Programming Fundamentals lecture 5

1. C Program Structure
REHAN IJAZ
By
ProgrammingFundamentals

2.  The main function serves as the starting point for program execution.
 It usually executes statements and controls program execution by
directing the calls to other functions in the program.
 A program usually stops executing at the end of main, although it can
terminate at other points in the program for a variety of reasons.
 At times, perhaps when a certain error is detected, you may want to force
the termination of a program. To do so, use the exit function.

3.  All C language programs must have a main() function.
 It's the core of every program. It's required.
 The main() function doesn't really have to do anything other than be
present inside your C source code.
 Eventually, it contains instructions that tell the computer to carry out
whatever task your program is designed to do. But it's not officially
required to do anything.

4.  When the operating system runs a program in C, it passes control of the
computer over to that program.
 In the case of a C language program, it's the main() function that the
operating system is looking for to pass the control.
 At a minimum, the main() function looks like this:
main()
{
}

5.  Like all C language functions,
 first comes the function's name, main,
 then comes a set of parentheses, and
 finally comes a set of braces, also called curly braces.

6. Command Explanation
1 #include <stdio.h> This is a preprocessor command that includes standard
input output header file(stdio.h) from the C library before
compiling a C program
2 int main() This is the main function from where execution of any C
program begins.
3 { This indicates the beginning of the main function.
4 printf(“Hello_World! “); printf command prints the output onto the screen.
5 getch(); This command waits for any character input from
keyboard.
6 return 0; This command terminates C program (main function) and
returns 0.
7 } This indicates the end of the main function.

7.  A statement is a command given to the computer that instructs the
computer to take a specific action, such as display to the screen, or
collect input.
 A computer program is made up of a series of statements.
 An assignment statement assigns a value to a variable. A printf()
statement writes desired text e.g.
 printf(“Hello_World! “);
 Int Stdregno, age;

8.  Compound Statement also called a "block“is the way C groups multiple
statements into a single statement with the help of braces (i.e. { and }).
 The body of a function is also a compound statement by rule.
#include <stdio.h>
void main()
{
printf("Hello, world!n");
getch();
}

9.  A comment is a note to yourself (or others) that you put into your source
code.
 Comments provide clarity to the C source code allowing yourself and
others to better understand what the code was intended to accomplish
and greatly helping in debugging the code.
 All comments are ignored by the compiler they are not executed as part
of the program.

10.  Comments are especially important in large projects containing hundreds
or thousands of lines of source code or in projects in which many
contributors are working on the source code.
 Comments are typically added directly above the related C source code.
 Adding source code comments to your C source code is a highly
recommended practice.

11.  Types
(i) Single line Comment
// comment goes here
(ii) Block Comment
/* comment goes here
more comment goes here */

12.  When we say Input, it means to feed some data into a program. An input
can be given in the form of a file or from the command line. C
programming provides a set of built-in functions to read the given input
and feed it to the program as per requirement.
 When we say Output, it means to display some data on screen, printer, or
in any file. C programming provides a set of built-in functions to output
the data on the computer screen as well as to save it in text or binary
files.

13.  INPUT: When we say Input, it means to feed some data into a program.
An input can be given in the form of a file or from the command line. C
programming provides a set of built-in functions to read the given input
and feed it to the program as per requirement.
 OUTPUT: When we say Output, it means to display some data on screen,
printer, or in any file. C programming provides a set of built-in functions
to output the data on the computer screen as well as to save it in text or
binary files.

14. printf()
 function writes the output to the standard output stream and produces
the output according to the format provided.
 The format can be a simple constant string, but you can specify %s, %d,
%c, %f, etc., to print or read strings, integer, character or float
respectively.

15. scanf()
 function reads the input from the standard input stream and scans that
input according to the format provided.

16. Example
#include <stdio.h>
int main( )
{
char str[100];
int i;
printf( "Enter values :");
scanf("%s %d", str, &i);
printf( "nYou entered: %s %d ", str, i);
return 0;
}
Enter a value : seven 7
You entered: seven 7

17.  C pre processor refers to a separate program which will be invoked by the
compiler as first part of translation.
 Before a C program is compiled in a compiler, source code is processed by a
program called preprocessor. This process is called preprocessing.
 Commands used in preprocessor are called preprocessor directives and they
begin with “#” symbol.
Source Code Preprocessor Compiler Object Code

18.  The preprocessor provides the ability for the inclusion of header files.
 Preprocessor directives are lines included in the code of programs preceded
by a hash sign (#).
 These lines are not program statements but directives for the preprocessor.
 The preprocessor examines the code before actual compilation of code
begins and resolves all these directives before any code is actually
generated by regular statements.

19.  No semicolon (;) is expected at the end of a preprocessor directive.
 Preprocessor directive can be extended by a backslash ().

20. Few examples of preprocessor directives
 #include<stdio.h>
 #include<conio.h>
 #include<math.h>
etc.

21. S.no Preprocessor Syntax Description
1
Header file
inclusion
#include
<file_name>
The source code of the file “file_name” is included in the
main program at the specified place
2 Macro #define
This macro defines constant value and can be any of the basic
data types.
3
Conditional
compilation
#ifdef,
#endif, #if,
#else,
#ifndef
Set of commands are included or excluded in source program
before compilation with respect to the condition
4
Other
directives
#undef,
#pragma
#undef is used to undefine a defined macro variable.
#Pragma is used to call a function before and after main
function in a C program

22.  #include pre processor directive is used to include header files in the
program.
 If header file name is enclosed within angle brackets (), then compiler
search this file only within its standard include directory.
 Similarly, if header file name is enclosed within double quotes (“ ”) then
compiler will search this file within and out side of its standard path.
1. Header file inclusion (#include)

23. #include <stdio.h>
void main()
{
printf("Hello, world!n");
getch();
}
1. Header file inclusion (#include)
Example

24.  In C/C++ programming language, Define directive is used for multi
functions such as to define a symbol, to define a name constant or to
define macros (short and fast processed code)
 (a) To define a symbol,
 (b)To define name and constants
 (c) To define macros
2. Macro - Define Directive (#define)

25. (a) To define a symbol,
 In the first statement, NULL is a symbol which would be replaced
with 0 in a program. Similarly, the symbol PLUS can be replaced with
+. So following both statements will be work in the same way.
2. Macro - Define Directive (#define)
C=A PLUS B;
it will work same as C=A+B;
#define NUL 0
#define PLUS +

26. (b) To define name and constants
 #define preprocessor can help to define a named constant. Such as
2. Macro - Define Directive (#define)
#define INTEGER 5
#define CHARACTER A

27. (c) To define macro
 In C/C++ programming language, a macro is defined as a segment of text
and it is very useful due to its readability or compact factors. Some
examples of macros are
2. Macro - Define Directive (#define)
#define pi 22.0/7
#define Area(r) 3.14*r*r
 The first macro is very simple and its name can be used in any expression such as
A= pi* 5. Similarly, second macro has a parameter r and it calculate the area.
 If a macro is defined at more than one line then you need to place at the end of
each line except last line. Symbol is used for continuation from next line.

28. 2. Macro - Define Directive (#define) (c) To define macro
value of height : 100
value of number : 3.140000
value of letter : A
value of letter_sequence : ABC
value of backslash_char : ?
#include <stdio.h>
#define height 100
#define number 3.14
#define letter 'A'
#define letter_sequence "ABC"
#define backslash_char '?'
void main()
{
printf("value of height : %d ", height );
printf("value of number : %f ", number );
printf("value of letter : %c ", letter );
printf("value of letter_sequence : %s ", letter_sequence);
printf("value of backslash_char : %c ", backslash_char);
}
Output:

29.  In this type of preprocessor, #ifdef directive is used to check the
existence of defined macros, constant name or symbol. For example,
when we write following statement
 The work of #ifndef directive is same as of #ifdef but there is slight
difference. For #ifdef directive, if a symbol or named constant or
macro has been undefined by using #undef then #ifdef will return
false valu, but #ifndef will return true value for such case.
3. Conditional Compilation(#ifdef, #ifndef, #endif)

30. 3. Conditional Compilation(#ifdef, #ifndef, #endif)
Khalid is defined. So, this line will be added in this C file
#include <stdio.h>
#define Khalid 10
int main()
{
#ifdef Khalid
printf("Khalid is defined. So, this line will be added in this C file");
#else
printf("Khalid is not defined");
#endif
return 0;
}
Output:

31.  #undef preprocessor directive is used to undefined a macro, named
constant or any symbol which has been defined earlier.
 Unlike #include and # define directives, #undef directive can be used
everywhere in a program but after the #define preprocessor.
#undef pi
#undef PLUS
4. Undefine Directives (#undef)