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Java, Java, Java: Object-Oriented Problem Solving, 2022E

Section 6.9 Principles of Loop Design

Subsection 6.9.1 Loop Summary

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 the table below.
Table 6.9.1. 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 while
Loop may not be entered at all
Do-while structure Number of iterations not known do-while
Loop must be entered at least once

Exercises Self-Study Exercise

1. Loop Choices.
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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