ICS 103:
Computer Programming in C
Handout-02
Topic:
Overview of C Language
Instructor:
M. Waheed Aslam.
Objective:
C LANGUAGE ELEMENTS:
(1) Preprocessor Directives:
Syntax:
#include
<standard header file>
e.g
#include <stdio.h>
#include <math.h>
Syntax:
#define NAME value
e.g
#define PI 3.141593
#define MILES_PER_KM 0.62137
#define MAX_LENGTH 100
In this declaration, each identifier NAME is replaced
by the constant value and this value will remain fixed (constant) throughout
the whole program.
/* This is a comment: Could be side comment, stand alone or in
between */
(2) Function main:
Syntax:
int main (void)
{
function body /* it has
two parts: declarations and executable statements */
}
Example:
int /* function returns an integer value (0) to the OS when
terminates normally.*/
main (void) /* void
indicates that main function receive no input data from the OS.*/
{ /* Open brace symbol marks start of function body. */
printf ("Assalamu Aleikum\n");
return (0);
} /*Closing
brace symbol marks end of function body*/
Outline of a C Program:
Then the general format of a
C program is:
preprocessor directives
int main (void )
{
variable declarations;
statements; /* Executable statements are
derived from the algorithm */
}
Example: C program to find addition of two numbers is:
/* To find sum of
two numbers */
#include <stdio.h> /* preprocessor directive*/
main () /* main function*/
{
int number1,
number2, sum; /* variable declarations*/
printf
("Please input two numbers: "); /* to display message*/
scanf ("%d
%d",&number1, &number2); /* to read 2 numbers*/
sum = number1+number2;
/* to calculate sum*/
printf ("The
sum of two numbers is: %d " ,sum);
/* to print sum*/
return (0);
} /*end of main*/
Overview of C Program:
Complete overview
of C Program is explained in following example:
/* Displays the user's nickname
and the current year in a welcoming message. */
#include <stdio.h>
/* printf, scanf definitions */
int main(void) // main function
{
char
letter_1, letter_2, letter_3 , letter_4; /* four
letters */
int year; /* current
year */
printf("Enter a 4-letter name and press return: ");
scanf("%c %c %c %c", &letter_1, &letter_2,
&letter_3, &letter_4); //to read 4 letters
from keyboard
printf("Enter the current year and press return:");
scanf("%d", &year); //
to read year from keyboard
printf("Welcome, %c%c%c%c. %d is a
great year to study C!\n", letter_1, letter_2, letter_3,letter_4, year);
return
(0);
} // end of
main
Sample
Output:
Enter a 4-letter name and press return: Omar
Enter the current year and press return: 2004
Welcome, Omar. 2004 is a great year to study
C!
(3) Reserved Words:
All reserved words in C language appears in lower case ; they have special meaning in C and cannot be used or redefined
for any other purpose at any place .
In our previous programs reserved key words are: int, char, void, double, return etc.
A complete list of ANSI C reserved words is present in your text
book at Appendix H .
(4) Standard
Identifiers:
(5) User Defined Identifiers:
Rules for
Making User Defined Identifier/Variable name:
· An
identifier name cannot begin with a digit.
· An identifier name must consist only of letters, digits, or underscores.
· A reserved word in C language
(e.g., return ,
double , int etc) cannot be used as an identifier
name.
· An identifier defined in a C
standard library (e.g., printf , scanf) should not be redefined.
This is an advice rather than ANSI C syntax.
Valid
User-Defined Identifiers: PI,
radius, area, letter_1, circum, year, main etc.
Invalid
User-Defined Identifiers: 4 name, int, two-by-four,
scanf.
Syntax rules for identifiers do not place a limit on
identifier length.
However, some ANSI C compilers put a limit of 31
characters as significant ones.
Uppercase and Lowercase Letters:
In C – Language there is big difference between
lowercase and uppercase letters.
e.g.,
int num;
int NUM;
int Num
are all considered to be different.
§ It is recommended that you should follow a consistent
pattern in using uppercase and lowercase letters in your program. This facilitate reading the program by users.
§ All reserved words in C are in lowercase.
§ All names of all standard library functions are in
lowercase letters.
§ Common practice adopted by professional programmers is
to use only uppercase letters in the names of constant macros.
§ Other identifiers generally use lowercase letters.
Variable declarations and Standard Data Types in C Programs:
The memory cells used for storing a program’s input
data and its computational results are called variables because the values
stored in variables may change (and usually do) as the program executes.
In a C program, the variable declaration tells the C
compiler:
Therefore, a variable declaration in C begins with an
identifier (e.g., double) that tells the C compiler, the type of data (e.g., an
integer, Character, real, etc.) stored in the associated variable location.
Variable names are also known as
identifiers. Use meaningful identifier names!!!
Syntax for
variable Declaration:
int variable_list;
double variable_list;
char variable_list;
float variable_list;
e.g.,
int count, large;
double x, y, z;
char first_initial, second_initial;
float a, b, c;
Basic Data Types
in C:
Data Type int:
Data Type double:
ANSI C provides for three floating-point types:
Data Type char:
Basic data types in C language are int, char, float and double:
|
Data Type |
Bytes
required for storing it in computer memory. |
Placeholder
/ Conversion specifier required with scanf or printf . |
|
int |
2 |
%d |
|
char |
1 |
%c |
|
float |
4 |
%f |
|
double |
8 |
%lf |