Objective-C Fundamentals

306 APPENDIX B The basics of C
anywhere between 0 and 1000 times. You’ll quickly find that you must write a large
amount of duplicative code, and the code is fairly hard to maintain, presenting a lot of
opportunity to introduce errors. It’s also hard to make changes, such as changing the
code to print "Hello, Objective-C!" rather than "Hello, World!".
Luckily, C provides a better alternative to such scenarios with a set of statements
that allow you to programmatically loop a specified number of times or while a certain
condition continues to hold true.
The first such statement we investigate is the while loop.
B.4.1
The while statement
The while statement allows you to execute a set of statements as long as a specific condition
continues to hold true. The general form of a while statement is as follows:
while (condition) {
statement block
}
When this statement is executed, the condition is evaluated. If the expression evaluates
to false, execution skips the statements in the block and proceeds to the
first statement after the while loop. If the condition evaluates to true, the statements
in the block are executed before execution returns to the top of the loop,
and the condition is reevaluated to determine if another pass through the loop
should be made.
As an example, the while loop in the following listing causes "Hello, World!" to
be printed 15 times.
Listing B.6
A flexible way to log an arbitrary number of messages
int x = 0;
while (x < 15)
{
NSLog(@"Hello, World!");
x = x + 1;
}
Execution through this loop is controlled by variable x, which is initialized to the
value 0. The while loop condition is evaluated to determine if x is less than 15, which
at this point it is, so the condition initially evaluates to true, and the statements in the
while loop are executed.
Each time through the loop, a call to NSLog is made to emit another copy of the
string "Hello, World!", and the value of x is incremented by 1. Execution then
returns to the top of the while loop, and the loop condition is reevaluated to determine
if another loop through the statement block is required.
After the 15th time through the while loop, the condition x < 15 no longer holds
true, so the looping stops and execution proceeds to the first statement after the
while loop.