Presentation on theme: "What have we learned so far? u Week 1 Summary u There are things called classes that describe things we are interested in representing in software. u There."— Presentation transcript:
What have we learned so far? u Week 1 Summary u There are things called classes that describe things we are interested in representing in software. u There are things called objects that are realisations or instantiations or instances of classes. u Only objects can be manipulated. u The only things we can do to an object are u Inspect the state using a relational expression u Alter the state using assignment u Ensuring objects have the correct initial state is absolutely crucial. u Weeks 2 and 3 Summary u Inspection and alteration can be combined in three patterns (or programming constructs) – sequence, selection and iteration u For convenience a variety of selection and iteration mechanisms are provided. u Sophisticated computing solutions are created using objects that have a large number of simple operations, or methods. u Methods may require (parameter) data and may provide result (return) data u Methods can be categorised as functions or procedures. u Frequently occurring utility objects (e.g. strings) do not need to be built from scratch and are provided by Java with associated API (method headers) descriptions
Relational Expressions u Relational (or boolean) expressions are interesting because when evaluated they produce one of only two possible results. u To evaluate a relational expression Java simply tests the truth of the expression and reports on the outcome. u Evaluating the test involves deciding whether it is TRUE or FALSE; Yes or No. u Relational expressions are sometimes called conditions.
Relational Expressions u The following binary operators are defined for relational expressions ==equal<=Less than or equal !=not equal>=Greater than or equal >Greater than
Relational Expressions u A simple relational expression involves one operator and two operands. u Examples of simple relational expressions 4 < 3 WeeklyPay > AverageIndustrialWage InterestRate <= 6.5 ApplicantsAge >= 18 LottoSlipNumber = = ChosenNumber CursorOffScreen = CursorPosition > ScreenWidth FoundIt = CurrentPageInBook = = PageToLookFor
Relational Expressions u The last two examples are important because they highlight the fact that, just like other expressions, the result of a relational expression can be assigned to a variable. u This practice can be a useful documentation aid because it allows us to name conditions thereby making the program a little easier to read.
Relational Expressions u More powerful relational expressions are possible using the Logical Operators AND and OR. u In effect, the logical operators are used toconnect two or more simple relational expressions thereby forming one composite relational expression. u Java uses && for specifying AND connections || for specifying OR connections
Relational Expressions u For example NumberOfPersons <= LiftCapacity && FloorSelected != CurrentFloor Age >= 18 && Age <= 65 EntryPoints >= CourseCutOff || Age >= 24 AccountType == Current || AccountType == Deposit u The Logical Operators have some very important behavioural characteristics which we need to remember. u These characteristics are often described pictorially by means of a TRUTH TABLE.
Truth Table for AND u The following truth table applies to the && operator RelExp1RelExp2Result TTT FTF TFF FFF u Notice that BOTH sub-expressions must be true for the result of the composite expression to be true.
Truth Table for AND u Truth Tables can easily be extended to describe composite expressions involving more than two sub-expressions. u NOTE, that no matter how many subexpressions are used the composite expression always produces a single result and this represents the result of the entire relational expression.
Truth Table for OR u The following truth table applies to the || operator RelExp1RelExp2Result TTT FTT TFT FFF u Notice that when ANY ONE of the sub- expressions is true the result of the composite expression is true.
Truth Table for OR u Again, the Truth Table can easily be extended to describe composite expressions involving more than two sub- expressions u NOTE, that no matter how many subexpressions are used the composite expression always produces a single result and this represents the result of the entire relational expression.
Scaling Up u When we create an algorithm to solve a particular problem we may find that the solution does not scale up for larger instances of the same problem. u For example, our solution for talking like a pirate is adequate (just about) for that particular instance of the problem but it would be completely inadequate, tedious and inefficient if the list had words or more!
Scaling Up u Ideally, we should try to develop solutions which lend themselves to modification for larger problems of the same type. u To create programs like this we need some more tools. u One of the major drawbacks of the pirate translator is that we had to include an if for each word so each one was unique. u Treating each one as unique makes adding new ones problematic.
Scaling Up u The pirate talker is an example of the type of situation we encounter quite frequently in programming environments. u Sometimes we would like to be able to treat a number of related items as a collection so that we would not have to explicitly reference each one. u At other times we would like to be able to treat them as separate and unique so that we can process them individually.
Scaling Up u Strings are an example of this type of requirement but they have limitations u We can only manipulate portions of the string u We cant change individual characters (we can of course inspect them but we cant alter them) u Alterations actually create new strings and discard the old ones String s = University ; String t = of Limerick ; s = s + t ;
Lists u A list is a collection of data items. u A list has three important aspects u The data in the list u The capacity of the list u The current usage of the capacity u What kind of things would we want to do to a list?
Lists u Add/Insert a new item u Remove/Delete an existing item u Modify an existing item u Show the list contents u Sorted in some order? u Find/Search the list for an item u Browse the list