Question

A criticism of the break statement and the continue statement is that each is unstructured. Actually, these statements can always be replaced by structured statements, although doing so can be awkward. Describe in general how you would remove any break statement from a loop in a program and replace it with some structured equivalent. [Hint: The break statement exits a loop from the body of the loop. The other way to exit is by failing the loop-continuation test. Consider using in the loop-continuation test a second test that indicates “early exit because of a ‘break’ condition.”] Use the technique you develop here to remove the break statement from the application in Fig. 5.12.

Short answer

Replace break by setting a control variable to exit, modifying the loop condition.

Step-by-step solution

Identify the break statement

Locate the break statement within the loop. This statement usually exits the loop when a certain condition is met.

Analyze the break condition

Determine the condition under which the break statement is activated. This is often an if statement that evaluates a specific condition and then uses break to exit the loop.

Introduce a control variable

Introduce a new boolean variable, let's call it 'exitLoop', and set it initially to false. This variable will be used to control when to exit the loop.

Modify the loop condition

Change the loop-continuation condition to include the control variable. For example, in a while loop, change 'while (condition)' to 'while (condition && !exitLoop)'.

Replace break with control variable

Replace the break statement with code that sets the control variable to true. When the original break condition is met, set 'exitLoop = true' instead of using break.

Test the rearranged structure

Run the program to ensure that the logic remains intact and that the loop exits as intended when the 'break' condition is met, now controlled by the control variable.

Key concepts

Break Statement

The break statement is a commonly used control structure in programming that allows you to exit a loop prematurely. While it's convenient, some argue that it leads to unstructured code. When a break statement is encountered in a loop, the loop stops immediately, and control moves to the statement following the loop. This feature can simplify code when used sparingly, but over-reliance on break can make the logic hard to follow.

To avoid using break, you can incorporate its functionality into other loop structures more gracefully. This requires modifying the loop conditions, so the loop ends naturally when a specific condition—for which you would use a break—is met. The key is to ensure the loop-continuation test anticipates the condition meant for following a break.

Loop Modification

Adjusting loops to replace break statements involves careful modifications. The goal is to redesign how the loop terminates. By default, loops like "while" or "for" have a single continuation test. Modifying the loop means adding a second condition that mimics the break behavior.

Here's how to approach this:

• Analyze why and when the break occurs.
• Determine an additional condition that indicates the same intent.
• Incorporate this condition into the loop-continuation test.

For instance, if a loop iterates while a variable is less than 10, and you want to include exit logic when another condition is met, alter the loop to break on either condition being false. This requires an additional boolean control, which brings us to the next step.

Boolean Control Variable

A Boolean control variable is a crucial tool in restructuring loops without breaks. It functions as an indicator of whether the loop should continue executing or conclude, aligning with specified conditions.

To implement this:

• Introduce a Boolean variable, often named something intuitive like 'exitLoop', and set it to false initially.
• Change your loop condition to factor in your control variable, such as modifying `while (condition)` to `while (condition && !exitLoop)`.
• Whenever the condition that would trigger a break is met, assign `exitLoop = true`.

This method keeps the loop structured and clear, ensuring you exit not by a break statement but by failing the loop condition naturally.

Loop Continuation Test

The loop continuation test is an essential part of any loop, determining whether another iteration should occur. When replacing a break statement, you must alter this test to consider additional conditions, ensuring seamless loop control.

Enhancing the loop continuation test involves:

• Monitoring the primary loop condition alongside a Boolean control variable.
• Embedding logic within the test that checks for the "early exit" symptom, paralleling a break scenario.

Let's break this down: if a loop originally ran with `while (some_condition)`, your modified version might look like `while (some_condition && !exitLoop)`. When the break criteria manifest, setting `exitLoop` to true naturally ends the loop without direct intervention. This structured approach helps maintain legibility and flow in your code, making it easier to understand and debug.