Fundamentals of Algorithmic problem solving

1.3 Fundamentals of Algorithmic problem solving

• Understanding the problem

• Ascertain the capabilities of the computational device

• Exact /approximate soln.

• Decide on the appropriate data structure

• Algorithm design techniques

• Methods of specifying an algorithm

• Proving an algorithms correctness

• Analysing an algorithm

Understanding the problem:The problem given should be understood completely.Check if it is similar to some standard problems & if a Known algorithm exists.otherwise a new algorithm has to be devised.Creating an algorithm is an art which may never be fully automated. An important step in the design is to specify an in- stance of the problem.

Ascertain the capabilities of the computational device: Once a problem is understood we need to Know the capabilities of the computing device this can be done by Knowing the type of the architecture,speed & memory availability.

Exact /approximate soln.: Once algorithm is devised, it is necessary to show that it computes answer for all the possible legal inputs. The solution is stated in two forms,Exact solution or approximate solution.examples of problems where an exact solution cannot be obtained are i)Finding a squareroot of number.

ii)Solutions of non linear equations.

Decide on the appropriate data structure:Some algorithms do not demand any in- genuity in representing their inputs.Someothers are in fact are predicted on ingenious data structures.A data type is a well-defined collection of data with a well-defined set of operations on it.A data structure is an actual implementation of a particular abstract data type. The Elementary Data Structures are ArraysThese let you access lots of data fast. (good) .You can have arrays of any other da ta type. (good) .However, you cannot make arrays bigger if your program decides it needs more space. (bad) .

RecordsThese let you organize non-homogeneous data into logical packages to keep every- thing together. (good) .These packages do not include operations, just data fields (bad, which is why we need objects) .Records do not help you process distinct items in loops (bad, which is why arrays of records are used) SetsThese let you represent subsets of a set with such operations as intersection, union, and equivalence. (good) .Built-in sets are limited to a certain small size. (bad, but we can build our own set data type out of arrays to solve this problem if necessary)

Algorithm design techniques: Creating an algorithm is an art which may never be fully au- tomated. By mastering these design strategies, it will become easier for you to devise new and useful algorithms. Dynamic programming is one such technique. Some of the techniques are especially useful in fields other then computer science such as operation research and electric- al engineering. Some important design techniques are linear, non linear and integer programming

Methods of specifying an algorithm: There are mainly two options for specifying an algorithm: use of natural language or pseudocode & Flowcharts.

A Pseudo code is a mixture of natural language & programming language like constructs. A flowchart is a method of expressing an algorithm by a collection of connected geometric shapes.

Proving an algorithms correctness: Once algorithm is devised, it is necessary to show that it computes answer for all the possible legal inputs .We refer to this process as algorithm validation. The process of validation is to assure us that this algorithm will work correctly independent of issues concerning programming language it will be written in. A proof of correctness requires that the solution be stated in two forms. One form is usually as a program which is annotated by a set of assertions about the input and output variables of a program. These assertions are often expressed in the predicate calculus. The second form is called a specification, and this may also be expressed in the predicate calculus. A proof consists of showing that these two forms are equivalent in that for every given legal input, they describe same out- put. A complete proof of program correctness requires that each statement of programming language be precisely defined and all basic operations be proved correct. All these details may cause proof to be very much longer than the program.

Analyzing algorithms: As an algorithm is executed, it uses the computers central processing unit to perform operation and its memory (both immediate and auxiliary) to hold the program and data. Analysis of algorithms and performance analysis refers to the task of determining how much computing time and storage an algorithm requires. This is a challenging area in which some times require grate mathematical skill. An important result of this study is that it allows you to make quantitative judgments about the value of one algorithm over another.

Another result is that it allows you to predict whether the software will meet any efficiency constraint that exits.

Performance analysis

There are any criteria upon which we can judge an algorithm for instance:

1. Does it do what we want to do?

2. Does it work correctly according to the original specifications to the task?

3. is there documentation that describes how to use it and how it works?

4. Are procedures created in such a way that they perform logical sub functions?

5. is the code readable?

The space complexity of an algorithm is the amount of memory it needs to run to completion. The time complexity of an algorithm is the amount of computer time it needs to run to completion.

Performance evaluation can be loosely divided into two major phases:

(1) A priory estimate and (2) a posteriori testing. We refer to these performance analysis and performance measurements respectively.

Space complexity

(1)A fixed part that is independent of characteristics (e.g., number, size) of the inputs and outputs this part typically includes the instruction space (i.e. space for code), space for simple variables and fixed size component variables (also called aggregate),space for constants and so on.

(2) A variable part that consist of space needed by component variable whose size is dependent on particular problem instance being solved, the space needed by referenced variables (to the extent that it depends on instance characteristics), and the recursion stack space (insofar and this space depends on the instance characteristics). The space requirement S(P) of an algorithm P may therefore be written as S(P) =c + SP (instance characteristics),where c is constant. When analyzing the space complexity of an algorithm, we concentrate solely on estimating SP (instance characteristics). For any given problem, we need first to determine which instance characteristics to use to measure the space requirement. Generally speaking our choices are related to the number and magnitude of the in- puts to and outputs from the algorithm at times, more complexity measures of the interrelationship among the data times are used.

Time complexity

The time T (P) taken by a program P is sum of compile time and run time. The compile time does not depend on the instance characteristics. Also, we may assume that a compiled pro- gram will be run several time of a program. This run time is denoted by tp.

Because of many of the factor tp depends on are not known at the time of a program is conceived, it is reasonable to attempt only to estimate tp. If we knew the characteristics of a compiler to be used, we could

Proceed to determine the number of additions, subtractions, multiplication, divisions, com- pares, loads, stores and so on, that would be made by the code for P. So, we could obtain an expression for tp (n) of the form

Where n denotes the instance characteristics ,and ca, cs , cm,cd, and so on, respectively, denote the time needed for an addition ,subtraction, multiplication, division and so on, and ADD, SUB,MUL,DIV, and so on are the functions whose values are numbers of additions ,subtractions, multiplication, division and soon ,that are performed when code for P is used on an instance with characteristics n.

Obtaining such an exact formula is in itself an impossible task, since the time needed for addition, subtraction, multiplication, and so on, depend on the number being added, and subtract, multiplication and so on. The value of tp (n) for any given n can be obtained only experimentally. The program is typed, compiled, and run on a particular machine. The execution time is physically clocked, and tp (n) obtained. Even with this experimental approach, one could face difficulties. In a multiuser system, the execution time depends on such factors as system load, the number of other programs running on the computer at the time program P is run, the characteristics of these programs, and so on.

Given the minimal utility of determining the exact number of additions, subtraction, and so on, that are needed to solve a problem instance with characteristics given by n, we might as well lump all the operations together and obtain a cont for the total number of operations .We can go one more step further and count only the number of program steps.

A program step is loosely defined as a syntactically or semantically meaningful segment of a program that has an execution time that is independent of the instance characteristics. For ex- ample, the entire statement

Of Algorithm 1.5 could be regarded as a step since its execution time is independent of the instance characteristics (this statement is not strictly true , since the time for a multiply and divide generally depends on the numbers involved in the operation).

The number of steps any program statement is assigned depends on the kind of statement. For example comments count as zero steps; an assignment statement which does not involve calls any to other algorithms is encountered as one step; in an iterative statement such as for, while and repeat until statement, we consider the step count only for control part of the statement. The control parts for and while statements have the following forms:

each execution of the control part of a while statement is a step count equal to the number of step counts assignable to <expr> .the step count for each execution of control part of a for statement is one, unless the count attribute to <expr> and <expr1> are functions of the in- stance characteristics . In this latter case the first execution of the control part of the for has step count equal to the sum of counts for <expr> and <expr1>.remaining executions of the for statement have a step count of one; and so on.

We can determine number of steps needed by program to solve a particular problem instance in one of the two ways. in the first method we introduce a new variable ,count ,into the pro- gram .this is the global variable with initial value 0.statement to increment count by appropri- ate amount are introduced into the program. this is done so that each time a statement in the original program is executed ,count is incremented by step count of that statement.