Data:
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:
Variables are named boxes for storing data. When we do operations oft 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.
Control flow:
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 front 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.
1. In sequential control flow, a sequence of statements are executed one after another in the same order as they are written.
2. 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.
Functions:
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.

1. m, n:= 2, 5
2. -- m, n = ? , ?
3. m,n: = m + 3, n - 1
4. -- m, n = ? , ?
The assignment in line (1) stores 2 in variable m, and 5 in variable n.
\(m\ n\\ \boxed { 2 } \ \boxed { 5 } \)
The assignment in line (3) evaluates the expressions m + 3 and n - 1 using the current values of m and n as
m+3,n-1
= 2+ 3,5-1
=5,4
and stores the values 5 and 4 in the variables m and n, respectively.
\(m\ n\\ \boxed { 4 } \ \boxed { 4 } \)
1. m, n:= 2,5
2. -- m, n = 2 , 5
3. m, n : = m + 3, n - 1
4. -- m, n = 2 + 3, 5-1 = 5, 4
Values of the variables after the two assignments are shown in line (2) and line (4).

There are a few basic principles and techniques for designing algorithms.
1. Specification:
The first step in problem-solving is to state the problem precisely. A problem is specified in terms of the input given and the output desired. The specification must also state the . properties of the given input; and the relation between the input and the output.
2. Abstraction:
A problem can involve a lot of details. Several of these details are unnecessary for solving the problem. Only a few details are essential. Ignoring or hiding unnecessary details and modeling an entity only by its essential properties is known as abstraction. For example, when we represent the state of a process, we select only the variables essential to the problem and ignore inessential details.
3. Composition:
An algorithm is composed of assignment and control flow statements. A control flow statement tests a condition of the state and depending on the value of the condition, decides the next statement to be executed.
4. Decomposition:
We divide the main algorithm into functions. We construct each function independently of the main algorithm and other functions. Finally, we construct the main algorithm using. the functions. When we use the functions, it is enough to know the specification of the function. It is not necessary to know how the function is implemented.

There are a few basic principles and techniques for designing algorithms.
(i) Specification:
The first step in problem solving is to state the problem precisely. A problem is specified in terms of the input given and the output desired. The specification must also state the properties of the given input, and the relation between the input and the output.
(ii) Abstraction:
A problem can involve a lot of details. Several of these details are unnecessary for solving the problem. Only a few details are essential. Ignoring or hiding unnecessary detail and modeling an entity only by its essential properties is known as abstraction.
(iii) Composition:
An algorithm is composed of assignment and control flow statements. A control flow statement tests a condition of the state and, depending on the value of the condition, decides the next statement to be executed.
(iv) Decomposition:
We divide the main algorithm into functions. We construct each function independently of the main algorithm and other functions. Finally, construct the main algorithm using the functions. When we use the functions. it is enough to know the specification of the function. It is not necessary to know how the function is implemented.

To construct algorithms using basic building blocks such as. Data, Variables, Control flow, Functions.
Data:
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:
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.
Control flow :
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.
Functions:
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.