Algorithms definition: it s conditions , Algorithm analysis methods

Algorithm
Once we design an algorithm, we need to know how well it performs on any input. In particular we would like to know whether there are better algorithms for the problem. An answer to this first demands away to analyze an algorithm in a machine-independent way. Algorithm design and analysis form a central theme in computer science.
Definition An Algorithm is a finite set of instructions that if followed accomplish particular task. It take some inputs, performs a well define sequence of steps, and produces some output.
Every algorithm must satisfy the following five characteristics of an algorithm:
-Input: there are zero or more quantities which are externally supplied.
-Output: at least one quantity is produced.
-Finiteness: terminates after a finite number of steps for all possible
inputs.
-Definiteness: each step must be clear and unambiguous. Example for clear instruction (add 6 to x) and example for ambiguous instruction (divide 5 on 0). i.e. all people must understand it at the same the understanding and solve it at the same solving.
-Effectiveness: every step must be sufficiently basic that it can, in principle, be carried out by a person using only pencil and paper.

We can distinguish between an algorithm and program as follow:
Each algorithm must contain the previous five conditions and can be described in many ways: