Department of Computer Science University of Pretoria. Introduction to Computer Science COS 151

Transcription

1 Department of Computer Science University of Pretoria Introduction to Computer Science COS 151 Practical 1 16 February Plagiarism Policy The Department of Computer Science considers plagiarism as a serious offence. Disciplinary action will be taken against students who commit plagiarism. Plagiarism includes copying someone else s work without consent, copying a friend s work (even with consent) and copying material (such as text or program code) from the Internet. Copying will not be tolerated in this course. For a formal definition of plagiarism, the student is referred to index.htm (from the main page of the University of Pretoria site, follow the Library quick link, and then click the Plagiarism link). If you have any form of question regarding this, please ask one of the lecturers, to avoid any misunderstanding. Also note that the principle of code re-use does not mean that you should copy and adapt code to suit your solution. Note that all assignments submitted for this module implicitly agree to this plagiarism policy, and declare that the submitted work is the students own work. Assignments will be submitted to a variety of plagiarism checks. Any typed assignment may be checked using the Turnitin system. After plagiarism checking, assignments will not be permanently stored on the Turnitin database. 2 Linux You will be expected to use the Linux operating system for this course. You can log into Linux on the lab computers inside the informatorium. You do so by selecting Linux when the computer starts and entering your username and password that you use when logging onto the cs website. 3 Basic Linux Commands Here are a few basic commands that you should familiarize yourself with. You type these commands into a terminal to instruct the computer to perform the action you want it to perform. You may need additional commands so we expect of you to use internet resources, such as Google, in the case that you do. ls - this command lists the files in a directory. cd dir - cd(change directory) is used to change directory. In this example it changes to a directory called dir. mkdir dir - mkdir(make directory) is used to make a directory. In this example a directory named dir is made. 1

2 touch file - this command creates or updates a file named file. rm file - this command deletes a file named file. rm -r dir - this command deletes a directory named dir. cp file1 file2 - this command that copies file1 to file2. mv file dir - this command moves the file to the specified directory. tar cvzf file.tar.gz file1 file this command creates a tar with gzip compresstion. This is what is uploaded with all relevant files contained within it. tar xzvf file.tar.gz - this command extracts files from a tar using gzip. This is an example of the terminal with the ls command entered. As you can see it lists the contents of the current directory. The white text contains names of files. The blue of directories. This varies from Linux distributions. 4 Task 1 In this task you will need to familiarize yourself with Linux, the terminal and terminal commands. To do this you will have to follow each step in creating a certain file structure. Next to each step is the command that you should make use of to accomplish the step. 1. Create a directory named LinuxPrac1 (mkdir dir) 2. Change directory to LinuxPrac1 (cd dir) 3. Create a file named musiclyrics.txt (touch file) 4. List contents of the current directory (ls) 5. Open the file using Scite and write Twinkle Twinkle to the file (scite file) 6. Save the file and close Scite 7. Create a copy of the file musiclyrics.txt and name it musiclyricsv2.txt (cp file1 file2) 8. Create a directory named Remix (mkdir dir) 9. List contents of the current directory (ls) 2

3 10. Move musiclyricsv2.txt to the Remix directory (mv file dir) 11. Change directory to Remix 12. List contents of the current directory (ls) 13. Delete musiclyricsv2.txt (rm file) 14. Change directory back to LinuxPrac1. Note: this directory is one level higher than the current one, to move one directory up use (cd../) 15. Delete the directory Remix (rm r dir) The result if all steps were followed correctly should be the directory LinuxPrac1 with a single file inside called musiclyrics.txt 5 Task 2 In this task you will create a simple Java program that prints to the screen Hello to my first COS151 practical. 1. Create a file named HelloWorld.java 2. Open the file in a text editor like Scite and enter the following lines of code: 01 public class HelloWorld { 02 public static void main(string [] args) { 03 System.out.println( Hello to my first COS151 practical ); 04 } 05 } Explanation of Code Line 01: This is the first line of our java program. Every java application must have at least one class definition that consists of class keyword(class) followed by class name(helloworld). Line 02: This is our next line in the program, lets break it down to understand it: public: This makes the main method public that means that we can call the method from outside the class. static: We do not need to create object for static methods to run. They can run itself. void: It does not return anything. main: It is the method name. This is the entry point method from which the JVM can run your program. (String[] args): Used for command line arguments that are passed as strings. Line 03: This method prints the contents inside the double quotes into the console and inserts a newline after. If you are interested in further learning about Java, there are many resources available online to assist you in doing so. 3. Save and close the file, then create a file named makefile. 4. Open the file in a text editor like Scite and enter the following Note that the space in front of javac is a single tab. This space is important when creating makefiles, so make sure you include it when writting the file. Makefiles are used to automate the compilation and running of code. You can run commands from a makefile using the make command. In this case it will run the javac HelloWorld.java command, this creates the executable code. The make run command will then run java HelloWorld, which instructs the computer to execute the program. HelloWorld: 3

4 run: javac HelloWorld.java java HelloWorld 5. Once this is done open the terminal. Navigate to the directory where these files are and enter the command make. This compiles the code. Then enter the command make run. This will run your program and output the message to the terminal. 6 Introduction to JavaBlock In some of your practical sessions you will be making use of JavaBlock to draw FlowCharts. This practical is to familiarize you with JavaBlock and how it works. Finding JavaBlock We assume that you are currently logged in to Linux. If you have difficulty doing this, please consult one of the tutors or teaching assistants. Find the JavaBlock icon on your desktop or click on the Applications Menu button Development JavaBlock. This will start the JavaBlock application. Getting started You will find the basic user interface of JavaBlock. To create a new flowchart, Click on File New. Now you can draw a flowchart using the symbols discussed in class. Refer to the help sheet at the end of this document for detailed instructions on drawing a flowchart. 4

5 Exiting If you want to exit click on File Exit. The following Dialog will appear in Polish. It asks if you want to save before exiting. If you haven t saved the flowchart yet, click on the Yes button, otherwise click on the No button. Logging Out Please note this is an important step!! The final step of this practical is an extremely important step. This is the procedure for logging off. The reason why this is important is that if you do not log off, the next person walking into lab might be able to abuse your account (this could include using your internet credit and performing various illegal activities you could be held responsible for). To log off, click on the Applications Menu button, and then Logout. This will cause a new window to pop up. Click Restart. 5

6 7 Task 3 Omelet Order Program In this practical you will create a flowchart for an omelet order handling program. The program should read in a persons name and the number of eggs they would like in their omelet. The program should then display the name of the person and the number of eggs that they would like in their omelet. Note that the name of the person and the number of eggs should be input separately. In other words, you must first prompt the user to type in the name, read the name into a variable, and then prompt the user for the number of eggs, which should be read into a second variable. Declare both the variables, and name them name and numeggs. Example: 1. Enter the persons name Sally 2. Enter the number of eggs 5 3. Sally ordered an omelet made with 5 eggs Important notes: Refer to the help sheet, below, for further details on how to construct the flowchart. Help Sheet The following points list details that will help you with this practical and the remaining practicals after this one. This list will not be re-published in every practical, so refer to this document if you need to refresh your memory in future: 1. To add a block to the scheme simply select the block you want to add from the left panel, or you can right-click in the empty workspace and select the block from the context menu. Before adding a new block, make sure nothing is selected by clicking anywhere on the empty page, otherwise it will create a flowline between the selected block and the newly added block. 2. To connect two blocks to one another using a flowline, simply select your first block by left-clicking on the block from which the arrow is to start from. Then press and hold the CTRL key on your keyboard, and select the block to which the arrow should point. For input and output parallelograms, processing blocks, and terminal symbols, this will also remove any previous outgoing connection the block had. For decision diamonds (only used in later practicals, the first connection will be the Yes or True branch, while the second will be the No or False branch). 3. To delete a flowline connection, select a block, and right-click on it. From the context menu that pops up, select Delete out connections, Delete in connection, or Delete all connections to remove arrows connected to the block that are either outputs, inputs, or both. 4. For clarity, you should declare your variables at any point before they are used. JavaBlock uses var to declare variables in a processing block. For example, var a declares a variable called a. You can declare more than one variable using a comma separated list. 6

7 5. You may not use the following input or output blocks in your flowchart: 6. You must make use of the following input/output parallelogram and insert the appropriate code into the parallelogram, in order to input or output data: 7. The input/output parallelograms can contain any of the following program code, depending on what the parallelogram should do. Note that the program code is case sensitive (so uppercase and lowercase letters must appear as they do below). The program text should be entered into the parallelogram by clicking on the parallelogram, and typing the text into the large text field that pops up on the right of the screen: 7

8 Read(message) Reads and returns string data entered by the user, where message is a prompt that is displayed in the window (while you can leave the prompt out, good program code always includes prompts). For example, a = Read("Input text for a") saves the string entered by the user into a variable called a. ReadNumber(message) The same as Read, except that a numeric value is read. For example, a = ReadNumber("Input a value for a") saves the numeric value entered by the user into a variable called a. Write(message) Writes the provided message to the console. For example, Write(a) writes the value of variable a to the console. If you wish to write output text to the screen, you can use a string literal like Write("This is text") in place of a variable. You can also write both at the same time like Write("This is text" + a) Writeln() The same as Write(), but the output will go to the next line when finished. 8. To run the flowchart, you need to use the following controls (in JavaBlock s toolbar): 8

9 To start the program, click on the button that is second from the right (the Play icon). To stop execution early once the program is executing, click on the leftmost button (the square Stop icon). To move ahead to the next step of the program, click on the third button from the left (the Skip forward icon). Skipping forward a step at a time is useful for following your flowchart s logic, and spotting semantic errors in your programs. Finally, to execute the whole program in one go from the current point in its execution, click the rightmost button (the Cog icon). 9. To see the output of your flowchart program, you will need to open JavaBlock s console. You can do this by clicking on the Hide console button in the toolbar. 9