Lecture 2

Analyzing C Program Structure

#include <stdio.h>

#include <conio.h>

void main(){

clrscr();

printf(“This is my first C program \n”);

getch();

}

#include <stdio.h>

C pre-processor. Lines beginning with # are processed by the pre-processor before the program is compiled. This line tells the pre-processor to include the contents of the standard input output header file. To compile library function printf(), it is required.

#include <conio.h>

Required for clrscr() and getch().

void main()

It is a part of every C program. The parenthesis after main indicates that main is a program building block called a function. C programs are composed of one or many functions like it but main is a must.

clrscr();

C library function. Clears the contents present in the screen.

printf(“This is my first C program \n”);

C statement. Statements are always terminated with a semi-colon. This statement has a library function called printf(). It takes a string inside of it within two quotation marks. Whatever is in between them, will be printed on the screen. The backslash (\) character is called Escape Character. When C encounters it, it looks for the next character (here it is n) to make an Escape Sequence. Here, the escape sequence \n means new line. After printing the line This is my first C program, the cursor goes to the new line.

getch();

Another C library function. When the program compiles and provides an output, it waits for getting a character from the keyboard. When it gets, it returns from the output screen to source code.

/* This program takes two integers

and provides the sum to the user */

#include <stdio.h> // header file

#include <conio.h> //header file

// main function starts

void main(){

clrscr(); // clearing the output screen

int a,b,sum; // declaring three integers a, b and sum

printf(“Enter integer one: \n”); // printing on output

scanf (“%d”,&a); //taking the input value into a

printf(“Enter integer two: \n”); // printing on output

scanf (“%d”,&b); // taking the input value into b

sum=a+b; // taking their summation into sum

printf(“Sum is: %d”, sum); // displaying the sum

getch(); //waiting for a character to return

}

/* This program takes two integers

and provides the sum to the user */

The characters /* … … … */ is called multi-line commenting. When C finds this character, it omits what is inside that and goes to the next instruction. Comments are required to make the program more readable and understandable if anyone analyzes the code.

// main function starts

The characters // is called single line commenting. Again, when C finds this character, it omits that line of code.

int a,b,sum;

It is a declaration. The letters a, b and sum are names of variables. A variable is a location in memory where a value can be stored for use by a program. This declaration specifies that the variables a, b and sum are of type int which means that these variables will hold integer values. All variables must be declared with a name and a data type immediately after the left brace that begins the body of main () before they can be used in program. There are other data types in C besides int. Several variables with the same data type can be declared in one declaration. We could declare that in separate declaration as well-

int a;

int b;

int sum;

Notes on variable names-

A variable name in C is any identifier. An identifier is a series of characters consisting of letters, digits and underscores (_) that does not begin with a digit.
It can be of any length. But C will understand the first 31 characters only.
C is case sensitive. Variable names like A1 and a1 are two different variables.

scanf (“%d”,&a);

It uses scanf () function to obtain a value from the user. The scanf () function takes input from the key-board. The scanf () here has two arguments. “%d” and &a. The first argument, the format control string, indicates that the data should be an integer (d stands for decimal integer). The % is Escape Character and %d is Escape Sequence. The second argument & (ampersand)- called address operator in C‑ followed by the variable name. The ampersand when combined with variable name tells scanf() the location in memory in which the variable a is stored. The computer then stores the values for a at that location.

When the computer executes the preceding scanf(), it waits for the user to enter a value for variable a. the user responds by typing an integer and then pressing the return key (Enter key) to send the number to computer. The computer then assigns the value to the variable a.

sum=a+b;

It calculates the sum of variables a and b and assigns the result to variable sum using the assignment operator =. The + and = operators are called binary operators as they require two operands. The two operands of + are a and b and two operands of = are sum and a+b.