Structured Programming

1. INTRODUCTION TO STRUCTURED PROGRAMMING

Basic Programming Concept  
Program and Programming  
A computer program is a series of organized instructions that directs a computer to perform 
tasks. Without programs, computers are useless.  
A program is like a recipe. It contains a list of variables (called ingredients) and a list of 
statements (called directions) that tell the computer what to do with the variables.  
Programming is a creation of a set of commands or instructions which directs a computer in 
carrying out a task.  
A programming language is a set of words, symbols and codes that enables humans to 
communicate with computers.  
Examples of programming languages are:  
 BASIC (Beginner‘s All Purpose Symbolic Instruction Code)  
 Pascal  
 C  
 Smalltalk.   
Structured programming and other programming languages 
Structured programming (sometimes known as modular programming) is a subset of 
procedural programming that enforces a logical structure on the program being written to 
make it more efficient and easier to understand and modify. Certain languages such as Ada, 
Pascal, and dBASE are designed with features that encourage or enforce a logical program 
structure. 
Structured programming frequently employs a top-down design model, in which developers map out 
the overall program structure into separate subsections.Program flow follows a simple hierarchical 
model that employs looping constructs such as "for," "repeat," and "while." Use of the "Go To" 
statement is discouraged. 
C is called a structured programming language because to solve a large problem, C 
programming language divides the problem into smaller modules called functions or 
procedures each of which handles a particular responsibility. The program which solves the 
entire problem is a collection of such functions. One major drawback of C language is that 
similar functions cannot be grouped inside a module or class. Also functions cannot be 
associated to a type or structure. Thus data and functions cannot be bound together. C++ 
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language overcomes these problems by introducing object oriented functionality in its 
programming capabilities.

1.1. Development of Programming Language

FIRST GENERATION OF PROGRAMMING LANGUAGE  
The first generation of programming language, or 1GL, is machine language. Machine language 
is a set of instructions and data that a computer's central processing unit can execute directly. 
Machine language statements are written in binary code, and each statement corresponds to one 
machine action.  
SECOND GENERATION PROGRAMMING LANGUAGE  
The second generation programming language, or 2GL, is assembly language. Assembly 
language is the human-readable notation for the machine language used to control specific 
computer operations. An assembly language programmer writes instructions using symbolic 
instruction codes that are meaningful abbreviations or mnemonics. An assembler is a program 
that translates assembly language into machine language.  
THIRD GENERATION PROGRAMMING LANGUAGE  
The third generation of programming language, 3GL, or procedural language uses a series of 
English-like words, that are closer to human language, to write instructions.  
High-level programming languages make complex programming simpler and easier to read, 
write and maintain. Programs written in a high-level programming language must be translated 
into machine language by a compiler or interpreter.  
PASCAL, FORTRAN, BASIC, COBOL, C and C++ are examples of third generation 
programming languages.  
FOURTH GENERATION PROGRAMMING LANGUAGE  
The fourth generation programming language or non-procedural language, often abbreviated as 
4GL, enables users to access data in a database.  
A very high-level programming language is often referred to as goal-oriented programming 
language because it is usually limited to a very specific application and it might use syntax that is 
never used in other programming languages.  
SQL, NOMAD and FOCUS are examples of fourth generation programming languages.  
FIFTH GENERATION PROGRAMMING LANGUAGE  
The fifth generation programming language or visual programming language is also known as 
natural language. Provides a visual or graphical interface, called a visual programming 
environment, for creating source codes. Fifth generation programming allows people to interact 
with computers without needing any specialized knowledge. People can talk to computers and 
the voice recognition systems can convert spoken sounds into written words.  
Prolog and Mercury are the best known fifth-generation languages.  
EXT : OPEN PROGRAMMING LANGUAGE  
The Open Programming Language (OPL) is an embedded programming language found in 
portable devices that run the Symbian Operating System. For example mobile telephones and 
PDAs. OPL is an interpreted language that is analogous to BASIC.  
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1.2. Programming Paradigms

A programming paradigm is a style or ―way‖ of programming. Some languages make it easy 
to write in some paradigms but not others. Never use the phrase ―programming language 
paradigm.‖ A paradigm is a way of doing something (like programming), and not a concrete 
thing (like a language). 
Some of the more common paradigms are 
 Imperative — Control flow is an explicit sequence of commands.  
 Declarative — Programs state the result you want, not how to get it.  
 Structured — Programs have clean, goto-free, nested control structures.  
 Procedural — Imperative programming with procedure calls.  
 Functional (Applicative) — Computation proceeds by (nested) function calls that avoid 
any global state.  
 Function-Level (Combinator) — Programs have no variables. No kidding.  
 Object-Oriented — Computation is effected by sending messages to objects; objects 
have state and behavior.  
o Class-based — Objects get their state and behavior based on membership in a 
class.  
o Prototype-based — Objects get their behavior from a prototype object.  
 Event-Driven — Control flow is determined by asynchronous actions (from humans or 
sensors).  
 Flow-Driven — Computation is specified by multiple processes communicating over 
predefined channels.  
 Logic (Rule-based) — Programmer specifies a set of facts and rules, and an engine 
infers the answers to questions.  
 Constraint — Programmer specifies a set of constraints, and an engine infers the 
answers to questions.  
 Aspect-Oriented — Programs have cross-cutting concerns applied transparently.  
 Reflective — Programs manipulate their own structures.  
 Array — Operators are extended to arrays, so loops are normally unnecessary.  
Paradigms are not meant to be mutually exclusive; you can program in a functional, object
oriented, event-driven style.

2. PROGRAM DEVELOPMENT AND  DESIGN 

CHAPTER 2: PROGRAM DEVELOPMENT AND 
DESIGN 
Introduction to program development 
Program development in computing; refer to the coding of an individual software program or 
to the creation on an entire information system and all related software. 
Program design is the activity of progressing from a specification of some required program to a 
description of the program itself. 
It is the process that organizations use to develop a program.  Ideally, the process is 
collaborative, iterative, and tentative—stakeholders work together to repeat, review, and refine a 
program until they believe it will consistently achieve its purpose. 
A program design is also the plan of action that results from that process.  Ideally, the plan is 
developed to the point that others can implement the program in the same way and consistently 
achieve its purpose. 

2.1. Program Development Cycle 

Program Development Cycle 

1. Analyze – Define the problem. 
You must have a clear idea of what data (or input) is given and the relationship between the 
input and the desired output. 
2. Design – Plan the solution to the problem. 
Find a logical sequence of precises steps that solve the problem (aka the algorithm). The logical 
plan may include flowcharts, psuedocode, and top-down charts. 
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3. Design the interface – Select objects (text boxes, buttons, etc.). 
Determine how to obtain input and how the output will be displayed. Objects are created to 
receive input and display output. Appropriate menus, buttons, etc. are created to allow user to 
control the program. 
4. Code – Translate algorithm into a programming language. 
During this stage that program is written. 
5. Test and debug – Locate and remove errors in program. 
Testing is the process for finding errors. Debugging is the process for correcting errors. 
6. Complete the documentation – Organize all materials that describe the program. 
Documentation is necessary to allow another programmer or non-programmer to understand 
the program. Internal documentation, known as comments, is created to assist a programmer. 
An instruction manual is created for the non-programmer. Documentation should be done 
during the coding stage.

2.2. Programming Techniques

1.  Structured Programming

This is the first programming approach used widely in beginning. Professor Edsger Wybe Dijkstra (1960) coins the term Structural Programming. Italian computer scientist C. Bohm and

G. Jacopini (1966) give the basic principal that supports this approach. The structured programming movement started in 1970, and much has been written about it. It is often regarded as ―goto-less‖ programming, because it is avoided by programmers.

 

The program is divided into several basic structures. These structures are called building blocks. These are following:

(a)  Sequence Structure: This module contains program statements one after another. This is a very simple module of Structured Programming.

 

Sequence Structure

 

(b)  Selection or Conditional Structure: The Program has many conditions from which correct condition is selected to solve problems. These are (a) if-else (b) else-if, and (c) switch-case

 

Conditional structure

 

(c)  Repetition or loop Structure: The process of repetition or iteration repeats statements blocks several times when condition is matched, if condition is not matched, looping process is terminated. In C, (a) goto, (b) for (), (c) do, (d) do – while are used for this purpose.

 

Loop Structure

 

Advantage:

·       Problem can be easily described using Flowchart and flowchart can be easily coded into program because the nature of this technique is like as flowchart.

·       The program is easily coded using modules.

·       The testing and debugging is easy because testing and debugging can be performed module-wise.

·       Program development cost low.

·       Higher productivity, high quality program production.

·       Easy to modify and maintain

 

Modular Programming

 

 

3.  Top down Approach

(a)  The large program is divided into many small module or subprogram or function or procedure from top to bottom.

 

(b)  At first supervisor program is identified to control other sub modules. Main modules are divided into sub modules, sub-modules into sub- sub- modules. The decomposition of modules is continuing whenever desired module level is not obtained.

 

(c)  Top module is tested first, and then sub-modules are combined one by one and tested.

 

Top down Approach

 

Example: The main program is divided into sub-program A, B, and C. The A is divided into subprogram A1, A2 and A3.The B is into B1, and B2. Just like these subprograms, C is also divided into three subprogram C1, C2 and C3. The solution of Main program is obtained from sub program A, B and C.

 

4.  Bottom up Approach

·       In this approach designing is started from bottom and advanced stepwise to top. So, this approach is called Bottom up approach.

·       At first bottom layer modules are designed and tested, second layer modules are designed and combined with bottom layer and combined modules are tested. In this way, designing and testing progressed from bottom to top.

A monolithic application is self-contained, and independent from other computing applications. The design philosophy is that the application is responsible not just for a particular task, but can perform every step needed to complete a particular function.

3. PROGRAM STRUCTURE

Introduction to program structure 
Program structure The overall form of a program, with particular emphasis on the individual 
components of the program and the interrelationships between these components. Programs are 
frequently referred to as either well-structured or poorly structured. 
Format of a structured programming language 
A C program basically has the following form: 
 Preprocessor Commands 
 Functions 
 Variables 
 Statements & Expressions 
 Comments 
The following program is written in the C programming language. Open a text file hello.c using 
vi editor and put the following lines inside that file. 
#include <stdio.h> 
int main() 
{ 
/* My first program */ 
printf("Hello, World! \n"); 
return 0; 
} 
Preprocessor Commands: These commands tells the compiler to do preprocessing before doing 
actual compilation. Like #include <stdio.h> is a preprocessor command which tells a C compiler 
to include stdio.h file before going to actual compilation. You will learn more about C 
Preprocessors in C Preprocessors session. 
Functions: are main building blocks of any C Program. Every C Program will have one or more 
functions and there is one mandatory function which is called main() function. This function is 
prefixed with keyword int which means this function returns an integer value when it exits. This 
integer value is retured using return statement. 
The C Programming language provides a set of built-in functions. In the above example printf() 
is a C built-in function which is used to print anything on the screen.  
Variables: are used to hold numbers, strings and complex data for manipulation.  
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Statements & Expressions : Expressions combine variables and constants to create new values. 
Statements are expressions, assignments, function calls, or control flow statements which make 
up C programs. 
Comments: are used to give additional useful information inside a C Program. All the comments 
will be put inside /*...*/ as given in the example above. A comment can span through multiple 
lines. 
Note:  
 There should be a main function somewhere in the program to determine where to 
start the executions.  
 Usually all C statements are entered in small case letters.  
 The group of statements in main are executed sequentially.  
 The left brace indicates the program opening.  
 The right brace indicates the program closing.   
 In C language Comments are enclosed with /* -- */ means these statements won‘t execute 
when the program is complied.    
A C program consists of one or more functions. Each function performs a specific task. A 
function is a group or sequence of C statements that are executed together.  
Every C program starts with a function called main(). This is the place where program 
execution begins. Hence, there should be main() function in every C program. The functions are 
building blocks of C program. Each function has a name and a list of parameters. 
The following are some rules to write C programs 
1. All C statements must end with semicolon.  
2. C is case – sensitive. That is, upper case and lower case characters are different. 
Generally the statements are typed in lower case.  
3. A C statement can be written in one line or it can split into multiple lines.  
4. Braces must always match upon pairs, i.e., every opening brace ‗{‗ must have a matching 
closing brace ‗}‘.  
5. A comment can be split into more than one line.  
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3.1. Basic C Programs - Variable and I/O instructions

1) Print Hello Word 
Let us look at a simple code that would print the words "Hello World": 
#include <stdio.h> 
int main() 
{ 
/* my first program in C */ 
printf("Hello, World! \n"); 
return 0; 
} 
Let us look various parts of the above program: 
1. The first line of the program #include <stdio.h> is a preprocessor command, which tells 
a C compiler to include stdio.h file before going to actual compilation. 
2. The next line int main() is the main function where program execution begins. 
3. The next line /*...*/ will be ignored by the compiler and it has been put to add additional 
comments in the program. So such lines are called comments in the program. 
4. The next line printf(...) is another function available in C which causes the message 
"Hello, World!" to be displayed on the screen. 
5. The next line return 0; terminates main()function and returns the value 0. 
Note: Compile & Execute C Program: 
Let‘s look at how to save the source code in a file, and how to compile and run it. Following are 
the simple steps: 
1. Open a text editor and add the above-mentioned code. 
2. Save the file as hello.c 
3. Open a command prompt and go to the directory where you saved the file. 
4. Type gcc hello.c and press enter to compile your code.  
5. If there are no errors in your code the command prompt will take you to the next line and 
would generate a.out executable file. 
6. Now, type a.out to execute your program. 
7. You will be able to see "Hello World" printed on the screen 
2) Declaring Variable Inputing and Outputting Value 
A variable is nothing but a name given to a storage area that our programs can manipulate.  
Each variable in C has a specific type, which determines the size and layout of the variable's 
memory; the range of values that can be stored within that memory; and the set of operations 
that can be applied to the variable. 
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a) Working with variables - Let's look at an example of how to declare an integer variable in 
the C language, assign value and read the value in the variable to output. 
For example: 
int age; 
In this example, the variable named age would be defined as an int. 
Below is an example C program where we declare this variable: 
#include <stdio.h> 
int main() 
{ 
int age; 
age = 10; 
printf("TechOnTheNet.com is over %d years old.\n", age); 
return 0; 
} 
This C program would print "TechOnTheNet.com is over 10 years old." 
Conversion specifiers are made up of two characters: % and a special character. 
The special character tells the program how to convert the data. 
Conversion Specifier Description 
%d 
%f 
%c 
Displays integer value 
Displays floating-point numbers 
Displays character 
Format specifiers are the operators used in printf() function to print the data which is referred by an 
object or a variable. when a value is stored in a particular variable, Then we cannot print the value 
stored in the variable directly with out the using format specifiers. We can retrieve the data stored in 
the variables and can print them on to the console screen by using these format specifiers in a printf 
function. Format specifiers start with a percentage(%) symbol and follows a special character to identify 
the type of the data. There are basically six types of format specifiers are available in c they are  
%d - represent integer values 
%f - represent  float values 
%c - represent single character values 
%s - represent  string values 
%u - represent the address of a variable  
%ld – represent Long  integer values 
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More examples: Use the printf function with formatting options. 
#include <stdio.h> 
main(){ 
int x; 
float y; 
char c; 
x = -4443; 
y = 554.21; 
c = 'M'; 
printf("\nThe value of integer variable x is %d", x); 
printf("\nThe value of float variable y is %f", y); 
printf("\nThe value of character variable c is %c\n", c); 
} 
The value of integer variable x is -4443 
The value of float variable y is 554.210022 
The value of character variable c is M 
More examples: Use the scanf function with formatting options assign to assign value to 
variables. 
This C program would print "Your Name is Kim and you age  is over 10 yrs old." 
#include <stdio.h> 
main(){ 
int Age; 
char Name[10]; 
printf("\nPlz enter your Name "); 
scanf(“%s”,&Name); 
printf("\nPlz enter your Age", y); 
scanf(“%d”,&Age): 
printf("\nYour Name is %s and your age is %d yrs old\n",Name,Age); 
} 
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3.2. Program Structures in Pascal

Before we study basic building blocks of the Pascal programming language, let us look a bare 
minimum Pascal program structure so that we can take it as a reference in upcoming chapters. 
Pascal Program Structure 
A Pascal program basically consists of the following parts − 
 Program name 
 Uses command 
 Type declarations 
 Constant declarations 
 Variables declarations 
 Functions declarations 
 Procedures declarations 
 Main program block 
 Statements and Expressions within each block 
 Comments 
Every pascal program generally has a heading statement, a declaration and an execution part 
strictly in that order. Following format shows the basic syntax for a Pascal program − 
program {name of the program} 
uses {comma delimited names of libraries you use} 
const {global constant declaration block} 
var {global variable declaration block} 
 
function {function declarations, if any} 
{ local variables } 
begin 
... 
end; 
 
procedure { procedure declarations, if any} 
{ local variables } 
begin 
... 
end; 
 
begin { main program block starts} 
... 
end. { the end of main program block } 
 
Pascal Hello World Example 
Following is a simple pascal code that would print the words "Hello, World!": 
program HelloWorld; 
uses crt; 
 
(* Here the main program block starts *) 
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begin 
   writeln('Hello, World!'); 
   readkey; 
end.  
This will produce following result − 
Hello, World! 
Let us look various parts of the above program − 
 The first line of the program program HelloWorld; indicates the name of the program. 
 The second line of the program uses crt; is a preprocessor command, which tells the 
compiler to include the crt unit before going to actual compilation. 
 The next lines enclosed within begin and end statements are the main program block. 
Every block in Pascal is enclosed within a begin statement and an end statement. 
However, the end statement indicating the end of the main program is followed by a full 
stop (.) instead of semicolon (;).  
 The begin statement of the main program block is where the program execution begins. 
 The lines within (*...*) will be ignored by the compiler and it has been put to add a 
comment in the program.  
 The statement writeln('Hello, World!'); uses the writeln function available in Pascal 
which causes the message "Hello, World!" to be displayed on the screen. 
 The statement readkey; allows the display to pause until the user presses a key. It is part 
of the crt unit. A unit is like a library in Pascal. 
 The last statement end. ends your program. 
Pascal - Basic Syntax 
You have seen a basic structure of pascal program, so it will be easy to understand other basic 
building blocks of the pascal programming language. 
 
Variables 
A variable definition is put in a block beginning with a var keyword, followed by definitions of 
the variables as follows: 
var 
A_Variable, B_Variable ... : Variable_Type; 
 
i.e. A, b, c:interger 
Pascal variables are declared outside the code-body of the function which means they are not 
declared within the begin and end pairs, but they are declared after the definition of the 
procedure/function and before the begin keyword. For global variables, they are defined after the 
program header. 
Functions/Procedures 
In Pascal, a procedure is set of instructions to be executed, with no return value and a function 
is a procedure with a return value. The definition of function/procedures will be as follows − 
Function Func_Name(params...) : Return_Value; 
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Procedure Proc_Name(params...); 
 
Comments 
The multiline comments are enclosed within curly brackets and asterisks as {* ... *}. Pascal 
allows single-line comment enclosed within curly brackets { ... }.  
{* This is a multi-line comments 
   and it will span multiple lines. *} 
 
{ This is a single line comment in pascal } 
 
Case Sensitivity 
Pascal is a case non-sensitive language, which means you can write your variables, functions and 
procedure in either case. Like variables A_Variable, a_variable and A_VARIABLE have same 
meaning in Pascal. 
 
Pascal Statements (I/O)  
Pascal programs are made of statements. Each statement specifies a definite job of the program. 
These jobs could be declaration, assignment, reading data, writing data, taking logical decisions, 
transferring program flow control, etc. 
For example − 
readln (a, b, c); 
s := (a + b + c)/2.0; 
area := sqrt(s * (s - a)*(s-b)*(s-c)); 
writeln(area);         
 
Note: All pascal functions and control structures start with name and follow with Begin..End in 
place of carry bracket “, -” used in C programs. 
1. Function Example 
function name(argument(s): type1; argument(s): type2; ...): function_type; 
local declarations; 
begin 
   ... 
   < statements > 
   ... 
   name:= expression; 
end; 
 
program exFunction; 
var 
   a, b, ret : integer; 
 
2. Function and control structure Example 
(*function definition *) 
function max(num1, num2: integer): integer; 
var 
   (* local variable declaration *) 
   result: integer; 
 
begin 
if (num1 > num2) then 
begin 
result := num1 
writeln(result); 
end; 
else 
begin 
result := num2; 
max := result; 
writeln(max); 
end; 
end; 
begin 
a := 100; 
b := 200; 
(* calling a function to get max value *) 
ret := max(a, b); 
writeln( 'Max value is : ', ret ); 
end.