STRUCTURED PROGRAMMING

• Designing and solving a programming problem using sequence, selection and iteration control instructions are called as structured programming
• It employs a top-down design model, in which the program is divided into modules 

Principles of Structured Programming

• At all times and under all circumstances, the programmer must keep the program within his intellectual grasp 
• The well known methods for achieving this : 
• Top-down design and construction 
• Limited control structures 
• Limited scope of data structures 
• Major elements of structured programming 
• Sequence control structure 
• Selection control structure 
• Iteration control structure 

Rules of Structured Programming

• Code no more than one statement per line. 
• Divide programs into functions. 
• Functions should perform only one task. 
• Variable names should be meaningful. 
• Use of global variables should be avoided. 
• All variables should be explicitly declared and given an initial value. 
• Hierarchical data structures should be used to keep the data and program structured. 
• Each function must have an initial comment 
• Compound conditional statements should be limited to avoid confusion. 
• Indentation should be consistent. 
• When used in functions, the use of { and } should be on their own lines to demark blocks of code. 
• Upper case should be used for defined static constants. 
• For all keywords and variables, use lower case. 

Advantages of Structured Programming

• Easy to write 
• Easy to debug 
• Easy to understand 
• Easy to change

BASIC CONTROL STRUCTURE

• Control structures are the basic entities of a structured programming language
• The mechanism that allow us to control the flow of execution are called control structures. 
• There are 4 main categories of control structures :
• Sequence
• Selection
• Iteration
• Branching

SEQUENCE

    

• Do one instruction the the next and the next
• It just executes them in the given sequence or in order listed
• Most lines of the code in the program belongs to this category.

e.g :

statement 1 is executed first, followed by statement 2, then finally statement 3.

SELECTION

    
• Selection structures are used to perform ‘decision making‘ and then branch the program flow based on the outcome of decision making
• Implemented in C/C++ with If, If Else and Switch statements
• If and If Else statements are 2 way branching statements 
• Switch is a multi branching statement.

e.g : If - then - else

if (condition) then

   Statement 1;

else

   Statement 2;

• Condition is a boolean expression 
• Evaluates either True / False
• If True - Statement 1 will be executed
• Otherwise - Statement 2 will be executed
• S1 & S2 may be single statement or group of statements

ITERATION

• Iterative construct means that some statements will be executed multiple times until some condition is met
• Such construct implements a loop structure in which action is executed multiple times, as long as some condition is true 
• In C, iterative constructs can be implemented using while, do-while, or for loop statements
• While is an entry controlled loop. Statement inside braces are allowed to execute only if condition inside while is TRUE.
• Do while is an exit controlled loop.
• For statement is an entry controlled loop. 
• The difference between while and for is in the number of repetitions.
• The for loop is used when an action is to be executed for a predefined number of times. 
• The while loop is used when the number of repetitions is not predefined.

e.g :

BRANCHING

• A control structure that allows the flow execution to jump to a different part of the program.
• This category is rarely used in modular structured programming.

e.g : Go - To Statement

• It provides an unconditional jump from the 'goto' to a labelled statement in the same function.
• Use of go-to statement is highly discouraged in any programming language because it makes difficult to trace the control flow of a program, making the program hard to understand and hard to modify. 
• Any program that uses a go-to can be rewritten to avoid them.

PRACTICE WITH FLOWCHARTS

1. Draw a flowchart for making a telephone call
2. Draw a flowchart to calculate C=A*B
3. Draw a flowchart to find out the smallest of 2 numbers
4. Draw a flowchart for adding integers from 1 to 100
5. Draw a flowchart to reverse a given number
6. Draw a flowchart to find out the largest of 3 numbers
7. Draw a flowchart to find the factorial of a number
8. Draw a flowchart to calculate the simple interest
9. Draw a flowchart to find the roots of a quadratic equation
10. Draw a flowchart to find FCF of 2 numbers
11. Draw a flowchart to print each 3 digit even number and its square(i.e., from 100 to 998)
12. Draw a flowchart to find all integer divisors of a given number
13. Draw a flowchart for preparing a bill of sales taking item, quantity as its input.
14. Draw a flowchart to find the area of a triangle for the given 3 sides of triangle.  

PROGRAM DESIGN TOOL - FLOWCHARTS

• A flowchart is a symbolic representation of the step by step solution of a given problem.
• It indicates the flow of the entire process, the sequence of data input, operations, computations, decisions, results and other relevant information pertaining to a particular problem.
• helpful in understanding the logic of complicated and lengthy problems.
• Once the flowchart is drawn, it becomes easy to write the program in any high level language.
• There are 2 types of flowcharts
• System flowcharts - showing controls at a physical or resource level
• Program flowchart - showing the controls in a program within a system

FLOWCHART SYMBOLS

ADVANTAGES OF A FLOWCHART

• Communication - Flowcharts are better way of communicating the logic of a system to all concerned or involved.
• Effective analysis - With the help of flowchart, problem can be analysed in more effective way therefore reducing cost and wastage of time.
• Proper documentation - Program flowcharts serve as a good program documentation, which is needed for various purposes, making things more efficient.
• Efficient Coding - The flowcharts act as a guide or blueprint during the systems analysis and program development phase.
• Proper Debugging - The flowchart helps in debugging process.
• Efficient Program Maintenance - The maintenance of operating program becomes easy with the help of flowchart. It helps the programmer to put efforts more efficiently on that part

DISADVANTAGES OF A FLOWCHART

• Complex logic - Sometimes, the program logic is quite complicated. In that case, flowchart becomes complex and clumsy. This will become a pain for the user, resulting in a waste of time and money trying to correct the problem
• Alterations and Modifications - If alterations are required the flowchart may require re-drawing completely. This will usually waste valuable time.
• Reproduction - As the flowchart symbols cannot be typed, reproduction of flowchart becomes a problem.

Practice with flowcharts.

PRACTICE WITH ALGORITHMS

1. Write an algorithm for making a telephone call
2. Write an algorithm for the average of 3 numbers
3. Write an algorithm to find out the discriminant of a quadratic equation
4. Write an algorithm for preparing a bill of sales
5. Write an algorithm to find the sum of all natural numbers between 1 and n
6. Write an algorithm to find the largest of 3 integers
7. Write an algorithm to find simple interest
8. Write an algorithm to find greatest of 2 numbers
9. Write an algorithm to find the greatest of 3 numbers
10. Write an algorithm to find the factorial of a number
11. Write an algorithm to find the LCM of 2 numbers
12. Write an algorithm to reverse a number
13. Write an algorithm to find all integer divisors of a given number
14. Given 3 numbers a, b, a the sides of a triangle. Check whether with the 3 sides given, it forms on Isosceles or equilateral Isosceles triangle

PROGRAM DESIGN TOOL - ALGORITHM

• An algorithm is a Characteristics of an algorithm

• Well defined set of steps for solving a problem.
• Logical concise list of procedures for solving a problem
• Complete unambiguous finite number of logical steps for solving any problem on the computer

• Characteristics of an algorithm 

• Precision  -  the steps are precisely defined.
• Uniqueness - results of each step are uniquely defined and only depend on the input and the result of the preceding steps.
• Finiteness - the algorithm stops after a finite number of instructions are executed.
• Input - the algorithm receives input.
• Output - the algorithm produces output.
• Generality - the algorithm applies to a set of inputs.

• Advantages of an algorithm

• Reduced complexity. 
• Increased flexibility. 
• Ease of understanding

• Disadvantages of an algorithm

• Multiple function calls are expensive.
• Inefficient when there is lots of branching from one state.
• Requires large amount of space as the each function state needs to be stored on system stack.

Practice with algorithms

UNIT 1 : INTRODUCTION

COMPUTATION

• The term Computation refers to the process of arriving at output from a given problem. 
• It should be logical and the process should follow a step by step approach to arrive at a meaningful output or result. 
• To achieve this, the basic computation model should be followed. 

STEPS IN PROBLEM SOLVING   

• The steps in problem solving are called as Program Development Life Cycle (PDLC).
• It is shown in the below diagram.

 

• Problem Definition

• Identify & define the problem
• It must be stated clearly, accurately & precisely
• It should not be ambiguous

• Problem Analysis 

• The following should be stated clearly 
• Input specification
• Output specification
• The processing steps required to convert given inputs to outputs.

• Designing A Problem

• Designing the solution for the problem 
• Obtained by using the program design tools 
• Algorithms - step by step procedure of solving a problem 
• Flowcharts - graphical representation of the algorithm. 
• The following ways are also followed 
• Top-down design approach 
• The problem is divided into sub-problems and further dividing them into smaller sub-problems until it is small enough to solve the problem individually. 
• Allows the programmer to build a solution in a step-wise manner 
• Bottom-up design 
• Modular design

• Coding

• Writing the instructions in a particular language to solve a problem 
• Data items, variables & constants are defined

• Testing A Program

• Testing is done to verify the quality of the program (completeness, correctness, reliability & maintainability)
• Testing can be done at the following levels
• Unit testing
• Program testing
• Verification testing
• Validation testing

• Maintaining A Program

• Modifications based on user requirements
• Periodic review of the programs & modifications based on user requirements
• Requires proper documentation
• Its a continuous activity