This lesson covers how to change the length of an array. Arrays are immutable by default, which means that once you define or declare an array, just like a variable, you normally cannot increase or reduce the number of array elements. Returning to a previous analogy, you cannot remove or add any more “buckets” strung up on the “broomstick”; it is of fixed size once that size is declared. We imprint on the broom-handle the size as precisely five elements:
Fortunately, the .NET framework class library defines arrays with some helper methods that can process the array, and return one with a new size – returning an array that is either larger or smaller than the original array. You can also access other helper methods that get the sum, minimum, maximum, and average values within the array, along with other generally useful operations.
Let’s begin this lesson by creating a new ASP.NET project called “CS-ASP_023” and create the following Controls and programmatic IDs:
This application will be a simple calculator that allows us to add the number of work hours on a given project and return some statistics. Each work session will be stored in an array, so one session might be eight hours, another is four hours, and another is six hours. Those would be three different sessions. From that, we can access array helper methods to average them out and find the session with the most hours, the fewest hours, the average session length, and so on.
Let’s first create the array with zero elements (to start off with) upon page load, and add it to the ViewState:
Then, in the addButton_Click event create another locally scoped variable called hours and populate it with all of the previously added elements preserved in ViewState. Remember that the ViewState["Hours"] is a generic object and therefore needs to be cast to an array of doubles.
Then resize the hours array (adding one more element) by passing it into the Resize() helper method found in the Array class. Note that the Resize() method takes in the current hours array, as well as its current length, plus one:
The ref keyword here means “reference” and essentially takes the actual variable/object hours into the Resize() method rather than just copying it. This means you can perform an operation on the variable and not have to return a copy of it back to the original variable. There will be a lot more said about passing by reference/value in further lessons.
It’s worth noting that the Array.Resize() method doesn’t violate the fundamental rule of array immutability. What the method actually does – behind the scenes – is make a new array with one more element than the one we passed into it (in this case hours), and then copies all of the elements from the one we passed in, into the new array.
Here’s a concept that may help you wrap your head around how Array.Resize() works: if you’re a Star Trek fan, you may be familiar with the concept of teleportation used throughout the series. With teleportation, no object actually moves from one place to another, but rather each particle (and its position in space) making up the source object gets copied (you can even imagine that the teleportation device uses three-dimensional arrays to represent each particle in three-dimensional space). A new object is simply built off of that copied information at the destination.
After that code, we will want to (1) return the highest numerical index in the hours array. And, since we know in advance that it will be the newest array element added in Array.Resize() – reference that index number (2) to store whatever the user entered in hoursTextBox.Text:
GetUpperBound() takes in the dimension of the current array that you are evaluating. Remember in the previous lesson we discussed multi-dimensional arrays, but in this case our array only has one dimension. Therefore, we pass in 0, since the arrays' indexes are zero-based.
And then simply replace the previous values in ViewState[“Hours”] with the new hours array, and then output the sum of all array elements:
When you run the application, you will now be able to add hours and store them in ViewState in between PostBack’s:
We can also call several other helper methods available to the array:
When you run the application, you will now see these various statistics. Here, we entered three values (6, 7, and 9) to represent the hours worked in three different sessions:
Lesson 23 - Changing the Length of an Array