We construct algorithms using basic building blocks such as
• Data
• Variables
• Control flow
• Functions

**Building Blocks of Algorithms**

We construct algorithms using
basic building blocks such as

• Data

• Variables

• Control
flow

• Functions

Algorithms take input data, process
the data, and produce output data. Computers provide instructions to perform
operations on data. For example, there are instructions for doing arithmetic
operations on numbers, such as add, subtract, multiply and divide. There are
different kinds of data such as numbers and text.

Variables are named boxes for
storing data. When we do operations on data, we need to store the results in
variables. The data stored in a variable is also known as the value of the
variable. We can store a value in a variable or change the value of variable,
using an assignment statement.

Computational processes in the
real-world have state. As a process evolves, the state changes. How do we
represent the state of a process and the change of state, in an algorithm? The
state of a process can be represented by a set of variables in an algorithm.
The state at any point of execution is simply the values of the variables at
that point. As the values of the variables are changed, the state changes.

**Example 6.4. State: **A traffic signal may** **be in one of the three states:
green, amber, or red. The state is changed to allow a smooth flow of traffic.
The state may be represented by a single variable signal which can have one of
the three values: green, amber, or red.

An algorithm is a sequence of
statements. However, after executing a statement, the next statement executed
need not be the next statement in the algorithm. The statement to be executed
next may depend on the state of the process. Thus, the order in which the
statements are executed may differ from the order in which they are written in
the algorithm. This order of execution of statements is known as the control
flow.

There are three important control
flow statements to alter the control flow depending on the state.

• In
sequential control flow, a sequence of statements are executed one after
another in the same order as they are written.

• In
alternative control flow, a condition of the state is tested, and if the
condition is true, one statement is executed; if the condition is false, an
alternative statement is executed.

• In
iterative control flow, a condition of the state is tested, and if the
condition is true, a statement is executed. The two steps of testing the
condition and executing the statement are repeated until the condition becomes
false.

Algorithms can become very
complex. The variables of an algorithm and dependencies among the variables may
be too many. Then, it is difficult to build algorithms correctly. In such
situations, we break an algorithm into parts, construct each part separately,
and then integrate the parts to the complete algorithm.

The parts of an algorithm are
known as functions. A function is like a sub algorithm. It takes an input, and
produces an output, satisfying a desired input output relation.

**Example 6.5. **Suppose we want to calculate** **the surface area of a cylinder of
radius r and height h.

*A
= 2πr ^{2} + 2πrh*

We can identify two functions, one
for calculating the area of a circle and the other for the circumference of the
circle. If we abstract the two functions as circle_ area(r) and
circle_circumference(r), then cylinder_area(r, h) can be solved as

*cylinder_area
(r,h) = 2 X circle_area (r) + circle_circumference (r) X h*

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11th Computer Science : Chapter 6 : Specification and Abstraction : Building Blocks of Algorithms |

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