Objective-C Fundamentals

308 APPENDIX B The basics of C
Listing B.8
Using a do-while loop to log an arbitrary number of messages
int x = 0;
do {
NSLog(@"Hello, World!");
x = n x 1;
} while (x < 15);
Notice that listings B.8 and B.6 are similar in style and behavior. In this example, the
only difference is in the behavior that occurs when the value of variable x is modified
so that it initially has a value of 15 or higher. If x is initially set to the value 16, for
example, the do-while loop version emits one "Hello, World!", whereas the while
loop version does not.
B.4.3
The for statement
Although while and do loops are flexible, they aren’t the most concise of statements
to understand at a glance. To determine how many times a given while loop may loop
or under what conditions it will exit, you must look for logic potentially spread across
a wide number of individual statements on a number of lines of source code.
A for loop is designed to make the common scenario of looping a specific number
of times easier, yet still provide a flexible construct capable of more complex
looping conditions.
Looking at the various code listings containing while and do loops, notice that
they generally have three things in common:
■
■
■
A statement that initializes a variable to an initial value
A condition that checks the variable to determine if the loop should continue
looping
A statement to update the value of the variable each time through the loop
A for loop can specify all three components in a single, concise statement. The general
form of a for loop is as follows:
for (initialization_expr; loop_condition; increment_expr)
{
statement block;
}
The three expressions enclosed in parentheses specify the three components previously
outlined and determine how many times the statements in the for loop will be executed.
The flexibility of three individual expressions allows for a number of variations, but the
most common arrangement is to configure a for loop to loop a fixed number of times.
In this scenario, the initialization expression is executed once to set the initial
value of a loop counter. The second component specifies the condition that must continue
to hold true in order for the loop to continue executing, and the last component
is an expression that’s evaluated at the end of each cycle through the loop to
update the value of the loop counter.