public readonly int nDaysInYear = 366; // this could also be static in C#.NET

Printer Code 128 Code Set C in C#.NET public readonly int nDaysInYear = 366; // this could also be static

As with const, after you assign the initial value, it can t be changed Although the reasons are too technical for this book, the readonly approach to declaring constants is preferable to const An alternative convention also exists for naming constants Instead of naming them like variables (as in nDaysInYear), many programmers prefer to use uppercase letters separated by underscores, as in DAYS_IN_YEAR This convention separates constants clearly from ordinary read-write variables

Variables that contain single values are all well and good Even class structures that can describe compound objects like a vehicle are critical But you also need a construct for holding a set of objects, such as Bill Gates s extensive

collection of vintage cars The built-in class Array is a structure that can contain a series of elements of the same type (all int values, all double values, and so on, or all Vehicle objects, Motor objects, and so on)

Consider the problem of averaging a set of 10 floating point numbers Each of the 10 numbers requires its own double storage (averaging int variables could result in rounding errors, as described in 3), as follows:

double double double double double double double double double double d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 = = = = = = = = = = 5; 2; 7; 35; 65; 8; 1; 9; 1; 3;

Now, you need to accumulate each of these values into a common sum, which you then divide by 10 (the number of values):

double dSum = d0 + d1 + d2 + d3 + d4 + d5 + d6 + d7 + d8 + d9; double dAverage = dSum / 10;

Listing each element by name is tedious Okay, maybe it s not so tedious when you have only 10 numbers to average, but imagine averaging 100 or even 1,000 floating point values

Fortunately, you don t need to name each element separately C# provides the array structure that can store a sequence of values Using an array, you can rewrite the preceding code segment as follows:

double[] dArray = {5, 2, 7, 35, 65, 8, 1, 9, 1, 3};

The Array class provides a special syntax that makes it more convenient to use The double brackets [] refer to the way you access individual elements in the array, as follows:

The FixedArrayAverage program (see the section The fixed-value array in this chapter) loops through an array of 10 elements Fortunately, the loop iterates through all 10 elements But what if you had made a mistake and didn t iterate through the loop properly You have the following two cases to consider: What if you had only iterated through 9 elements C# would not have considered this an error If you want to read 9 elements of a 10element array, who is C# to say any differently Of course, the average would be incorrect, but the program wouldn t know that What if you had iterated through 11 (or more) elements Now, C# cares a lot C# does not allow you to index beyond the end of an array, for fear that you will overwrite some important value in memory To test this, change the comparison in the for loop to the following, replacing the value 10 with 11 in the comparison:

for(int i = 0; i < 11; i++)

When you execute the program, you get a dialog box with the following error message:

At first glance, this error message seems imposing However, you can get the gist rather quickly: An IndexOutOfRangeException was reported Clearly, C# is telling you that the program tried to access an array beyond the end of its range accessing element 11 in a 10-element array (Buried in the message details under StackTrace is information about the exact line from which the access was made, but you haven t progressed far enough in the book to understand the entire message completely)

The 0th element of the array corresponds to d0, the 1th element to d1, and so on It s common to refer to the 0th element as dArray sub-0, the 1st element as dArray sub-1, and so on The array s element numbers 0, 1, 2, are known as the index In C#, the array index starts at 0 and not at 1 Therefore, you typically don t refer to the element at index 1 as the first element but the oneth element or the element at index 1 The first element is the zeroth element If you insist on using normal speech, just be aware that the first element is at index 0 and the second element is at index 1

dArray wouldn t be much of an improvement were it not for the fact that the index of the array can be a variable Using a for loop is easier than writing each element out by hand, as the following program demonstrates: