Page 63 - Introduction to Programming with Java: A Problem Solving Approach

Figure 2.3 shows that each of the three variables is a container that holds a value. Figure 2.3 also visu- ally suggests that assignment goes in a right-to-left direction. Assignment in our pseudocode has no direc- tionality, but in Chapter 3, you’ll see that assignment in Java code actually does go right to left. So if you are a person who likes to visualize things, visualize assignment going right to left, as Figure 2.3 suggests.
2.5 Input
In the preceding Rectangle algorithm, the algorithm itself supplied the values for the length and width variables. We did it that way to make the introductory discussion as simple as possible. Sometimes this is an appropriate strategy, but in this particular case, it’s silly, because the algorithm solves the problem only for one particular set of values. To make the algorithm more general, instead of having the algorithm sup- ply the values for length and width, you should have the user (the person who runs the program) supply the values. When a user supplies a value(s) for a program, that’s called user input, or just input. Figure 2.4 presents an improved Rectangle algorithm, where input length and input width perform user input operations.
Figure 2.4 Rectangle algorithm that gets length and width values from a user
Note the first two print statements in Figure 2.4—they’re called prompts because they tell (or prompt) the user what to enter. Without prompts, most users would be left with an unpleasant sensation and the puz- zling question, “What do I do now?”
Throughout the book, we provide sample sessions as a means of showing what happens when an al- gorithm or program is run with a typical set of inputs. When there is space, we include the sample session in the figure with the algorithm or program that generates it. Can you identify the user-input values in the sample session in Figure 2.4? Our convention is to italicize sample session input values to distinguish them fromoutput.Thus,10and4 areuser-inputvalues.
The combination of a pseudocode algorithm and a sample session represents a con-
venient and efficient way to specify a simple algorithm or program. The sample session
shows the format of desired inputs and outputs. It also shows representative input and out-
put numerical values, which allow a programmer to verify that his/her completed pro-
gram actually behaves as required. In many of the book’s projects (projects are on the Web site), we provide some combination of pseudocode and sample session to specify the problem we are asking you to solve.
2.6 Input 29
print “Enter a length in meters: ” input length
print “Enter a width in meters: ” input width
set rectangleArea to length * width
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print “The area is” rectangleArea “square meters.”
Sample session:
Enter a length in meters: 10
Enter a width in meters: 4
The area is 40 square meters.
User inputs are italicized.
Write what you’ll do and how you’ll do it.