1 Software Development Topic 2 Software Development Languages and Environments

2 Translators ► As mentioned in the CS topic computers can only understand instructions and data that are written in binary form. ► Programs written in high level languages use lots of English like terms and need to be translated so that the computer can understand them. ► This is done by a piece of systems software called a translator. ► We will look at 2 types of translator: - ► Compiler ► Interpreter

3 Compiler This program converts high-level language code into machine code. The compiler takes all the source code (the high-level language program before anything is done to it) as one block and translates it into machine code. This translated program can be run at any time without the source code or the compiler because the translated program is stored as machine code and this machine code can be understood by the processor.

4 If a change is made to the source code then the program must be compiled again so that the change is incorporated in the finished compiled program. The finished compiled program is called object code. Object code is created every time the source code is compiled.

5 Interpreter An interpreter translates and executes each program instruction one at a time as the program is run. In simple terms the interpreter does the following: 1.Fetch instruction 2.Translate instruction into machine language equivalent(s) 3.Execute Programs translated using an interpreter can only be run in conjunction with the interpreter. If the interpreter is not present then the source code instructions cannot be translated into their machine code equivalents.

6 Compilers V Interpreters. 1. Compilers produce executable programs. E.g. They produce programs that can be run on a computer system without the need for the source code or the compiler/assembler to be present in the system. 2. Compiled programs run much faster than interpreted programs. This is because interpreted programs read and translate each instruction in turn and this takes time. Compiled programs already have all their instructions converted to machine code.  Loops run very slowly in interpreted programs because every line in the loop has to be translated every time it operates.

7 3. Interpreted programs take less time to develop (write) because if a change is made to the source code then this change is included in the next program run. However, with compilers any changes to the source code are not included in the executable code until the source code is re-compiled. Each time a change is made a new version of the executable program must be made.

8 ► The choice of which type of translator to use depends on the type of programs being written and the experience of the program writer. ► E.g. Complex programs written by experienced programmers for system software or applications tend to use compilers. TranslatorSpeedEasy to AlterEasy to Understand InterpreterSlowYes CompilerFastNoYes Summary

9 Programming Languages SDP Theory Booklet pages ► There are 4 main classifications of programming languages that we will concentrate on. These are  Procedural Languages  Declarative Languages  Event Driven Languages  Scripting Languages

Procedural Languages Key features ► Carried out in a fixed sequence with a start and end point ► Can be split into separate functions and procedures ► Have control structures such as If statements, for..next loops and do..while loops Examples of procedural languages are: ► BASIC ► Pascal ► C A lot of programmers are familiar with at least one procedural language

Example Pascal Procedural Code

Declarative Languages Key features ► Consist of facts and rules ► No fixed sequence of instructions ► Are queried (asked questions) which they try to find a solution to ► Uses pattern matching to answer queries  facts and rules combined in different ways until a solution, or no solution, is found Examples of declarative languages are: ► Prolog ► LISP ► ML ► Scheme ► Erlang languages such as Prolog and LISP are used predominantly in the field of Artificial Intelligence.

Here are some facts: Parent (Liz, Tom). means that Liz is a parent of Tom Female (Liz). means that Liz is female Male(Tom). Parent(Bert, Tom). Here is a rule: Mother(X, Y) :- means that X is a mother of Y IF Parent(X, Y), Female(X) X is a parent of Y AND X is female! Query could be Mother(X, Tom) which would give the answer X=Liz Example Prolog Declarative Code

Event Driven Languages Key Features ► no fixed order of instructions ► wait for user input (clicking a button) before performing an action ► front end for creating graphical user interfaces ► share the same type of language features as procedural languages Examples of event driven languages are: ► Visual Basic ► Delphi (Object Pascal) ► Smalltalk Used for creating a wide range of desktop applications for modern OS’s such as Mac OSX and Windows

Example Visual Basic Event Driven Code

Scripting Languages Key Features ► usually built-in to existing applications ► automate or extend the functionality of applications ► have extra functions, procedures and data types related to the application  Selection.Font.Name  Range(“A1”).Select ► run slower than other programs because they’re interpreted ► share the same type of language features as procedural languages Examples of scripting languages are: ► VBA (Visual Basic for Applications) ► Javascript ► TCL Lots of people create and use macros who wouldn’t consider themselves programmers

Example VBA Scripting Code

Macros Benefits of Macros ► Can extend and add new features to a program that the developers didn’t think of ► Can simplify and repeat complex or frequently used commands ► Can be assigned to a keyboard shortcut to make it easier to use when required Creating a Macro 1. Record a series of actions in an application you are using 2. Code gets generated and saved as a program script 3. Script can then be assigned to a keystroke Or 1. Write and save the script in an editor 2. Run the script in the application and find and fix any errors 3. Script can then be assigned to a keystroke

Example Exam Questions 1. Describe two characteristics of a scripting language not commonly found in a procedural language (2) 2. A program contains the following statement: Is_a(rover, dog). State which type of programming language is being used. (1) 3. State two benefits of using a macro to create an alphabetical list of customers whose account balance is over £5000. (2) 4. Macros are written in a high level language. State the type of high level language that is used to write macros. (1) 5. State two features of a declarative programming language (2) 6. State two methods of creating a macro (2)