Lining Up Your Ducks with Collections in C#

Encoder Code 3 of 9 in C# Lining Up Your Ducks with Collections

The program begins by initializing a variable sum to 0 Then it loops through the values stored in doublesArray, adding each one to sum By the end of the loop, sum has accumulated the sum of all values in the array The resulting sum is divided by the number of elements to create the average The output from executing this program is the expected 46 (You can check it with your calculator)

Fortunately, the FixedArrayAverage program (in the preceding section The fixed-value array ) loops through all ten elements But what if you goof and don t iterate through the loop properly You have these two cases to consider: You iterate through only nine elements: C# doesn t consider it an error If you want to read nine elements of a ten-element array, who is C# to say any differently Of course, the average is incorrect, but the program doesn t know that You iterate through 11 (or more) elements: Now C# cares a lot It doesn t let you index beyond the end of an array, for fear that you may overwrite an important value in memory To test it, change the comparison in FixedArrayAverage s for loop to the following, replacing the value 10 with 11:

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

When you execute the program, you see a dialog box with this error message:

At first glance, this error message seems imposing However, you can get the gist rather quickly: Clearly, the IndexOutOfRangeException tells you that the program tried to access an array beyond the end of its range accessing element 11 in a 10-element array (In 9 of this minibook, I show you how to find out more about that error)

The array used in the FixedArrayAverage program example suffers from these two serious problems: The size of the array is fixed at ten elements Worse, the elements values are specified directly in the program A program that could read in a variable number of values, perhaps determined by the user during execution, would be much more flexible It would work not only for the ten values specified in FixedArrayAverage but also for any other set of values, regardless of their number The format for declaring a variable-size array differs slightly from that of a fixed-size, fixed-value array:

double[] doublesArrayVariable = new double[N]; double[] doublesArrayFixed = new double[10]; // Variable, versus // Fixed

Here, N represents the number of elements to allocate The updated program VariableArrayAverage enables the user to specify the number of values to enter (N has to come from somewhere) Because the program retains the values entered, not only does it calculate the average, but it also displays the results in a pleasant format, as shown here:

// VariableArrayAverage -- Average an array whose size is // determined by the user at runtime, accumulating the values // in an array Allows them to be referenced as often as // desired In this case, the array creates an attractive output namespace VariableArrayAverage { using System; public class Program { public static void Main(string[] args) { // First read in the number of doubles the user intends to enter ConsoleWrite( Enter the number of values to average: ); string numElementsInput = ConsoleReadLine(); int numElements = ConvertToInt32(numElementsInput); ConsoleWriteLine(); // Now declare an array of that size double[] doublesArray = new double[numElements]; // Here s the N // Accumulate the values into an array for (int i = 0; i < numElements; i++) { // Prompt the user for another double ConsoleWrite( enter double # + (i + 1) + : ); string val = ConsoleReadLine(); double value = ConvertToDouble(val); // Add this to the array using bracket notation doublesArray[i] = value; } // Accumulate numElements values from // the array in the variable sum double sum = 0; for (int i = 0; i < numElements; i++) { sum = sum + doublesArray[i]; } // Now calculate the average double average = sum / numElements; // Output the results in an attractive format ConsoleWriteLine(); ConsoleWrite(average + is the average of ( + doublesArray[0]); for (int i = 1; i < numElements; i++) { ConsoleWrite( + + doublesArray[i]); } ConsoleWriteLine( ) / + numElements); // Wait for user to acknowledge the results ConsoleWriteLine( Press Enter to terminate ); ConsoleRead(); } } }