Algorithm related desciptions, Study notes of Computer Science

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Problem Solving Process

Step 1 - Analyze the problem

Outline the problem and its requirements Design steps (algorithm) to solve the problem

Step 2 - Implement the algorithm

Implement the algorithm in code Verify that the algorithm works

Step 3 - Maintenance

Use and modify the program if the problem domain changes

What is an algorithm?

The idea behind the computer program

Stays the same independent of

Which kind of hardware it is running on Which programming language it is written in

Solves a well-specified problem in a general

way

Is specified by

Describing the set of instances (input) it must work on Describing the desired properties of the output

What is an algorithm? (Cont’d)

Before a computer can perform a task, it must

have an algorithm that tells it what to do.

Informally: “An algorithm is a set of steps that

define how a task is performed.”

Formally: “An algorithm is an ordered set of

unambiguous executable steps, defining a

terminating process.”

Ordered set of steps: structure! Executable steps: doable! Unambiguous steps: follow the directions! Terminating: must have an end!

Important Properties of Algorithms

Correct

always returns the desired output for all legal instances of the problem.

Unambiguous

Precise

Efficient

Can be measured in terms of  (^) Time  (^) Space Time tends to be more important

Representation of Algorithms

A single algorithm can be represented in

many ways:

Formulas: F = (9/5)C + 32 Words: Multiply the Celsius by 9/5 and add 32. Flow Charts. Pseudo-code.

In each case, the algorithm stays the same;

the implementation differs!

Pseudocode

Pseudocode is like a programming language

but its rules are less stringent.

Written as a combination of English and

programming constructs

Based on selection (if, switch) and iteration (while, repeat) constructs in high-level programming languages

Design using these high level primitives

Independent of actual programming language