop as normal.

Flow-control diagram of a simple while loop with a break statement. If testA is true, the loop will terminate immediately.

This way of writing while loops is common because you can check the condition anywhere in the loop (not just at the top) and you can express the stop condition affirmatively (stop when this happens) rather than negatively (keep going until that happens.).

continue statement

In Python, continue is a keyword used in loops (such as for and while loops) to skip over the remaining iterations of the loop and move on to the next iteration.

When the continue statement is encountered inside a loop, it immediately stops executing the rest of the statements in that iteration of the loop and moves on to the next iteration. This means that any code following the continue statement in that iteration will be skipped as shown in the figure below. In this diagram, if testB is True, the statements M is skipped and the loop moves on to the next iteration. Note that at the next iteration, testB might be False and statements M might be executed.

Flow-control diagram of a simple while loop with a continue statement. if testB is True, the statements M is skipped and the loop moves on to the next iteration.

Here's an example to illustrate the use of continue in a for loop:

numbers = [1, 2, 3, 4, 5]

for num in numbers:
    if num % 2 == 0:
        continue
    print(num)

In this example, we have a list of numbers and we're using a for loop to iterate over them. Inside the loop, we have an if statement that checks if the current number is even (i.e., divisible by 2). If it is, the continue statement is executed and the loop immediately moves on to the next iteration without executing the print statement. If the number is odd, the print statement is executed and the number is printed to the console.

The output of this code would be:

1
3
5

This is because the continue statement skipped over the even numbers (2 and 4) and only printed the odd numbers (1, 3, and 5).

In general, you should use the continue statement when you want to skip over certain iterations of a loop based on a particular condition, without exiting the loop entirely.

Some considerations

In case you are wondering, you should not avoid using break and continue statements in Python programs. These statements are important flow-control statements that can make your code more efficient and easier to read.

break and continue statements are used in loops to change the normal flow of control. break is used to exit a loop prematurely, while continue is used to skip the current iteration of the loop and move on to the next one.

Using break and continue statements can make your code more efficient by avoiding unnecessary computations. For example, if you are searching for a particular value in a large list, you can use a for loop with a break statement to exit the loop as soon as you find the value, rather than continuing to iterate through the rest of the list.

Here's an example that illustrates the use of break to exit a loop prematurely:

numbers = [1, 2, 3, 4, 5]

for num in numbers:
    if num == 3:
        break
    print(num)

In this example, we're using a for loop to iterate over a list of numbers. Inside the loop, we have an if statement that checks if the current number is equal to 3. If it is, the break statement is executed, and the loop is exited prematurely. If the number is not equal to 3, the print statement is executed, and the number is printed to the console.

The output of this code would be:

1
2

As you can see, the loop exited prematurely when it encountered the number 3, and did not print the remaining numbers in the list.

In general, you should use break and continue statements when they make your code more efficient and easier to read. However, it's also important to use them judiciously and not to overuse them, as this can make your code harder to understand and maintain.