The first form of the if statement is an all-or-nothing
choice: if some condition is satisfied, do something; otherwise,
do nothing. For example:
if (condition) statement;
or
if (condition)
{
compound statement
}
In the second example, instead of a single statement, a whole block of
statements is executed. In fact, wherever you can place a single
statement in C, you can place a compound statement instead: a
block of statements enclosed by curly brackets.
A condition is usually an expression that makes some sort of
comparison. It must be either true or false, and it must be enclosed in
parentheses: (...). If the condition is true, then the
statement or compound statement following the condition will be
executed; otherwise, it will be ignored. For example:
if (my_num == 0)
{
printf ("The number is zero.\n");
}
if (my_num > 0)
{
printf ("The number is positive.\n");
}
if (my_num < 0)
{
printf ("The number is negative.\n");
}
The same code could be written more compactly
in the following way:
if (my_num == 0) printf ("The number is zero.\n");
if (my_num > 0) printf ("The number is positive.\n");
if (my_num < 0) printf ("The number is negative.\n");
It is often a good idea stylistically to use curly brackets in an
if statement. It is no less efficient from the compiler's
viewpoint, and sometimes you will want to include more statements
later. It also makes if statements stand out clearly in the
code. However, curly brackets make no sense for short statements such as
the following:
if (my_num == 0) my_num++;
The if command by itself permits only limited decisions. With the
addition of else in the next section, however, if becomes
much more flexible.
