*422*

An algorithm is a well-ordered collection of unambiguous and effectively computable operations that when executed produces a result and halts in a finite amount of time.

## Characteristics of Algorithms

- Algorithms are well-ordered.
- Algorithms have unambiguous operations.
- Algorithms have effectively computable operations.
- Algorithms produce a result.
- Algorithms halt in a finite amount of time.

These characteristics need a little more explanation, so we will look at each one in detail.

### Algorithms are well-ordered

Since an algorithm is a collection of operations or instructions, we must know the correct order in which to execute the instructions. If the order is unclear, we may perform the wrong instruction or we may be uncertain which instruction should be performed next. This characteristic is especially important for computers. A computer can only execute an algorithm if it knows the exact order of steps to perform.

### Algorithms have unambiguous operations

Each operation in an algorithm must be sufficiently clear so that it does not need to be simplified. Given a list of numbers, you can easily order them from largest to smallest with the simple instruction “Sort these numbers.” A computer, however, needs more detail to sort numbers. It must be told to search for the smallest number, how to find the smallest number, how to compare numbers together, etc. The operation “Sort these numbers” is ambiguous to a computer because the computer has no basic operations for sorting. Basic operations used for writing algorithms are known as primitive operations or primitives. When an algorithm is written in computer primitives, then the algorithm is unambiguous and the computer can execute it.

### Algorithms have effectively computable operation

Each operation in an algorithm must be doable, that is, the operation must be possible to do. Suppose you were given an algorithm for planting a garden where the first step instructed you to remove all large stones from the soil. This instruction may not be done if there is a four-ton rock buried just below ground level. For computers, many mathematical operations such as division by zero or finding the square root of a negative number are also impossible. These operations are not effectively computable so they cannot be used in writing algorithms.

### Algorithms produce a result

In our simple definition of an algorithm, we stated that an algorithm is a set of instructions for solving a problem. Unless an algorithm produces some result, we can never be certain whether our solution is correct. Have you ever given a command to a computer and discovered that nothing changed? What was your response? You probably thought that the computer was malfunctioning because your command did not produce any type of result. Without some visible change, you have no way of determining the effect of your command. The same is true with algorithms. Only algorithms which produce results can be verified as either right or wrong.

### Algorithms halt in a finite amount of time

Algorithms should be composed of a finite number of operations and they should complete their execution in a finite amount of time. Suppose we wanted to write an algorithm to print all the integers greater than 1. Our steps might look something like this.

- Print the number 2.
- Print the number 3.
- Print the number 4.

.

.

.

While our algorithm seems to be pretty clear, we have two problems. First, the algorithm must have an infinite number of steps because there are an infinite number of integers greater than one. Second, the algorithm will run forever trying to count to infinity. These problems violate our definition that an algorithm must halt in a finite amount of time. Every algorithm must reach some operation that tells it to stop.

## Advantages of Algorithm

- It is a step-by-step rep. of a solution to a given problem ,which is very easy to understand.
- It has got a definite procedure.
- It easy to first develop an algorithm, then convert it into a flowchart & then into a computer program.
- It is independent of programming language.
- It is easy to debug as every step is got its own logical sequence.

## Disadvantages of Algorithm

- It is time consuming & cumbersome as an algorithm is developed first which is converted into flowchart & then into a computer program.

## Algorithm Example

Algorithm to find out number is odd or even

step 1 : start

step 2 : input number

step 3 : rem=number mod 2

step 4 : if rem=0 then

print “number even”

else

print “number odd”

endif

step 5 : stop