In this lesson, we're going to look at how to loop through a block of code using the for() iteration statement. A looping statement is very similar to other conditional statements (such as the if() statement), except at the end of its code block it loops back to the top where the initial condition is evaluated, and continues to do so until the condition evaluates as false.
Step 1: Create a New Project
For this lesson, you should create an ASP.NET project called “CS-ASP_026” and create a single resultLabel Control:
And in the Default.aspx.cs file write the following in the Page_Load() method:
And then run the application to see the result:
Step 2: Breaking Down the for() Iteration Statement
Let’s break down what’s happening in the for() iteration statement here:
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We initialize, to zero, an int variable that is local to the for loop, called i.
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We create a conditional evaluation at the start of every loop (here it checks if i is less than 10).
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If the evaluation in (2) returns true, increment i by one, else end the loop.
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For each time the conditional evaluates true (10 times) execute this line of code.
Perhaps the best way to illustrate how this loop executes is to set a break-point and watch it progress – line by line – in debug mode. Remember to press the F10 key to step through each part of code as it executes. Also, it would be worthwhile to pin the i variable in order to see exactly where and when it increments:
Tip:
Naming the counter variable’s as “i” is quite common throughout computer programming. It can stand for “iterator” or, when used as the index for an array could stand for “indexer.” You might also see code that uses “c”, as in “counter” or any other single-letter character. This is a coding convention that is supposed to improve readability, but you are free to use whatever convention makes most sense to you.
Step 3: Code Snippets for Recalling Syntax
It may be a bit difficult, at first, for beginners to remember the exact syntax for constructing for() loops. Luckily, Visual Studio has a short-cut for creating a for() loop template by using the following sequence: type in the word “for” and on the keyboard hit the tab key twice:
This kind of preset template is called a "code snippet" and you can even find it referenced in Intellisense when typing in the for keyword:
Another shortcut you can exploit by using this code snippet is renaming the iterator variable, which then automatically renames it for the rest of the for() statement:
Tip:
There are a lot of useful code snippets you will encounter as we move forward. Eventually, you can even learn how to create your own custom code snippets!
Step 4: Setting Up the Iterator
It’s worth noting that you can, of course, (1) start with any value you want for the iterator, (2) perform any legal conditional evaluation on the iterator, and (3) perform any calculation on the iterator, as well:
Step 5: Using for() Loops with Arrays
One of the greatest uses of looping statements is to use the iterator as an index for an array (you will often hear this stated as “looping through an array”):
Tip:
Notice that when looping through the names[] array, we loop only when i is less than the length of the array. Why not less than or equal to? Because arrays are zero-based, whereas names.Length begins at one. If we were to use less than or equal to, we would receive an index out of range exception.
You will begin to see the power of arrays and loops when combined with all of the different ways of manipulating and sorting arrays:
Step 6: Breaking Out of the Loop
A common usage for looping through an array would be to go through a database or list of items, searching for a particular item you had in mind. You might imagine that if you have a very large array of items, you would not want the loop to continue after it finds the item you are looking for. By default, that’s exactly what would happen, however, you can prematurely break-out of the loop by using the break keyword:
To show the iteration process, you can set a break-point and watch it in Debug mode, or you can add this else() clause. Notice how even though there are four items in the list the for() loop stops executing after it found the item we are looking for at the third position in the list:
Now that you know how to work with arrays and create a for loop, you should be able to see the value of using them together. Keep these techniques fresh, review what you've learned, and then move on to the next challenge. Good luck!