34,333
edits
Changes
New page: In Objective-C Flow Control with if and else we looked at how to control program execution flow using the ''if'' and ''else'' statements. Whilst these statement constructs work well fo...
In [[Objective-C Flow Control with if and else]] we looked at how to control program execution flow using the ''if'' and ''else'' statements. Whilst these statement constructs work well for testing a limited number of conditions they quickly become unwieldy when dealing with larger numbers of possible conditions. To simplify such situations Objective-C has inherited the ''switch'' statement from the C programming language. In this chapter we will explore the ''switch'' statement in detail.
== Why Use a switch Statement? ==
For a small number of logical evaluations of a value the ''if ... else if ...'' construct outlined in [[Objective-C Flow Control Using if and else]] is perfectly adequate. Unfortunately, any more than two or three possible scenarios can quickly make such a construct both time consuming to write and difficult to read. As a case in point consider the following code example. The program is designed to evaluate integer between 0 and 5 entered at the keyboard and to output the word version of the number (zero, one, two etc):
<pre>
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[])
{
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
int value;
printf ("Enter a number between 0 and 5: ");
scanf ("%i", &value);
if (value == 0)
NSLog (@"zero");
else if (value == 1)
NSLog (@"one");
else if (value == 2)
NSLog (@"two");
else if (value == 3)
NSLog (@"three");
else if (value == 4)
NSLog (@"four");
else if (value == 5)
NSLog (@"five");
else
NSLog (@"Integer out of range");
[pool drain];
return 0;
}
</pre>
As you can see, whilst the code is not too excessive it is already starting to become somewhat hard to read and also took more time to write than should really be necessary. Imagine, however, if instead of five numbers we had to test for more. Clearly an easier solution is needed, and that solution is the ''switch'' statement.
== Using the switch Statement Syntax ==
The syntax for an Objective-C ''switch'' statement is as follows:
switch (''expression'')<br>
{
case ''match1'':<br>
''statements''<br>
break;<br>
case ''match2'':<br>
''statements''<br>
break;<br>
default:<br>
''statements''<br>
break;<br>
}
<google>ADSDAQBOX_FLOW</google>
This syntax needs a little explanation before we embark on creating a ''switch'' based version of the above ''if ... else'' construct.
In the above syntax outline ''expression'' represents either a value, or an expression which returns a value. This is the value against which the ''switch'' operates. Using our example, this would be the integer to be evaluated.
For each possible match a ''case'' statement is required, followed by a ''match'' value. Each potential match must be of the same type as the governing expression. Following on from the case line are the Objective-C statements that are to be executed in the event of the value matching the case match.
After the ''statements'' comes a ''break'' statement. This statement breaks out of the switch statement. Failure to provide a break statement results in every case after the matching case evaluating to true (regardless of whether the match is made or not) and the corresponding Objective-C statements executing.
Finally, the ''default'' section of the construct defines what should happen if none of the case statements present a match to the ''expression''.
== A switch Statement Example ==
With the above information in mind we may now construct a ''switch'' statement which provides the same functionality as our previous and somewhat unwieldy ''if ... else if ...'' construct:
<pre>
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[])
{
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
int value;
printf ("Enter a number between 0 and 5: ");
scanf ("%i", &value);
switch (value)
{
case 0:
NSLog (@"zero");
break;
case 1:
NSLog (@"one");
break;
case 2:
NSLog (@"two");
break;
case 3:
NSLog (@"three");
break;
case 4:
NSLog (@"four");
break;
case 5:
NSLog (@"five");
default:
NSLog (@"Integer out of range");
break;
}
[pool drain];
return 0;
}
</pre>
== Why Use a switch Statement? ==
For a small number of logical evaluations of a value the ''if ... else if ...'' construct outlined in [[Objective-C Flow Control Using if and else]] is perfectly adequate. Unfortunately, any more than two or three possible scenarios can quickly make such a construct both time consuming to write and difficult to read. As a case in point consider the following code example. The program is designed to evaluate integer between 0 and 5 entered at the keyboard and to output the word version of the number (zero, one, two etc):
<pre>
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[])
{
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
int value;
printf ("Enter a number between 0 and 5: ");
scanf ("%i", &value);
if (value == 0)
NSLog (@"zero");
else if (value == 1)
NSLog (@"one");
else if (value == 2)
NSLog (@"two");
else if (value == 3)
NSLog (@"three");
else if (value == 4)
NSLog (@"four");
else if (value == 5)
NSLog (@"five");
else
NSLog (@"Integer out of range");
[pool drain];
return 0;
}
</pre>
As you can see, whilst the code is not too excessive it is already starting to become somewhat hard to read and also took more time to write than should really be necessary. Imagine, however, if instead of five numbers we had to test for more. Clearly an easier solution is needed, and that solution is the ''switch'' statement.
== Using the switch Statement Syntax ==
The syntax for an Objective-C ''switch'' statement is as follows:
switch (''expression'')<br>
{
case ''match1'':<br>
''statements''<br>
break;<br>
case ''match2'':<br>
''statements''<br>
break;<br>
default:<br>
''statements''<br>
break;<br>
}
<google>ADSDAQBOX_FLOW</google>
This syntax needs a little explanation before we embark on creating a ''switch'' based version of the above ''if ... else'' construct.
In the above syntax outline ''expression'' represents either a value, or an expression which returns a value. This is the value against which the ''switch'' operates. Using our example, this would be the integer to be evaluated.
For each possible match a ''case'' statement is required, followed by a ''match'' value. Each potential match must be of the same type as the governing expression. Following on from the case line are the Objective-C statements that are to be executed in the event of the value matching the case match.
After the ''statements'' comes a ''break'' statement. This statement breaks out of the switch statement. Failure to provide a break statement results in every case after the matching case evaluating to true (regardless of whether the match is made or not) and the corresponding Objective-C statements executing.
Finally, the ''default'' section of the construct defines what should happen if none of the case statements present a match to the ''expression''.
== A switch Statement Example ==
With the above information in mind we may now construct a ''switch'' statement which provides the same functionality as our previous and somewhat unwieldy ''if ... else if ...'' construct:
<pre>
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[])
{
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
int value;
printf ("Enter a number between 0 and 5: ");
scanf ("%i", &value);
switch (value)
{
case 0:
NSLog (@"zero");
break;
case 1:
NSLog (@"one");
break;
case 2:
NSLog (@"two");
break;
case 3:
NSLog (@"three");
break;
case 4:
NSLog (@"four");
break;
case 5:
NSLog (@"five");
default:
NSLog (@"Integer out of range");
break;
}
[pool drain];
return 0;
}
</pre>
