Building blocks of algorithms (statements, state, control flow, functions)

BUILDING BLOCKS OF ALGORITHMS (statements, state, control flow, functions)

Algorithms can be constructed from basic building blocks namely, sequence, selection and iteration.

Statements:

Statement is a single action in a computer.

In a computer statements might include some of the following actions

Ø   input data-information given to the program

Ø   process data-perform operation on a given input

Ø   output data-processed result

State:

Transition from one process to another process under specified condition with in a time is called state.

Control flow:

The process of executing the individual statements in a given order is called control flow.

The control can be executed in three ways

1.              sequence

2.              selection

3.              iteration

Sequence:

All the instructions are executed one after another is called sequence execution.

Example:

Add two numbers:

Step 1: Start

Step 2: get a,b

Step 3: calculate c=a+b

Step 4: Display c

Step 5: Stop

Selection:

A selection statement causes the program control to be transferred to a specific part of the program based upon the condition.

If the conditional test is true, one part of the program will be executed, otherwise it will execute the other part of the program.

Example

Write an algorithm to check whether he is eligible to vote?

Step 1: Start

Step 2: Get age

Step 3: if age >= 18 print “Eligible to vote”

Step 4: else print “Not eligible to vote”

Step 6: Stop

Iteration:

In some programs, certain set of statements are executed again and again based upon conditional test. i.e. executed more than one time. This type of execution is called looping or iteration.

Example

Write an algorithm to print all natural numbers up to n

Step 1: Start

Step 2: get n value.

Step 3: initialize i=1

Step 4: if (i<=n) go to step 5 else go to step 7

Step 5: Print i value and increment i value by 1

Step 6: go to step 4

Step 7: Stop

Functions:

v   Function is a sub program which consists of block of code(set of instructions) that performs a particular task.

v   For complex problems, the problem is been divided into smaller and simpler tasks during algorithm design.

Benefits of Using Functions

v   Reduction in line of code

v   code reuse

v   Better readability

v   Information hiding

v   Easy to debug and test

v   Improved maintainability

Example:

Algorithm for addition of two numbers using function

Main function()

Step 1: Start

Step 2: Call the function add()

Step 3: Stop

sub function add()

Step 1: Function start

Step 2: Get a, b Values

Step 3: add c=a+b

Step 4: Print c

Step 5: Return