Programming Logic and Design, Seventh Edition презентация

Programming Logic and Design Seventh Edition Chapter 3 Understanding Structure

Objectives In this chapter, you will learn about: The disadvantages of unstructured spaghetti code The three basic structures—sequence, selection, and loop Using a priming input to structure a program The need for structure Recognizing structure Structuring and modularizing unstructured logic

The Disadvantages of Unstructured Spaghetti Code Spaghetti code Logically snarled program statements Often a complicated mess Programs often work but are difficult to read and maintain Confusing and prone to error Unstructured programs Do not follow the rules of structured logic Structured programs Follow the rules of structured logic

Understanding the Three Basic Structures Structure Basic unit of programming logic Sequence structure Perform actions in order No branching or skipping any task Selection structure (decision structure) Ask a question, take one of two actions Dual-alternative ifs or single-alternative ifs Loop structure Repeat actions while a condition remains true

Understanding the Three Basic Structures (continued)

Understanding the Three Basic Structures (continued)

Understanding the Three Basic Structures (continued) Dual-alternative ifs Contain two alternatives The if-then-else structure

Understanding the Three Basic Structures (continued) Single-alternative ifs An else clause is not required null case Situation where nothing is done

Understanding the Three Basic Structures (continued)

Understanding the Three Basic Structures (continued) Loop structure Repeats a set of actions while a condition remains true Loop body Also called repetition or iteration Condition is tested first in the most common form of loop The while…do or while loop

Understanding the Three Basic Structures (continued)

Understanding the Three Basic Structures (continued) Loop structure

Understanding the Three Basic Structures (continued) All logic problems can be solved using only sequence, selection, and loop Structures can be combined in an infinite number of ways Stacking structures Attaching structures end-to-end End-structure statement Indicates the end of a structure The endif statement ends an if-then-else structure The endwhile statement ends a loop structure

Understanding the Three Basic Structures (continued)

Understanding the Three Basic Structures (continued) Any individual task or step in a structure can be replaced by a structure Nesting structures Placing one structure within another Indent the nested structure’s statements Block A group of statements that execute as a single unit

Understanding the Three Basic Structures (continued)

Understanding the Three Basic Structures (continued)

Understanding the Three Basic Structures (continued)

Understanding the Three Basic Structures (continued) Structured programs have the following characteristics: Include only combinations of the three basic structures Each structure has a single entry point and a single exit point Structures can be stacked or connected to one another only at their entry or exit points Any structure can be nested within another structure

Using a Priming Input to Structure a Program Priming input (or priming read) Reads the first input data record Is outside the loop that reads the rest of the records Helps keep the program structured Analyze a flowchart for structure one step at a time Watch for unstructured loops that do not follow this order First ask a question Take action based on the answer Return to ask the question again

Using a Priming Input to Structure a Program (continued)

Using a Priming Input to Structure a Program (continued)

Understanding the Reasons for Structure Clarity—unstructured programs are confusing Professionalism—other programmers expect it Efficiency—most languages support it Ease of maintenance—other programmers find it easier to read Supports modularity—easily broken down into modules It can be difficult to detect whether a flowchart is structured

Recognizing Structure (continued)

Summary Spaghetti code Statements that do not follow rules of structured logic Three basic structures Sequence, selection, and loop Combined by stacking and nesting Priming input Statement that reads the first input value prior to starting a structured loop

Summary (continued) Structured techniques promote: Clarity Professionalism Efficiency Modularity Flowcharts can be made structured by untangling Logical steps can be rewritten to conform to the three structures