6: What is a programming language Eventdriven programming

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6 What is a programming language? /
Event-driven programming

• Mark Elsom-Cook 1999

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Programming vs. Description

• Programming language is not a description
  language
• It does things
• Define things to do, then do them (execute)
• Instructions vs. control structure
• Interpreted vs. compiled

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What makes a programming language?

• Instructions
• Conditionals
• Repeated behaviour
• Variables and data structures
• Input and Output
• Data plus Algorithms that work on that data

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Classes of languages

• Command
• Procedural
• Functional
• (Logic)
• Object-oriented

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Command languages

• Linear sequence
• Jump, go to and Return
• e.g. Basic, Assembly language
• 1 x 3
• 2 y 3
• 3 z x y
• 4 print z

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Procedural languages

• Procedure is named and has parameters
• Local and Global variables
• reusable
• Top procedure
• e.g. C, Pascal
• procedure greet(name)
• writeln(Hello there name)
• end procedure

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Functional languages

• Functions
• like procedures but return values
• Embed functions in functions
• Top function
• Can ignore values and treat like procedural
• e.g. Lisp, JavaScript
• (defun hypot (x y)
• (sqrt (plus (times x x) (times y y))))

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Object-oriented languages

• Objects contain data and procedures
• Objects communicate via messages
• e.g. Java, Smalltalk, C
• class triangle extends shape
• int base, height
• int angle
• int area ()
• area height base / 2

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So

• We will be learning
• Functional programming
• in JavaScript
• Java and JavaScript are proper languages except
  they do not allow reading and writing files

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Event-Driven programming
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What is event-driven?

• Old languages go from beginning to end
• Event-driven languages call procedures/fns
• from other procedures
• when an event happens
• Event-driven essential for interfaces and GUI
• e.g. Mac OS, Windows
• Most modern language support events

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Event concepts

• An event
• type data
• keypress a
• click 230, 421
• Event Loop
• Processes User Events
• Calls correct procedures

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Events, HTML and JavaScript

• Some HTML tags have event attributes
• These attributes can have Javascript values
• onclickalert(you clicked me)
• onloadalert(You are loading a page)
• onmouseenteralert(Hello mouse)
• Definitely need string quotes round code

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Easy JavaScript code for events

• Three very useful bits for events
• alert(This is a message)
• prompt(What day is it?)
• confirm(Fancy a coffee? Y/N)
• Attaching these to bits of tag
• good for debugging
• quick incremental development

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Some useful tag events

• onClick
• onDblClick
• onKeyDown
• onKeyPress
• onKeyUp
• onError
• onAbort
• onLoad
• onUnload

• onMouseDown
• onMouseUp
• onMouseOver
• onMouseEnter
• onMouseOut
• onFocus
• onBlur

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Summary

• JavaScript is a functional programming language
• Events in HTML tags allow an event-driven
  interface
• We will combine HTML and JavaScript to create
  interactive web pages