6.9: Principles of Loop Design

Before moving on, it will be useful to summarize the main principles involved in correctly constructing a loop.

• A counting loop can be used whenever you know in advance exactly how many iterations are needed. Java’s for statement is an appropriate structure for coding a counting loop.
• A while structure should be used when the problem suggests that the loop body may be skipped entirely. Java’s while statement is specially designed for the while structure.
• A do-while structure should be used only when a loop requires one or more iterations. Java’s do-while statement is specially designed for the do-while structure.
• The loop variable is used to specify the loop-entry condition. It must be initialized to an appropriate initial value, and it must be updated on each iteration of the loop.
• A loop’s bound may be a count, a sentinel, or, more generally, a conditional bound. It must be correctly specified in the loop-entry expression, and progress toward the bound must be made in the updater.
• An infinite loop may result if the initializer, loop-entry expression, or updater expression is not correctly specified.

The loop types are also summarized in Table 6.1.

Table 6.1: A summary of the design decisions required when coding a loop

Use	If	Java Statement
Counting loop	Number of iterations known in advance	for
While structure	Number of iterations not known Loop may not be entered at all	while
Do-while structure	Number of iterations not known Loop must be entered at least once	do-while

Exercise \(\PageIndex{1}\)

For each of the following problems, decide whether a counting loop structure, a while structure, or a do-while structure should be used, and write a pseudocode algorithm.

• Print the names of all visitors to your Web site.
• Validate that a number input by the user is positive.
• Change all the backslashes (\(\backslash\)) in a Windows Web page address to the slashes (/) used in a Unix Web page address.
• Find the car with the best miles-per-gallon ratio among the cars in the Consumer Reports database.

 

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