Exercise Session 3 Ilinca Ciupa, Hermann Lehner 2 October 2007 Chair of Software Engineering
Assignments
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Deadline for assignment 1 over Deadline for assignment 2 on Thursday
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Today
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Purpose of the exercise session: Gi Give you the h intuition i i i b hi d object-oriented behind bj i d (OO) programming Teach you about formatting your code Answer questions to Assignment 2
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A class text Softwar e machine Operations
Featur e name
Commen t
class Extend an PREVIEW existing inherit class TOURISM feature explore is -- Show city info and route. do “To be filled in ((by y you!)” y ) end end
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Feature declaratio n
Pseudocod Keywords have a special meaning: class, inherit, feature,e is, do, end.
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Da style rules!
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For indentation, use tabs, not spaces Use this property to highlight the structure of the program, particularly through indentation • CLASS NAMES: ALL UPPERCASE • Period in feature call: no space before or after Chair of Software Engineering
Exercise: format
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class bank_account bank account feature deposit (sum: INTEGER) is -- Add `sum' to the account. do balance := balance + sum end balance: INTEGER end
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Solution: format
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class BANK_ACCOUNT feature deposit (sum: INTEGER) is -- Add `sum' to the account. do balance := balance + sum end balance: INTEGER end Chair of Software Engineering
Features, commands and queries
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Feature: an operation available on a certain class of objects Three kinds: Command Query Creation procedure (seen later)
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Defining and classifying features
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A feature is an operation that programs may apply to certain classes of objects. • A feature that accesses an object is a query • A feature that may modify an object is a command
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Command-query separation principle
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“A ki “Asking a question i shouldn’t h ld ’ change h the h answer””
i.e. “query”
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Queries
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Goal: obtain properties of objects Sh ld not modify Should dif the h object, bj or any other h Examples, for “route” objects:
What is the origin (first station) of Route1? What is the end point of Route1? How many steps does Route1 use? Which stations does Route1 traverse?
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Queries – other examples
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What is the name of a person? What is the age of a person? Wh iis the What h id off a student? d ? Is a student registered for a particular course? Are there any places left in a certain course?
Can you think of any other examples?
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Commands
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Goal: produce a change on an object, or several Examples, for “route” objects:
Animate Route1 Prepend (add at the beginning) a segment to Route1 Append (add at the end) a segment to Route1.
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Commands – other examples
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Register a student to a course Assign an id to a student Record R d the h grade d a student d got in i an exam
Can you think of any other examples?
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Exercise: query or command?
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What is Wh i the h balance b l off a bank b k account? ? Withdraw some money from a bank account Who is the owner of a bank account? Get all clients of a bank whose deposits are over 100,000 CHF Change the account type of a client How much money can a client withdraw at a time? Set a minimum limit for the balance of accounts Is Steve Jobs a client of Credit Suisse?
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Syntax and semantics
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The syntax of a program is the structure and form of its text. The semantics of a program is the set of properties of its potential executions.
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Syntax and semantics
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Syntax is the way you write a program: characters grouped into words grouped into bigger structures Semantics is the effect you expect from this program
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Exercise: Syntax or semantics?
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Describe the syntax and semantics for…
Assignment A i Feature Query Expression Return Value Command Indentation
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Solution: Syntax or semantics?
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Describe the syntax and semantics Assignment A i Syntax: “v := e” Semantics: “The assignment statement assigns the value of the evaluation of expression ‘e’ e to variable ‘v’.”
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Solution: Syntax or semantics?
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Describe the syntax and semantics Return R V Value l Syntax: “Result” Semantics: “The value of Result at the end of a feature-body feature body execution represents the return value of a feature.”
