STATEMENTS IN C# CAN BE either simple statements or compound statements. Simple statements, such as assignments statements and subroutine call statements, are the basic building blocks of a program. Compound statements, such as while loops and if statements, are used to organize simple statements into complex structures, which are called control structures because they control the order in which the statements are executed. Now we'll explore the details of the control structures I introduced at the beginning of this chapter. At the same time, we'll look at examples of programming with each control structure and apply the techniques for designing algorithms that were introduced in the previous section.
The while statement was already introduced in Section 1, “Blocks, Loops, and Branches”. A while loop has the form
while ( comparison ) {
statements
}The statement can, of course, be a block statement consisting of several statements grouped together between a pair of braces. This statement is called the body of the loop. The body of the loop is repeated as long as the boolean-expression is true. This boolean expression is called the continuation condition, or more simply the test, of the loop. There are a few points that might need some clarification. What happens if the condition is false in the first place, before the body of the loop is executed even once? In that case, the body of the loop is never executed at all. The body of a while loop can be executed any number of times, including zero. What happens if the condition is true, but it becomes false somewhere in the middle of the loop body? Does the loop end as soon as this happens? It does not, because the computer continues executing the body of the loop until it gets to the end. Only then does it jump back to the beginning of the loop and test the condition, and only then can the loop end.
Let's look at a typical problem that can be solved using a while loop: finding the average of a set of positive integers entered by the user. The average is the sum of the integers, divided by the number of integers. The program will ask the user to enter one integer at a time. It will keep count of the number of integers entered, and it will keep a running total of all the numbers it has read so far. Here is a pseudocode algorithm for the program:
Let sum = 0 Let count = 0 while there are more integers to process: Read an integer Add it to the sum Count it Divide sum by count to get the average Print out the average
But how can we test whether there are more integers to process? A typical solution is to tell the user to type in zero after all the data have been entered. This will work because we are assuming that all the data are positive numbers, so zero is not a legal data value. The zero is not itself part of the data to be averaged. It's just there to mark the end of the real data. A data value used in this way is sometimes called a sentinel value. So now the test in the while loop becomes "while the input integer is not zero". But there is another problem! The first time the test is evaluated, before the body of the loop has ever been executed, no integer has yet been read. There is no "input integer" yet, so testing whether the input integer is zero doesn't make sense. So, we have to do something before the while loop to make sure that the test makes sense. Setting things up so that the test in a while loop makes sense the first time it is executed is called priming the loop. In this case, we can simply read the first integer before the beginning of the loop. Here is a revised algorithm:
Let sum = 0 Let count = 0 Read an integer while the integer is not zero: Add the integer to the sum Count it Read an integer Divide sum by count to get the average Print out the average
Notice that I've rearranged the body of the loop. Since an integer is read before the loop, the loop has to begin by processing that integer. At the end of the loop, the computer reads a new integer. The computer then jumps back to the beginning of the loop and tests the integer that it has just read. Note that when the computer finally reads the sentinel value, the loop ends before the sentinel value is processed. It is not added to the sum, and it is not counted. This is the way it's supposed to work. The sentinel is not part of the data. The original algorithm, even if it could have been made to work without priming, was incorrect since it would have summed and counted all the integers, including the sentinel. (Since the sentinel is zero, the sum would still be correct, but the count would be off by one. Such so-called off-by-one errors are very common.) Counting turns out to be harder than it looks!
We can easily turn the algorithm into a complete program. Note that the program cannot use the statement "average = sum/count;" to compute the average. Since sum and count are both variables of type int, the value of sum/count is an integer. The average should be a real number. We've seen this problem before: we have to convert one of the int values to a double to force the computer to compute the quotient as a real number. This can be done by type-casting one of the variables to type double. The type cast "(double)sum" converts the value of sum to a real number, so in the program the average is computed as "average = ((double)sum) / count;". Another solution in this case would have been to declare sum to be a variable of type double in the first place.
One other issue is addressed by the program: If the user enters zero as the first input value, there are no data to process. We can test for this case by checking whether count is still equal to zero after the while loop. This might seem like a minor point, but a careful programmer should cover all the bases.
Here is the program:
Example 3.3. ComputeAverage.cs
/* This program reads a sequence of positive integers input
by the user, and it will print out the average of those
integers. The user is prompted to enter one integer at a
time. The user must enter a 0 to mark the end of the
data. (The zero is not counted as part of the data to
be averaged.) The program does not check whether the
user's input is positive, so it will actually work for
both positive and negative input values.
*/
using System;
class ComputeAverage {
static void Main() {
int inputNumber; // One of the integers input by the user.
int sum; // The sum of the positive integers.
int count; // The number of positive integers.
double average; // The average of the positive integers.
/* Initialize the summation and counting variables. */
sum = 0;
count = 0;
/* Read and process the user's input. */
Console.Write("Enter your first positive integer: ");
inputNumber = Convert.ToInt32(Console.ReadLine());
while (inputNumber != 0) {
sum += inputNumber; // Add inputNumber to running sum.
count++; // Count the input by adding 1 to count.
Console.Write("Enter your next positive integer, or 0 to end: ");
inputNumber = Convert.ToInt32(Console.ReadLine());
}
/* Display the result. */
if (count == 0) {
Console.WriteLine("You didn't enter any data!");
} else {
average = ((double)sum) / count;
Console.WriteLine();
Console.WriteLine("You entered " + count + " positive integers.");
Console.WriteLine("Their average is " + average + ".");
}
} // end main()
} // end class ComputeAverage