Introduction to Algorithms: Understanding Basic Principles

1. Chapter 5 Algorithms Introduction to computer, 2nd semester, 2010/2011 Mr.Nael Aburas nras@iugaza.edu.ps Faculty of Information Technology Islamic University of Gaza

2. Introduction • We have seen that before a computer can perform a task, it must be given an algorithm telling it what to do. • Algorithm: set of ordered steps that define how a task is performed. • Algorithm: an ordered set of executable steps that defines a terminating process • We have seen some examples of algorithms such as converting numeric representation, as well as algorithm for the machine cycle that is followed by the CPU.

3. Introduction • Machine cycle: • As long as the halt instruction not executed • Continue to execute the following steps: • Fetch an instruction • Decode the instruction • Execute the instruction

4. Introduction • As mentioned in the definition, the steps required to be ordered, that means the steps must have a well-established structure in terms of the order of their execution.

5. The abstract nature of algorithm • It is important to differentiate between the algorithm and its representation. • It is similar to different between story and book. • Story is abstract, book is a physical representation of a story, is a book is translated in another language or different format, it still the representation of the story. • A single algorithm can be represented in many ways.

6. Process, algorithm, and program • Program is a presentation of an algorithm • Process is activity of executing a program “algorithm”.

7. Algorithm representation • The goal of this section is to introduce the basic of primitives and Pseudocode • Primitives: • The representation of an algorithm requires some form of language, human language (English, Arabic), or picture language. • Such natural channels of communications “natural language” leads to misunderstandings (communication problems).

8. Algorithm representation • Computer science approaches this problem by establishing a well-defined set of building blocks from which algorithm representation can be constructed, such building block is called a primitive. • A collection of primitives a long with a collection of rules stating how the primitives can be combined constitutes a programming language. • Each primitive has its own syntax and semantic. • Syntax: primitive's symbolic representation • Semantic: meaning of the primitive

9. Pseudocode • Notational system in which ideas can be expressed informally during the algorithm development process. • Pseudocode is a kind of structured English for describing algorithms. • Pseudocode generally does not actually obey the syntax rules of any particular language.

10. Pseudocode • Example: saving of a computed value. • If we have computed the sum of two value, and we want to save the result. • General syntax: • Name expression • We read it as assign name the value of the expression (assignment statement) • Result A+B • Grade mid+final • So that the grade can be used on future to refer to that sum.

11. Pseudocode • Another recurring semantic structure is: • If (condition) then (activity) else (activity) • Selection one of two possible activites. • If (sales have decreased) then (lower the price by 5%) • If (not raining ) then (if (temperature = hot) then ( go swimming) else (play golf) ) else (watch TV)

12. Pseudocode • Another recurring semantic structure repeated execution of a statement: • While (condition) do (activity) • While (tickets remain to be sold) do (sell a ticket) X  3; while (X < 5) do (X  X + 1)

13. Pseudocode, procedure • We want to use out Pseudocode to describe activities that can be used as abstract tools in other applications • Computer science has a variety of terms for such programs units, including sub-program, procedure, model, and functions. • We will use the term procedure • Syntax: • Procedure name

14. Figure 5.4 The procedure Greetings in pseudocode

15. Iterative Structures • Pretest loop: while (condition) do (loop body) • Posttest loop: repeat (loop body) until(condition)

16. Figure 5.7 Components of repetitive control

17. Recursion • The execution of a procedure leads to another execution of the procedure. • Multiple activations of the procedure are formed, all but one of which are waiting for other activations to complete.

18. Flow chart • A flowchart is a common type of diagram, that represents an algorithm or process. • A flow chart is a graphical or symbolic representation of a process. Each step in the process is represented by a different symbol. • The flow chart symbols are linked together with arrows showing the process flow direction.

19. Flowchart symbols

20. Construct Flowchart • To construct an effective flowchart • Define the process boundaries with starting and ending points. • Complete the big picture before filling in the details. • Clearly define each step in the process

21. Example 1 • Draw a flowchart to find the sum of first 50 natural numbers

22. Start Read A, B,C IS B>C? IS A>C? IS A>B? NO YES YES NO YES NO Print B Print A Print C END Example 2 • Draw a flowchart to find the largest of three numbers A, B, and C.