Looping Logic
Loops allow you to perform an action over and over again until some condition is met. There are three types of
loops
in VB.NET and C#:
While
,
Do
, and
For
.
While
Loops
While
loops are useful when you know don't know how many times you need to do something, such as when you want to loop through an unknown number of
cars
until you run into a Mercedes. These loops are based on conditional expressions, and they continue to run until the condition becomes false. The syntax for a
while
loop in VB.NET is as
follows
:
While
condition
code
End While
In C#:
while (
condition
) {
code
}
For example, the following code counts from 1 to 10 and prints out the number to the browser:
int intCount = 1;
while (intCount < 10) {
Response.Write(intCount + "<br>");
intCount += 1;
}
On the second line, the program checks if
intCount
is greater than 10. It shouldn't be, because you just set it to be equal to 1. The program executes the code inside the
while
loop, writing the number to the browser and adding 1 to
intCount
. It then goes back to the second line and checks if
intCount
is now greater than 10.
intCount
is 2 now, so the loop continues. When it finishes, you'll see the
numbers
1 through 9 displayed in the browser. Figure 3.5 illustrates this process.
Why do we use 9 instead of 10? Once
intCount
hits 10, the statement on line 2 is no longer true—it
evaluates
to false—so the loop exits and continues after line 5.
The
do
loop is the same as the
while
loop; the syntax is just slightly different. The
do
loop comes in many forms, such as the following:
'VB.NET
Do
code
Loop While
condition
//C#
do {
code
} while
condition
Here's another one:
Do While
condition
code
Loop
The different forms produce different output depending on how many times you want them to execute. Let's look at an example of each:
dim intCount as Integer = 10
Do
Response.Write(intCount & "<br>")
intCount = intCount + 1
Loop While intCount < 10
Do While intCount < 10
Response.Write(intCount & "<br>")
intCount = intCount + 1
Loop
The first loop prints out the number 10, while the second loop prints out nothing. Why? The first loop evaluates the condition on the last line of the loop. Therefore, the code inside the loop will be executed once no matter what. The second loop evaluates the condition right up front, and in this case it stops the code from ever executing.
You should use a
Do
loop when you want the code inside the block to execute at least once. Conversely, you should use a
Do While
loop if you don't care whether the code is executed at all. In VB.NET, you can also use
until
in place of
while
:
dim intCount as Integer = 1
Do
Response.Write(intCount & "<br>")
intCount = intCount + 1
Loop Until intCount >= 10
This loop does exactly the same thing as the first loop in the previous listing. The difference is that using
until
will let the loop go until the condition is true, whereas using
while
stops when the condition is false. In this case, the loop stops when
intCount
is greater than or equal to 10.
For
Loops
Use a
for
loop when you know how many times you want to execute your code. This loop
increments
a counter to tell itself when to stop executing. The
for
loop syntax is as follows:
'VB.NET
For
variable
=
startvalue
To
stopvalue
[Step
step-size
]
code
Next [variable]
//C#
for (
variable
=
startvalue
;
variable comparison stopvalue
;
step
) {
code
}
This loop uses the
step-size
to increment
variable
until it
reaches
the
stopvalue
, at which point the loop stops executing. (In VB.NET, you can leave the
step
parameter out if you want—it defaults to a value of 1.) For example:
For intCount = 1 to 10
Response.Write(intCount & "<br>")
Next
or:
For intCount = 10 to 1 step -1
Response.Write(intCount & "<br>")
Next
The first loop prints out the numbers 1 through 10, while the second prints them in reverse order.
In C#, the previous code snippet would be
for (intCount = 10; intCount == 1; intCount--) {
Response.Write(intCount + "<br>");
}
Another form of the for loop is the for...each loop (foreach in C#). This loop iterates through all of the items in a collection, such as an array. For example:
dim arrWeekDays() as String = {"Monday", "Tuesday", _
"Wednesday", "Thursday", "Friday"}
For Each strDay in arrWeekDays
Response.Write(strDay & "<br>")
Next
On the first line, you create an array of strings containing the weekdays. On the fourth line, you loop through each item in the array. The variable
strDay
is simply a counter that the loop uses—you could specify any variable here. The loop
assigns
each variable in the array to
strDay
, and you print that value out on the fifth line. The result is that the days of the week are displayed in the browser, as shown in Figure 3.6.
Infinite Loops
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Notice that you had to manually increment the counter in your
while
and
do
loops, whereas the
for
loops did so automatically. It's very important to increment your counter in these loops. Otherwise, you'd end up with an
infinite loop
—a loop that never exits. Not only is this a pain for
visitors
to your site, but it also drains system resources very quickly and can cause the site to crash.
|
If you ever need to exit a loop before it finishes, you can use the
exit
(
break
in C#) keyword to stop the loop. The syntax for this keyword in VB.NET depends on where you use it. For example:
Do While intCount < 10
Response.Write(intCount & "<br>")
intCount = intCount + 1
if intCount = 7 then
exit do
end if
Loop
For intCount = 1 to 10
Response.Write(intCount & "<br>")
if intCount = 7 then
exit for
end if
Next
The first loop iterates through the numbers 1 through 10, but if the number equals 7 (which it eventually will, in this case), you stop the loop by using
exit do
(note that it prints out only 1-6). The second loop does exactly the same thing, but using
exit for
(this one prints out 1-7). The
exit
statement that's used will depend on the type of loop you're in.
In C#, the syntax doesn't change no matter where the keyword is used:
for (intCount = 1; intCount == 10; intCount++) {
Response.Write(intCount + "<br>");
if (intCount == 7) {
break;
}
}
|
