Algorithm and Pseudo Codes

Algorithm and PseudocodesPrepared By:Mr. Richard R. BasilioBSECE Dip ICTAlgorithmWhat is Algorithm?By wikipedia definition:an algorithm is a sequence of finite instructions, often used for calculation and data processing.It is formally a type of effective method in which a list of well-defined instructions for completing a task will, when given an initial state, proceed through a well-defined series of successive states, eventually terminating in an end-state.What is Algorithm

Euclidean AlgorithmFirst documented algorithm by Euclid (300 B.C.) to compute greatest common divisor (gcd).Example: gcd(3,21)=3Condition:1.Let A and B be integers with A > B 0.2.If B = 0, then the gcd is A and the algorithm ends.3.Otherwise, find q and r such thatA = qB + r where 0 r < BNote that we have 0 r < B < A and gcd(A,B) = gcd(B,r).Replace A by B, B by r. Go to step 2.Euclidean AlgorithmExample No. 2:Find the greatest common divisor of A=40, B=15; using Euclidean algorithm;A = 2B + 10 A = 15 ; B = 10 A = 1B + 5 A = 10 ; B = 5 A = 2B + 0 A = 5 ; B = 0 gcd is 5Properties of AlgorithmThere are three properties of algorithm that must have to consider in solving a certain problem in programming:FinitenessAbsence of AmbiguitySequence DefinitionInput and Output DefinitionEffectivenessScope of Definition

Properties of AlgorithmFinitenessThe execution of a programmed algorithm must be complete after a finite number of operations have been performed. Otherwise, we cannot claim that the execution produces a solution.Properties of AlgorithmAbsence of AmbiguityThe representation of every step of an algorithm should have a unique interpretation which also understand by the human.It is convenient to deal with algorithms presented in notational with sparse detail:Example:Pseudo codeFlowchartsProperties of AlgorithmSequence of DefinitionThe sequence in which the steps of the algorithm are to carried out should be clearly specified.In algorithmic specifications, the instructions are performed from top to button, unless the instruction themselves otherwise specified.Properties of AlgorithmInput and Output DefinitionInputs are the data items that is presented in the algorithm.Outputs are the data items presented to the outside world as the result of the execution of a program based on the algorithm.An algorithm ought to produce at least one output (otherwise, what use is it?...)

Properties of AlgorithmEffectivenessit consists of basic instructions that are realizable. This means that the instructions can be performed by using the given inputs in a finite amount of time. The instructions of an algorithm may order the computer only to perform tasks that is capable of carrying out.Properties of AlgorithmScope DefinitionAn algorithm applies to the following:Specific problem or class of problemThe range of inputs has to be predefinedThe range determines the generality of the algorithm.How to express an ALGORITHM?Algorithms can be expressed in many kinds of notation, including:Natural languagePseudo CodeFlowchartsProgramming LanguagePseudocodeWhat is Pseudocode?Pseudo means imitation or false and code refers to the instructions written in a programming language.Pseudocode is another programming analysis tool that is used for planning a program.Pseudocode is also called Program Design Language (PDL).

What is Pseudocode?By wikipedia definition:Pseudocode is a compact and informal high-level description of a computer programming algorithm that uses the structural conventions of some programming language, but is intended for human reading rather than machine reading.Logical Structure of PseudocodePseudocode is made up of the following logic structures that have been proved to be sufficient for writing any computer program:Sequence LogicSelection LogicIteration Logic

Sequence LogicIt is used to perform instructions in a sequence, that is one after another.Thus, for sequence logic, pseudocode instructions are written in an order in which they are to be performed.The logic flow of pseudocode is from top to bottom.Sequence Logic

Selection LogicIt is used for making decisions and for selecting the proper path out of two or more alternative paths in program logic.It is also known as decision logic.Selection logic is depicted as either an IF...THEN or an IF...THEN...ELSE structure.Selection Logic

Selection Logic

Iteration LogicIt is used to produce loops when one or more instructions may be executed several times depending on some of the conditions.It uses structures called the DO_WHILE, FOR and the REPEAT_UNTIL.Iteration Logic

Iteration Logic

Iteration Logic

Rules for Pseudocode1.Write only one statement per line. Each statement in your pseudocode should express just one action for the computer. If the task list is properly drawn, then in most cases each task will correspond to one line of pseudocode.

Rules for PseudocodeExamples

Rules for Pseudocode2.Capitalized initial keyword. In the example above, READ and WRITE are in caps. There are just a few keywords we will use: READ, WRITE, IF, ELSE, ENDIF, WHILE, ENDWHILE, REPEAT, UNTIL Rules for Pseudocode3. Indent to show hierarchy. We will use a particular indentation pattern in each of the design structures: SEQUENCE: keep statements that are stacked in sequence all starting in the same column. SELECTION: indent the statements that fall inside the selection structure, but not the keywords that form the selection LOOPING: indent the statements that fall inside the loop, but not the keywords that form the loop Rules for PseudocodeExamples:

Rules for Pseudocode4. End multi-line structures.All the initial keyword must always in line with the last or end of the structure.5. Keep statement language independent.Resist the urge to write in whatever language you are most comfortable with. There may be special features available in the language you plan to eventually write the program in; if you are SURE it will be written in that language, then you can use the features. If not, then avoid using the special features.

Rules for PseudocodeIn summary:Write only one statement per line.Capitalized initial keyword.Indent to show hierarchy.End multi-line structures.Keep statement language independent.

Standard for good pseudocodeThese are follows:Number each instruction.This is to enforce the notion, well-ordered collection of ... operations.Each instruction should be unambiguous.It means the computing agent, in this case the reader, should be capable of carrying out the instructions. And also, each instruction should be effectively computable (do-able).Completeness.Nothing should be left out.

Advantage of PseudocodeFollowing are some of the advantages of using pseudocode:Converting a pseudocode to a programming language is much more easier than converting a flowchart.As compared to flowchart, it is easier to modify a pseudocode of a program logic when program modifications are necessary.Limitations of PseudocodeIt also suffers from some of the limitations. These limitations are as follows:In the cases of pseudocode, a graphic representation of program logic is not available.There are no standard rules to follow for using a pseudocode. Different programmers use their own style of writing pseudocode and hence, communication problem occurs due to lack of standardization.Calculation SymbolsTo symbolize the arithmetic operators we use these symbols:Note: There is a precedence or hierarchy implied in this symbols.

Selection SymbolsWhen we have to make a choice between actions, we almost always base that choice on a test. There is a universally accepted set of symbols used to represent these phrases:

Selection Symbols

Logical Operators

Logical Operators

For the beginnersIt is more difficult to follow the logic of or write pseudocode as compared to flowcharting.ExamplesPrepare crosswise yellow paper for seatwork after the discussion.