quick guide

Example:1

main() {
int num;
printf("Enter a number lessthan 10");
scanf("%d",&num);
if(num<=10)
printf(" How nice you are");
}
Description:
On execution of this program, if you type a
number less than or equal to 10, you get a message on the
screen through printf(). If you type some other number the
program doesn't do anything.

To make you comfortable with the decision
control instruction one more example has been given below:

Example:2

While purchasing certain items,a discount of
10% is offered if the quantity purchased is more than 1000.

If quantity and price per item are input through the
keyboard , while a program to calculate the total expenses.


main()

{
int qty,dis=0;

float rate,tot;

printf("Enter quantity and rate");

scanf("%d%f",&qty,&rate);

if(qty>1000)

dis=10;

tot=(qty*rate)-(qty*rate*dis/100);

printf("total Expenses = Rs.%f",tot);

}

Here is some sample interaction with the program:

Enter quantity and rate 1200 15.50

Total expenses = Rs 16740.000000

Enter quantity and rate 200 15.50

Total expenses = Rs 3100.00000

Description:
In the first run of the program, the
condition evaluates to true, as 1200 (value
of qty) is
greater than 1000. Therefore, the variable dis, which
was earlier set to 0,now gets a new value 10. using this
new values total expenses are calculated and printed.

Is the statement dis=0 necessary ?

The answer is YES, since in C, a variable
if not specifically initialized contains some unpredic
table values( garbage value).

We can even use Arithmetic Expressions in the "IF"
statement. For example all the following "IF" statements
are valid.

if(3+2%5)
printf("This works");

if(a=10)
printf("Even this works");

if(-5)
printf("Surprisingly even this also works");

Note That in C a non-zero value is considered
to be true, where as a '0' is considered to be FALSE.In the
first "IF" , the expression evaluates to 5 and since 5 is a
non-zero,it is considered to be TRUE. Hence the printf()
gets executed In the second "IF" , 10 gets assigned to a
so the if is now reduced to if(a) or if(10) Since 10 is a
non-zero, it is TRUE hence again printf() goes to work.
In the third "IF", -5 is a non-zero number, hence TRUE.
So again printf() goes to work. In place of -5 even if a
float like 3.14 were used it would be considered to be
TRUE.So the issue is not whether the number is integer or
float, or whether the number is positive or negative.
Issue is whether it is zero or non-zero.