Use these bookmarks to jump around the tutorial:
And now on to something new. In this installment we’ll make you dizzy by talking about loops. We’ll also take a look at arrays and how they relate to tables.
The For .. Next Loop
For those of you that tend to be repetitive, you’ll love this installment of the ASP Primer. Using loops we can thumb through data, automatically generate a list of numbers or dates, traverse an array or any number of other things.
Let’s take a look at the
FOR .. NEXT loop first. This particular loop is about as straight-forward as it
gets. Its job is to start at a certain number and count until it reaches its
designated stopping point.
In order to demonstrate
how this works, let’s take a look at our practical example for this installment
of the ASP Primer. Below we will use a FOR .. NEXT loop to populate a drop-down
list box on a form. We will be putting in the year, beginning with this year,
plus 7 more years for a total of 8. This is a handy little piece of code for
populating a credit card drop-down so that you will never have to go back and
change the years listed as time goes by. Here it is:
<% OPTION
EXPLICIT %>
<HTML>
<HEAD>
<TITLE>Auto-generate Year</TITLE>
</HEAD>
<BODY>
<FORM METHOD="POST" ACTION="thispage.asp">
<P>
<SELECT SIZE="1" NAME="year">
<OPTION>Please select a year …</OPTION>
<% DIM intYearCounter, intCurrentYear %>
<% intCurrentYear = Year(Date) %>
<% FOR intYearCounter = intCurrentYear TO
(intCurrentYear + 7) %>
<% Response.Write "<OPTION>" &
CStr(intYearCounter) & "</OPTION>" %>
<% NEXT %>
</SELECT>
</P>
</FORM>
</BODY>
</HTML>
You’ll notice we are
using some of the elements from the last tutorial like the DIM statement.
Next you will notice we set our newly created variable, intCurrentYear,
to Year(Date). This sets our variable to the current year. This is one of
those nifty little functions that is built into VBScript and ASP. We’ll cover this function
and more later in this series.
Then we have our FOR
statement. This is where we define where we will start our loop and how long it
will continue looping. First we give it the variable we created above,
intYearCounter, so that it can use that variable for storage as it counts.
Next we define where we want to begin counting. In our case we will begin
counting with the current year which is what is being stored in
intCurrentYear. Now that the FOR statement knows where we want to
begin we need to tell it when to stop. In our case, we want it to stop after it
has run though the loop 8 times. So, we finish the FOR statement by using
TO (intCurrentYear + 7).
If you are wondering why
we are doing (intCurrentYear + 7) instead of (intCurrentYear + 8)
imagine this: suppose our starting year is 2001. We want to display a total of 8
years starting with 2001. Therefore, our last year listed should be 2008. The
difference between 2008 and 2001 is 7 which is why we add 7 to intCurrentYear.
The next line that you
come to is Response.Write "<OPTION>" & CStr(intYearCounter) & "</OPTION>".
What this actually does is take the current number in intYearCounter and combine
it with HTML to send to the browser when the page is rendered. We’ll get into
the particulars of CStr and Response.Write later in this series.
Then there is the NEXT
statement. This simply tells the server that this is the end of our FOR .. NEXT
loop. The FOR .. NEXT loop only executes the code
between the FOR and the NEXT. So, when our loop runs it will only repeatedly
execute the line starting with Response.Write since it is the only line inside
the loop. Even though we only have one line inside our loop, you can have as many
lines as you like.
| A Word of Caution … Don’t ever mess with the |
Now that you know how
the basic loop structure works, here’s another little thing you can do. You can
use STEP to change how your loop increments. Let’s say you wanted every
other year to be displayed above instead of every year. Try this:
<% FOR intYearCounter = intCurrentYear TO
(intCurrentYear + 7) STEP 2 %>
Now instead of adding 1
(the default) to intYearCounter each time through the loop you will add 2
thus displaying every other year.
Let’s suppose you wanted
to go backwards and list the years in reverse order. Try this:
<% FOR intYearCounter =
(intCurrentYear + 7) TO
intCurrentYear
STEP -1 %>
In the example above we
will run our loop backwards in a sense. We will start with a larger number and
STEP backwards by -1 until we get to the smallest number.
Feel free to take the
code above and play around with it using STEP and changing the numbers. Be
careful not to get yourself stuck in an infinite loop, though. Make sure your
loops will have a predictable beginning and ending.
| Cutting and Pasting Code … Because of our limited column space we often have to |
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The
Do .. Loop
The DO .. LOOP is
a combination of the IF .. THEN statement and the FOR .. NEXT loop. It’s a
conditional loop. In other words, it loops repeatedly (like a FOR .. NEXT loop)
until some specific criteria is met (like an IF .. THEN statement).
Here is a basic example
of the DO .. LOOP:
<% DIM intDice,
intRollCounter %>
<% intRollCounter
= 0 %>
<% Randomize %>
<% DO UNTIL
intDice = 6 %>
<%
intRollCounter = intRollCounter + 1 %>
<%
intDice = Int(Rnd * 6) + 1 %>
<% LOOP %>
<% Response.Write
"It took " & CStr(intDice)
& " tries to roll a 6!" %>
This example is a simple
dice game. We will keep automatically rolling a single die until we get a six
and then output how many tries it took. Let’s walk through the code.
The first new thing that
you will encounter is the Randomize command. This command simply sets a random
number seed so that you can generate random numbers. We’ll cover both Randomize
and Rnd in more detail later in this series.
After Randomize comes
the DO statement. Like FOR, this simply tells the server that we
are beginning our loop here. Then comes UNTIL. UNTIL is the
conditional part and works just as you would expect it to. Each time through the
loop it checks to see if our criteria is met, i.e. intDice = 6. When it
does equal 6 the loop stops executing.
Next you will see
intRollCounter be incremented by one each time we go through the loop. We
are doing our incrementing by hand this time instead of using a FOR .. NEXT
loop because we don’t know how many times the loop will execute before
intDice equals 6.
Then we generate a
random number between 1 and 6. We’ll talk about how this works when we cover
Randomize and Rnd later in this series.
The last part of our
loop is designated by, you guessed it, LOOP. This tells the server that
we have reached the bottom of the loop and to try it again UNTIL we get
it right.
Lastly, after our loop
is done looping we simply write a line that says how many times we had to loop
through to get a 6.
There’s also another way
to approach this. We could use WHILE instead. WHILE is the polar
opposite of UNTIL. WHILE keeps going until a condition is
not met. Let’s change the DO line to use WHILE instead:
<% DO WHILE intDice
<> 6 %>
We are accomplishing the
same task as before but we are just wording it a bit different. Now we are
saying keep looping as long as intDice does not equal 6.
Obviously, each
situation will logically lend itself to either WHILE or UNTIL.
You’ll have to determine for yourself which makes your code easier to read and
more intuitive. In our case UNTIL makes the most sense.
Now, there’s one other
little twist you will need to know. WHILE and UNTIL can be used
either at the top or the bottom of the loop. That means we could move the
UNTIL in our example down to the bottom of the loop like this:
<% LOOP UNTIL intDice
= 6 %>
Now instead of doing our
test at the top of the loop we are doing it at the bottom. I know it doesn’t
sound like a big deal but it can be very important. If you place UNTIL or
WHILE at the bottom you are ensured that your loop will execute at least
one time. If you put them at the top, your loop may never execute if your
condition is already met when the loop begins. This can be a very important
distinction depending on your circumstance.
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All About Arrays
And now for a new way to
store information .. arrays. Arrays are a very handy storage method. Think of
arrays as variables on steroids.
Imagine you wanted to
store 20 different people’s last names in variables. It would be quite a pain to
create and keep track of strLastName1, strLastName2, etc. Not very efficient,
right? That’s where arrays come in. With an array you can create one variable
that will have 20 slots, one for each person’s name. Here is how you would
declare that array:
<% DIM
strLastNames(20) %>
Now you have a variable
ready to receive your twenty names. Think of it as a table with 1 column and 20
rows. To refer to each slot in your array simply refer to its index number like
this:
<%
strLastNames(1) = "Smith" %>
<%
strLastNames(2) = "Hatfield" %>
For an example of how
arrays can be used let’s go back to our FOR .. NEXT example. Change the
DIM statement to this:
<% DIM
intYearArray(8), intYearCounter,
intCurrentYear %>
Now you have an array to
store the years that you will generate in the FOR .. NEXT loop. Add this
line inside the FOR .. NEXT loop:
<% intYearArray (intYearCounter
– intCurrentYear + 1)
= intYearCounter %>
While that looks
incredibly complicated, it’s not. Since our loop doesn’t count from 1 to 8 we
have to reduce our year counter down to single index numbers 1 through 8. That’s
what our little formula inside the parentheses does.
Now, if you want to get
a bit more complicated you can use a two dimensional array. Think of this as a
table with x number of rows and y number of columns. Here is how
you would declare a two dimensional array:
<% DIM
strMyArray(20,10) %>
Think of strMyArray
as a "table" that is 20 rows by 10 columns. With a two dimensional array it is a
bit more complicated to keep track of your index numbers for each item. However,
if you imagine your data in a table and you remember that strMyArray(1,1)
is the top left corner then it shouldn’t be too difficult. If you really
want to get complicated you could go for the three dimensional effect like this:
<% DIM
strMyArray(5,5,5) %>
Now you more or less
have a cube that is 5 blocks wide by 5 blocks tall by 5 blocks deep.
So, what about getting
stuff back out of an array? That’s pretty easy actually. All you need to do is
refer to the index number of the item that you want to retrieve. If we wanted to
get an item out of our intYearArray we would just refer to an index
number like this:
intTheLastYear =
intYearArray(8)
In a two dimensional
array like this:
strData =
strMyArray(7,2)
You get the idea. But if
you wanted to get the data out of each element of your array you could save
yourself some time by using a specialized FOR .. NEXT loop like this:
<% DIM
intEachYear %>
<% FOR EACH
intEachYear IN intYearArray %>
<%
Response.Write intEachYear %>
<% NEXT %>
You’ll notice we defined
our FOR very differently. Here we are telling the server to loop through
EACH element in intYearArray (from the example above). Each time
through the loop we want the current element to be temporarily placed in the
variable intEachYear. Then within the loop we will simply print out each
element as it loops through. What we will end up with is our series of years all
printed on the same line.
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What’s Next?
We’re getting somewhere
now! Next it’s on to some of the different functions of VBScript and ASP. These
little guys will do a lot of your programming work for you. Learn them well.
In the next part of this series
we will:
-
Do the Math Functions
-
Messing with String
Functions -
Nailing Down the Date
& Time Functions
