Execution-Time Errors
In this lesson, we will focus on finding logical errors in a program.
We'll cover the following...
An error in the logic of a program usually goes undetected by the compiler. Rather, such errors cause problems during the program’s execution. As we indicated in the previous lesson of this debugging interlude, a logical error might or might not cause an error message during execution.
Incorrect arithmetic
The program in the previous lesson had a syntax error. The following program corrects that error by inserting the * operator between 2 and y in the computation of z. Execute this program by clicking the RUN button.
public class Debug17{public static void main(String[] args){double x = 50.5;double y = 4.8;double z = 7 / 8 * x + 2 * y;System.out.println("x is " + x + ", y is " + y + ", z is " + z);} // End main} // End Debug17
This program compiles without error, and its execution produces the following output:
x is 50.5, y is 4.8, z is 9.6
As tempting as it might be to feel successful, we should check our answer. Substituting 50.5 and 4.8 for the variables x and y, respectively, and using a calculator, we would compute 53.7875 as the value of z. The program computed 9.6. How can we be so far off? Should we trust the computer or our calculator?
We certainly can—and ...