Pseudo Code and Flow Charts. Chapter 1 Lesson 2

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1 Pseudo Code and Flow Charts Chapter 1 Lesson 2

2 Pseudocode Using Pseudocode Statements and Flowchart Symbols English-like representation of the logical steps it takes to solve a problem Flowchart Pictorial representation of the logical steps it takes to solve a problem 2

3 Writing Pseudocode Pseudocode representation of a numberdoubling problem start stop input mynumber set myanswer = mynumber * 2 output myanswer 3

4 Writing Pseudocode (continued) 4

5 Drawing Flowcharts Create a flowchart Draw geometric shapes that contain the individual statements Connect shapes with arrows Input symbol Indicates input operation Parallelogram Processing symbol Contains processing statements such as arithmetic Rectangle 5

6 Drawing Flowcharts (continued) Output symbol Represents output statements Parallelogram Flowlines Arrows that connect steps Terminal symbols Start/stop symbols Shaped like a racetrack Also called lozenges 6

7 Drawing Flowcharts (continued) 7 Figure 1-6 Flowchart and pseudocode of program that doubles a number

8 Repeating Instructions Program in Figure 1-6 only works for one number What if I wanted to obtain and double 10,000 numbers instead of just one? Not feasible to run the program over and over 10,000 times We could repeat the lines of code that get a number, double it and output 10,000 times in the program Not feasible to add these lines of code to the program 10,000 more times! This is a code maintenance nightmare. The solution: Create a loop (repetition of a series of steps) instead Avoid an infinite loop (repeating flow of logic that never ends) 8

9 Repeating Instructions (continued) Figure 1-7 Inefficient pseudocode for program that doubles 10,000 numbers 9

Repeating Instructions (continued) In the flowchart to the right, we see a loop in action. Notice the flowline bending back up to get another number, double it and output the result.

10 Repeating Instructions (continued) In the flowchart to the right, we see a loop in action. Notice the flowline bending back up to get another number, double it and output the result. The action keeps repeating. The memory locations used to store mynumber and myanswer are reused each time through the loop to process the next number (variables) The idea to use a loop is good, but notice this loop never ends. It is called an infinite loop and is usually something we want to avoid. Figure 1-8 Flowchart of infinite numberdoubling program 10

11 Using a Sentinel Value to End a Program So how can we create logic that repeats but eventually stops? The key is the ability to make a decision within the program We can test a value to determine whether the loop should continue Flowchart Decision symbol Diamond shape There are always two possible outcomes of a decision yes or no (true or false) Use a dummy value to indicate that the loop should end Data-entry value that the user will never need as a real value User enters the value to indicate that the loop should end To do this the user must know what the value is to type when they are done processing This dummy value is called a Sentinel value The decision symbol above tests to see if mynumber is equal to the sentinel value of zero 11

12 Using a Sentinel Value to End a Program So the example in the previous slide showed a decision symbol that checks to see if mynumber is zero (or not) The idea here is that the user will enter zero when they are done using the program. However, not all programs rely on user data entry from a keyboard. Many computer programs read data from an input device, such as a disk. Our program may be reading data from a file on the C drive, for example. When this is the case, we often need to check to see if we have reached the end of the data in the file When we reach the end of the file, we want to stop the loop Many programming languages used the term eof ( end of file ) to indicate that we have reached the end of the file Each programming language has a way to determine if we have reached the end of the file. We will use eof to represent this in our logic. The eof marker at the end of a file automatically acts as a sentinel 12

13 Using a Sentinel Value to End a Program The next slide shows how to represent a loop that checks for the mynumber value of 0. When the value of 0 is encountered, the loop (and program) ends The second flowchart is almost identical but checks for eof and stops the loop (and program) if eof is encountered Although the figure from the book indicates don t do it, we are going to accept this solution for now. The book is indicating that there is a better way to structure the solution and we will learn how to do it in Chapter 3! For now, let s just get acquainted with the flowcharting symbols and how they can represent our logic. 13

14 Using a Sentinel Value to End a Program (continued) Figure 1-9 Flowchart of number-doubling program with sentinel value of 0 14 Figure 1-10 Flowchart using eof

15 Understanding Programming Environments Understanding Programming Environments Recall that you begin solving a problem with a pseudocode or flowchart solution, however eventually you will need to translate that logic into a programming language A programming environment is where you can go to actually create your programming solution and execute (run) it Text Editor is used to create simple text files Examples are Notepad or Notepad++ Doesn t require a lot of disk space Integrated Development Environment (IDE) provides an editor, compiler, and other programming tools Microsoft Visual Studio IDE Recall that we are using this for the class! Regardless of whether a text editor or an IDE is used, the program itself and its logic is the same. 15

Understanding Programming Environments (continued) 16 Figure 1-12 A C# number-doubling program in Visual Studio One of the major advantages of using an IDE is that different colors are used by

16 Understanding Programming Environments (continued) 16 Figure 1-12 A C# number-doubling program in Visual Studio One of the major advantages of using an IDE is that different colors are used by the IDE to make items easier to find. It will also highlight syntax errors for you, offers code completion, includes debugging tools, and allows you to run your program from within the software.

17 Understanding User Environments Understanding User Environments A user might execute a program you have written in any number of environments. One example is command line. Command Line is a location on your computer screen at which you type text entries to communicate with the computer s operating system Many programs are not run at the command line, but are run using a graphical user interface. A graphical user interface, or GUI (pronounced gooey), allows users to interact with a program in a graphical environment Regardless of the type of program, the logical process behind the scenes is the same. 17

18 Understanding User Environments (continued) Figure 1-13 Executing a number-doubling program in a command-line environment 18

19 Understanding User Environments (continued) Figure 1-14 Executing a number-doubling program in a GUI environment 19

20 Understanding the Evolution of Programming Models People have been writing modern computer programs since the 1940s Newer programming languages Look much more like natural language Are easier to use Create self-contained modules or program segments that can be pieced together in a variety of ways 20

21 Understanding the Evolution of Programming Models (continued) Major models or paradigms used by programmers Procedural programming Focuses on the procedures that programmers create Object-oriented programming Focuses on objects, or things, and describes their features (or attributes) and their behaviors 21

22 Summary Hardware and software accomplish input, processing, and output Logic must be developed correctly Logical errors are much more difficult to locate than syntax errors Use flowcharts, pseudocode, IPO charts, and TOE charts to plan the logic Avoid infinite loops by testing for a sentinel value Use a text editor or an IDE to enter your program statements 22

Chapter 1: An Overview of Computers and Logic

Chapter 1: An Overview of Computers and Logic Programming Logic and Design, Third Edition Comprehensive Objectives After studying Chapter 1, you should be able to: Understand computer components and operations