understanding structure of program,variables and operators in c

A simple c program

/* Purpose: display the message */

#include

int main()

{
printf("%s"," welcome to c programming writting");

return0;
}


Anatomy of a C Program

/*program header comment*/

preprocessor directives (if any)

int main ( ) /*it is the main function, program execution begins from here*/
{
statement(s)
return 0 ;
}

Program Header Comment
 
• A comment is descriptive text used to help a
  reader of the program understand its
  content.
• All comments must begin with the characters
     /* and end with the characters */
• These are called comment delimiters
• The program header comment always
   comes first.


Preprocessor Directives
• Lines that begin with a # in column 1 are
  called preprocessor directives
  (commands).
• Example: the #include directive
  causes the preprocessor to include a copy of
  the standard input/output header file stdio.h at
  this point in the code.
• This header file was included because it
  contains information about the printf ( )
  function that is used in this program.
  When we write our programs, there are
  libraries of functions to help us so that we
  do not have to write the same code over
  and over again.
• Some of the functions are very complex
  and long. Not having to write them
  ourselves make it easier and faster to
  write programs.
• Using the functions will also make it easier
  to learn to program!

int main ( ) or void main()
 
• Every program must have a function called
  main. This is where program execution begins.
• main() is placed in the source code file as the
  first function for readability.
• The reserved word “int” indicates that main()
  returns an integer value.
• The parentheses following the reserved word
  “main” indicate that it is a function.
• The reserved word “void” means nothing is
  there.

The Function Body

• A left brace (curly bracket) -- { -- begins the
  body of every function. A corresponding
  right brace -- } -- ends the function body.
• The style is to place these braces on
  separate lines in column 1 and to indent the
  entire function body 3 to 5 spaces.



printf (“Hello, World!\n”) ;

• This line is a C statement.
• It is a call to the function printf ( ) with a
  single argument (parameter), namely the
  string “Hello, World!\n”.
• Even though a string may contain many
  characters, the string itself should be
  thought of as a single quantity.
• Notice that this line ends with a semicolon.
  All statements in C end with a semicolon.

return 0 ;
 
• Because function main() returns an integer value,
there must be a statement that indicates what this
value is.
• The statement
return 0 ;
indicates that main() returns a value of zero to
the operating system.
• A value of 0 indicates that the program successfully
terminated execution.
• Do not worry about this concept now. Just
remember to use the statement.





Tokens

• The smallest element in the C language is
the token.
• It may be a single character or a sequence
of characters to form a single item.

Tokens are:
• Tokens can be:
– Numeric constants
– Character constants
– String constants
– Keywords
– Names (identifiers)
– Punctuation
– Operators

Numeric Constants

• Numeric constants are an uninterrupted
sequence of digits (and may contain a
period). They never contain a comma.
• Examples:
– 123
– 98.6
– 1000000


Character Constants
 
• One character from a defined character
set.
• Surrounded on the single quotation mark.
• Examples:
– ‘A’
– ‘a’
– ‘$’
– ‘4’

String Constants
• A sequence characters surrounded by
  double quotation marks.
• Considered a single item.
• Examples:
– “Akv”
– “I like ice cream.”
– “123”
– “D-skY”

Keywords
• Sometimes called reserved words.
• Are defined as a part of the C language.
• Can not be used for anything else!
• Examples:
– int
– while
– for

Names

• Sometimes called identifiers.
• Can be of anything length, but on the first 31 are
  significant (too long is as bad as too short).
• Are case sensitive:
– abc is different from ABC
• Must begin with a letter and the rest can be
  letters, digits, and underscores.
• There can be one exception to beginning letter
  that variable name can start with underscore( _ )
  but it is used by C library.

Punctuation
• Semicolons, colons, commas,
apostrophes, quotation marks, braces,
brackets, and parentheses.
• ; : , ‘ “ [ ] { } ( )

Operators
• There are operators for:
– assignments
– mathematical operations
– relational operations
– Boolean operations
– bitwise operations
– shifting values
– calling functions
– subscripting
– obtaining the size of an object
– obtaining the address of an object
– referencing an object through its address


What Are Variables in C?
• Variables in C have the same meaning as
Variables in algebra. That is, they represent
Some unknown, or variable, value.
x = a + b
z + 2 = 3(y - 5)

• Remember that variables in algebra are
  represented by a single alphabetic
  character.

Naming Variables
• Variables in C may be given representations
  containing multiple characters. But there are
  rules for these representations.
• Variable names (identifiers) in C
– May only consist of letters, digits, and
   underscores
– May be as long as you like, but only the first 31
   characters are significant
– May not begin with a digit
– May not be a C reserved word (keyword)


Reserved Words (Keywords) in C
• auto • break
• case • char
• const • continue
• default • do
• double • else
• enum • extern
• float • for
• goto • if
• int • long
• register • return
• short • signed
• sizeof • static
• struct • switch
• typedef • union
• unsigned • void
• volatile • while

Naming Conventions
 
• C programmers generally agree on the
following conventions for naming variables.
– Begin variable names with lowercase letters
– Use meaningful identifiers
– Separate “words” within identifiers with
underscores or mixed upper and lower case.
– Examples: surfaceArea surface_Area
surface_area
– Be consistent!
Use all uppercase for symbolic constants
(used in #define preprocessor directives).
• Note: symbolic constants are not variables,
but make the program easier to read.
• Examples:
#define PI 3.14159
#define AGE 52


Case Sensitivity
 

• C is case sensitive
– It matters whether an identifier, such as a
variable name, is uppercase or lowercase.
– Example:
area
Area
AREA
ArEa
are all seen as different variables by the
compiler


Which Are Legal Identifiers?

AREA
area_under_the_curve
3D
num45
Last-Chance
#values
x_yt3
pi
num$
%done
lucky***


Declaring Variables
 
• Before using a variable, you must give the
compiler some information about the variable;
i.e., you must declare it.
• The declaration statement includes the data
type of the variable.
• They must be declared just after the start of
block (i.e. start of a function) and before any
other executable statement.
• Examples of variable declarations:
int meatballs ;
float area ;

• When we declare a variable
– Space is set aside in memory to hold a value of
   the specified data type
– That space is associated with the variable name
– That space is associated with a unique address

 

Notes About Variables
 
• You must not use a variable until you
somehow give it a value.
• You can not assume that the variable will
have a value before you give it one.
– Some compilers do, others do not! This is the
source of many errors that are difficult to find.

Using Variables: Assignment

• Variables may have values assigned to them through

the use of an assignment statement.

• Such a statement uses the assignment operator =

• This operator does not denote equality. It assigns

the value of the right-hand side of the statement (the

expression) to the variable on the left-hand side.

• Examples:       

diameter = 5.9 ;

area = length * width ;

Note that only single variables (LValue) may appear

on the left-hand side of the assignment operator.

Getting Input from User

• Every process requires some input from

the user. Variables hold the input values.

• We have a function called scanf( ) that will

allow us to do that.

• The function scanf needs two pieces of

information to display things.

– The data type of input values

– Address where to store the values• scanf( “%f”, &diameter );

Displaying Variables

• Variables hold values that we occasionally

want to show the person using the

program.

• We have a function called printf( ) that will

allow us to do that.

• The function printf needs two pieces of

information to display things.

– How to display it

– What to display• printf( “%f\n”, &diameter );

Both printf and scanf Returns a Value

• We can call printf as

          i=810;

       n=printf(“%d”,i);

• We also can call a scanf

     m=scanf(“%d%f”,&i,&f)

check out the value of n & m if every thing goes fine?

Good Programming Practices

• Place each variable declaration on its own

   line with a descriptive comment.

• Place a comment before each logical

  “chunk” of code describing what it does.

• Do not place a comment on the same line as

   code (with the exception of variable

   declarations).

• Use spaces around all arithmetic and

   assignment operators.

• Use blank lines to enhance readability.

• Place a blank line between the last

  variable declaration and the first

  executable statement of the program.

• Indent the body of the program 3 to 5

  spaces -- be consistent!

• Comments should explain why you are

   doing something, not what you are doing

   it.

a = a + 1 /* add one to a */           /* WRONG */

              /* count new student */ /* RIGHT*/