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New page: The Objective-C ''for'' loop described previously works well when you know in advance how many times a particular task needs to be repeated in a program. There will, however, be instances...
The Objective-C ''for'' loop described previously works well when you know in advance how many times a particular task needs to be repeated in a program. There will, however, be instances where code needs to be repeated until a certain condition is met, with no way of knowing in advance how many repetitions are going to be needed to meet that criteria. To address this need, Objective-C provides the ''while'' loop (yet another construct inherited by Objective-C from the C Programming Language).
== The Objective-C ''while'' Loop ==
Essentially, the ''while'' loop repeats a set of tasks until a specified condition is met. The ''while'' loop syntax is defined follows:
<pre>
while (''condition'')
{
// Objective-C statements go here
}
</pre>
In the above syntax, ''condition'' is an expression that will return either ''true'' (1) or ''false'' (0) and the ''// Objective-C statements go here'' comment represents the code to be executed while the ''condition'' expression is ''true''. For example:
<pre>
int myCount = 0;
while ( myCount < 100 )
{
myCount++;
}
</pre>
<google>ADSDAQBOX_FLOW</google>
In the above example, the ''while'' expression will evaluate whether the ''myCount'' variable is less than 100. If it is already greater than 100 the code in the braces is skipped and the loop exits without performing any tasks.
If, on the other hand, ''myCount'' is not greater than 100 the code in the braces is executed and the loop returns to the ''while'' statement and repeats the evaluation of ''myCount''. This process repeats until the value of ''myCount'' is greater than 100, at which point the loop exits.
== Objective-C do ... while loops ==
It is often helpful to think of the ''do ... while'' loop as an inverted ''while'' loop. The ''while'' loop evaluates an expression before executing the code contained in the body of the loop. If the expression evaluates to ''false'' on the first check then the code is not executed. The ''do .. while'' loop, on the other hand, is provided for situations where you know that the code contained in the body of the loop will ''always'' need to be executed at least once. For example, you may want to keep stepping through the items in an array until a specific item is found. You know that you have to at least check the first item in the array to have any hope of finding the entry you need. The syntax for the ''do ... while'' loop is as follows:
<pre>
do
{
// Objective-C statements here
} while (''conditional expression'')
</pre>
In the ''do ... while'' example below the loop will continue until the value of a variable named ''i'' equals 0:
<pre>
int i = 10;
do
{
i--;
} while (i > 0)
</pre>
== Breaking from Loops ==
As with the ''for'' loop, it is also possible to exit from a ''while'' or ''do while'' loop at any time through the use of the ''break'' statement. When the execution path encounters a ''break'' statement the looping will stop and execution will proceed to the code immediately following the loop. In the following example, the loop is coded to exit when the value of ''i'' matches the value of ''j'':
<pre>
int i = 0;
int j = 5;
while (i < 100)
{
i++;
if (i == j)
break;
}
</pre>
It is important to note that in the case of nested loops the ''break'' statement only exits the current loop leaving the outer loop to continue executing (and most likely once again executing the inner loop).
== The ''continue'' Statement ==
The ''continue'' statement causes all remaining code statements in a loop to be skipped, and execution to be returned to the top of the loop. In the following example, the NSLog function is only called when the value of variable ''i'' is an even number (i.e divisible by 2 with no remainder):
<pre>
int i = 1;
while (i < 20)
{
i++;
if ((i % 2) != 0)
continue;
NSLog ("i = %i", i);
}
</pre>
The ''continue'' statement in the above example will cause the NSLog call to be skipped unless the value of ''i'' can be divided by 2 with no remainder. If the ''continue'' statement is triggered, execution will skip to the top of the while loop and the statements in the body of the loop will be repeated (until the value of ''i'' exceeds 19), resulting in the following output:
<pre>
i = 2
i = 4
i = 6
i = 8
i = 10
i = 12
i = 14
i = 16
i = 18
i = 20
</pre>
== The Objective-C ''while'' Loop ==
Essentially, the ''while'' loop repeats a set of tasks until a specified condition is met. The ''while'' loop syntax is defined follows:
<pre>
while (''condition'')
{
// Objective-C statements go here
}
</pre>
In the above syntax, ''condition'' is an expression that will return either ''true'' (1) or ''false'' (0) and the ''// Objective-C statements go here'' comment represents the code to be executed while the ''condition'' expression is ''true''. For example:
<pre>
int myCount = 0;
while ( myCount < 100 )
{
myCount++;
}
</pre>
<google>ADSDAQBOX_FLOW</google>
In the above example, the ''while'' expression will evaluate whether the ''myCount'' variable is less than 100. If it is already greater than 100 the code in the braces is skipped and the loop exits without performing any tasks.
If, on the other hand, ''myCount'' is not greater than 100 the code in the braces is executed and the loop returns to the ''while'' statement and repeats the evaluation of ''myCount''. This process repeats until the value of ''myCount'' is greater than 100, at which point the loop exits.
== Objective-C do ... while loops ==
It is often helpful to think of the ''do ... while'' loop as an inverted ''while'' loop. The ''while'' loop evaluates an expression before executing the code contained in the body of the loop. If the expression evaluates to ''false'' on the first check then the code is not executed. The ''do .. while'' loop, on the other hand, is provided for situations where you know that the code contained in the body of the loop will ''always'' need to be executed at least once. For example, you may want to keep stepping through the items in an array until a specific item is found. You know that you have to at least check the first item in the array to have any hope of finding the entry you need. The syntax for the ''do ... while'' loop is as follows:
<pre>
do
{
// Objective-C statements here
} while (''conditional expression'')
</pre>
In the ''do ... while'' example below the loop will continue until the value of a variable named ''i'' equals 0:
<pre>
int i = 10;
do
{
i--;
} while (i > 0)
</pre>
== Breaking from Loops ==
As with the ''for'' loop, it is also possible to exit from a ''while'' or ''do while'' loop at any time through the use of the ''break'' statement. When the execution path encounters a ''break'' statement the looping will stop and execution will proceed to the code immediately following the loop. In the following example, the loop is coded to exit when the value of ''i'' matches the value of ''j'':
<pre>
int i = 0;
int j = 5;
while (i < 100)
{
i++;
if (i == j)
break;
}
</pre>
It is important to note that in the case of nested loops the ''break'' statement only exits the current loop leaving the outer loop to continue executing (and most likely once again executing the inner loop).
== The ''continue'' Statement ==
The ''continue'' statement causes all remaining code statements in a loop to be skipped, and execution to be returned to the top of the loop. In the following example, the NSLog function is only called when the value of variable ''i'' is an even number (i.e divisible by 2 with no remainder):
<pre>
int i = 1;
while (i < 20)
{
i++;
if ((i % 2) != 0)
continue;
NSLog ("i = %i", i);
}
</pre>
The ''continue'' statement in the above example will cause the NSLog call to be skipped unless the value of ''i'' can be divided by 2 with no remainder. If the ''continue'' statement is triggered, execution will skip to the top of the while loop and the statements in the body of the loop will be repeated (until the value of ''i'' exceeds 19), resulting in the following output:
<pre>
i = 2
i = 4
i = 6
i = 8
i = 10
i = 12
i = 14
i = 16
i = 18
i = 20
</pre>
