Higher Computing. Software Development. LO2 Software Development languages and environments

Transcription

1 Software Development LO2 Software Development languages and environments Ian Simpson Inverurie Academy 2006

2 Software Development The candidate must demonstrate knowledge and understanding, practical skills and problem solving based on the following content statements: Software Development languages and environments Description and comparison of procedural, declarative and event-driven languages Comparison of the functions, uses and efficiency of compilers and interpreters Description of the features and uses of scripting language (including creating and editing a macro) Explanation of the need for and benefits of scripting languages Description of the use of module libraries ISK 2006

3 Types of Languages In Higher Computing you have to know about three types of languages: procedural, declarative and event-driven. The table below describes, sets out the main features of and gives examples of each type of language. Description Procedural Declarative Event-driven The user sets out a list of instructions in the correct sequence (an algorithm) in order to solve a problem. Has a definite start and end point. Also known as imperitive languages as imperitive (in computing) means giving or expressing commands. Features Different variable types Arithmetic and logical operations Program control using sequence, iteration and selection Subprograms and procedures Data flow using parameters Examples BASIC COMAL Pascal May be thought of as the opposite of a procedural language. States what to do rather than how to do it. Also known as a rule-based language. Contain statements or clauses which describe facts Contain rules which apply to the facts Recursion (equivalent to repetition/looping in a procedural language) Collection of facts and rules known as a knowledge base PROLOG These programs have no definite start or end. Clicking on a screen object (eg. button) in the GUI activates the program code associated with that object. An event is the action of clicking on a screen object and because it is detected by the program it is called event-driven. Quick to design a program with a GUI Similar in operation to menu-driven OS with a GUI as program always running in the computer and waiting for the user to select a screen object. Visual Basic Delphi Yes, Visual Basic is an event-driven language even though it has all the features of a procedural language. ISK

4 Program control using sequence, iteration and selection Procedural languages have three basic control structures: sequence, iteration and selection Sequence is where a number of statements follow on from each other in sequential order. For example: Name = InputBox( Please enter your name ) Age = InputBox( Please enter your age ) lbldisplay.caption = Hello & Name &, you are & Age & years old. x Iteration (which also means repetition) is where looping is used to repeat a number of lines of code. There are a number of different methods used, including FOR NEXT, DO LOOP UNTIL and WHILE WEND. For example: FOR counter = 1 TO 100 picdisplay.print I must always do my homework NEXT counter Selection is where a choice has to be made in a program. This is often done with an IF control structure, although a CASE control structure could also be used. For example: IF pupil_mark > 49 THEN lstresults.additem(pupil_mark & = Pass ) ELSE lstresults.additem(pupil_mark & = Fail ) END IF ISK

5 Scripting Languages Scripting languages can be divided into two types: a) those which are embedded within an application package and which make use of only those commands that are available within the package b) those which work with the operating system and across and between suitable (scriptable) applications A scripting language allows the user to customise an application package to carry out additional operations other than those provided in the original menus. Scripting languages use commands from application packages in order to do this. Scripting languages are becoming more widely used within web pages, for example to make them more accessible to each individual user. Scripts do this by detecting which type of computer, operating system or even browser the user has and then change the look of the web page to suit. Examples of scripting languages Examples of scripting languages include JavaScript, VBScript, CGI, Perl, AppleScript and FileMaker Pro. JavaScript is a scripting language that is used with web browsers. It was developed by Netscape and Sun Microsystems but is different to the Java programming language. The most common use of JavaScript is for client-side scripting in web pages. Microsoft have their own version of this language called Jscript. VBScript is a scripting language written by Microsoft. Similar to Visual Basic, VBScript is widely used on the Web for both client-side processing with a Web page and server-side processing in Active Server Pages (ASPs). VBScript can also be used with the Windows Script Host (WSH) to perform functions on the user s own machine if they have a Windows OS installed. Uses of scripting languages An expert user of an application package would use a scripting language to automate a repetitive task that had to be carried out. Programs written using a scripting language are often called scripts. Once programmed, a script may be activated by clicking on a button or by selecting from a menu. If you perform a task repeatedly in Microsoft Word, you can automate the task by using a macro. A macro is a series of Word commands and instructions that you group together as a single command to accomplish a task automatically. Instead of manually performing a series of time-consuming, repetitive actions in Word, you can create and run a single macro in effect, a custom command that accomplishes the task for you. ISK

6 Here are some typical uses for macros: To speed up routine editing and formatting To combine multiple commands; for example, inserting a table with a specific size and borders, and with a specific number of rows and columns To make an option in a dialog box more accessible To automate a complex series of tasks See the handout LO2: VBScript macros for examples of macros created in Microsoft Word. Benefits of scripting languages Scripting languages, whether used for macros or with web pages, have a number of benefits. Increased rate of work for the user through use of macros Less training necessary as macros can automate complex tasks Web pages or applications can be customised to suit the user s computer system ISK

7 Programming languages on the Internet Scripting languages are not the only type of programming languages which are used in Internetwork programming. Similarly the programs created are not always run on the user s computer. This section describes the where the program is executed when the user is connected over the Internet and gives two further examples of programming languages. Note that Java is not a scripting language JavaScript is! Server-side solutions CGI (Common Gateway Interface) is a specification for transferring information between a World Wide Web server and a CGI script. CGI scripts are the most common way for web servers to interact with users. Many HTML programs that contain forms, for example, use a CGI program to process the data on the form after it has been submitted. CGI is known as a server-side solution because the process occurs on the web server and not the client computer. Client-side solutions Java was developed to take advantage of the increasing use and importance of the World Wide Web. Java source code is compiled into an intermediate code called bytecode which can be executed by a Java interpreter called a Java Virtual Machine. The advantage of Java is that it is platform independent, which means that it can run on any operating system that has a Java Virtual Machine e.g. Unix, Windows and Mac OS. Small Java applications are called Java applets and can be downloaded from a web server and run on a user s computer using a Java-compatible web browser such as Microsoft Internet Explorer. Using Java applets (or ActiveX control programs) to provide feedback to web users is known as client-side solutions because processing takes place on the user s own computer. ISK

8 Compilers A compiler translates a high level language program, the source code, into machine code, the object code. After compilation is complete, the object code can then be run. The object code runs very fast because it is in the computer s own language, machine code. The object code program may be saved and run separately. The source code should also be saved because without it it is impossible to edit the program since the object code, once produced, cannot be easily changed. When a compiled program is being created, the source code and the compiler are needed at the time of writing and editing stages; the source code, object code and compiler are required at the testing stage, and only the object code is required to use the completed program. The compiler changes each single high-level language instruction into several machine code instructions. Compilation involves analysing the language structure of the source program, checking to see if it is correct, and producing machine code. These separate compilation stages are known as lexical analysis, syntax analysis and code generation. Source code Lexical analysis Syntax analysis Code generation Object code Compilation stages Lexical analysis: each recognised keyword in the source code is changed into a token. This tokenising process resembles the representation of characters by their ASCII values, in that each token is a unique code. The keyword PRINT for instance has the token (decimal 241) and IF has the token (decimal 231). Any errors detected at this stage are reported. Syntax analysis: the language statements are checked against the rules or grammar of the language. This grammar checking process is known as parsing and that part of the compiler program that carries it out is the parser. Typical errors that might be detected and reported by the parser are missing brackets from an arithmetical expression or inverted commas from a PRINT statement. Code generation: this stage produces the object code, which is a machine code repesentation of the source code. The programming language Pascal is an example of a language which is normally compiled before it can be run. ISK

9 Interpreters An interpreter translates and runs a high-level language program one instruction or statement at a time. No separate object code is produced. Interpreted programs run slower than compiled programs because each line must be translated every time the program is run. When an interpreted program is being created both the source code and the interpreter are required to write, edit, test and use the completed program. If the program being interpreted contains a loop which is to be exectured, say 1000 times, then each line of code inside the loop will need to be repeatedly interpreted 1000 times. An interpreter will report any mistakes such as syntax errors in the code immediately, rather than waiting until the end of the translation process the way compilers do. Using an interpreter is a single process, unlike using a compiler, which requires the program to be compiled separately and then run. For these reasons, interpreted programming languages are popular with students and others who are learning to program. Unlike compilers, it is not possible to save the translated version which is produced by an interpreter since each program line is executed immediately upon translation. Interpreted programs tend to use more memory than compiled programs because the interpreter must also be present in the computer s memory when the program is run. Programming languages such as BASIC and COMAL are normally interpretted, although nowadays most languages are also provided with a compiler or similar translator program capable of generating a stand alone version of the user s program. The efficiency of compilers and interpreters Compiler Program speed Once compiled, the program runs much faster than an interpreted program as it is in machine code. Memory usage Uses much less memory than interpre ted programs. Debugging Harder to trace errors as program waits until end of compilation process before reporting. Interpreter Runs slower than compiled programs because each line must be translated every time the program is run. Takes up more memory than compiled programs as the interpreter needs to be running at the same time. User can trace syntax errors more quickly as program stops interpretting at the offending line of code. ISK

10 Module Libraries A module library is part of a software library. A software library is a collection of software held permanently accessible on backing storage. A software library will typically include complete programs, a module library and a set of machine code routines which may be loaded into user programs. A module library will contain modules (procedures or pre-defined routines) which a user can add to his or her programs when required. A programmer would not write a procedure to sort numbers each time this was required in a new program, the procedure would simply be loaded from a module library. This greatly speeds up the software development process. See the handout LO2: Using module libraries for examples of Visual Basic programs which use module libraries. ISK