Theory：Branching statements

Branching statements are used to alter the standard behavior of loops; they can terminate a loop or skip some iterations.

# The break statement

The break statement has two uses:

it terminates the current loop of any type (for, while, do-while);
it terminates a case in the switch statement;

In this topic, we will learn how to use it to terminate loops.

The following example demonstrates a loop that includes one break.

int i = 10;
while (true) { // the condition to continue the loop
    if (i == 0) { // the condition to perform the break that stops this loop 
        break;
    }
    i--;
}

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In the code above, the condition to continue the loop is always true, but it will be successfully stopped when the variable i becomes 0 through the use of break inside the conditional statement.

The break statement terminates only the loop in which it is currently located. If this loop is performed inside another loop, the outer loop won't be stopped.

The following code prints a ladder of numbers.

for (int i = 0; i < 10; i++) {
    for (int j = 0; j < 10; j++) {
        System.out.print(j + " ");
        if (i == j) {
            break;
        }
    }
    System.out.println();
}

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The break statement can't stop the outer loop (with variable i) and the code prints:

0 
0 1 
0 1 2 
0 1 2 3 
0 1 2 3 4 
0 1 2 3 4 5 
0 1 2 3 4 5 6 
0 1 2 3 4 5 6 7 
0 1 2 3 4 5 6 7 8 
0 1 2 3 4 5 6 7 8 9

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To stop the outer loop we'd like to declare a Boolean variable stopped and use it as a special Boolean flag.

boolean stopped = false;
for (int i = 0; (i < 10) && !stopped; i++) {
    for (int j = 0; j < 10; j++) {
        System.out.print(j + " ");
        if (i == j) {
            stopped = true;
            break;
        }
     }
    System.out.println();
}

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Now, the program's output is not the same:

There is another way to stop the outer loop: the labeled break operator. However, it's not good practice to use it. Google it if you are really interested.

# The continue statement

It causes a loop to skip the current iteration and go to the next one.

This statement can be used inside any kind of loops.

inside the for-loop, the continue causes control to immediately move to the increment/decrement statement;
inside the while or do-while loop, control immediately moves to the condition.

In the following example, a sequence of numbers is output. Odd numbers are skipped.

int n = 10;
for (int i = 0; i < n; i++) {
    if (i % 2 != 0) {
        continue;
    }
    System.out.print(i + " ");
}

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The output:

0 2 4 6 8

The continue statement and the break statement only affect the loop in which they are located. The continue statement cannot skip the current iteration of the outer loop.

Often, we can rewrite our loop without using the continue statement. Here is an example:

int n = 10;
for (int i = 0; i < n; i++) { 
    if (i % 2 == 0) {
        System.out.print(i + " ");
    } 
}

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The result is the same as above, but the code became shorter and more readable.

It is important to note that the widespread use of branching statements leads to poorly-structured code because conditions in your loops are not actually what you need to do. So, use them wisely — only when it helps to make code shorter and easier to understand for humans.

编辑

#Java basic

上次更新: 2022/09/26, 16:55:15

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