Debugging

For any given problem, we design a solution and then implement it.

As an example, let’s say that we are writing a program that gives you the number of digits in an integer. We expect the following input-output mappings.

Now, it’s possible that the actual outputs you get from your code are as follows,

We need to find out why do some inputs have incorrect outputs. So we go through our design and implementation looking for possible bugs. A logical way to do this is to trace every variable at every stage and see where does the program deviate from the expected.

Example 1

Consider the following code that is supposed to return the product of all integers from 1 to n (n >= 1).

The input-output mappings are -

If you trace the program, you’ll see that the loop executes when i=1 and result becomes result (0) * i (1) = 0. And every subsequent time, result becomes 0 * i = 0. A trace for n=4 using logic table is provided below,

Thus, the first bug is result should be initalized to 1 and not 0.

Our partially fixed code:

The new input-output mappings are -

A trace for n=4 using logic table is provided below,

It can now be seen that the loop should execute for i=4 and multiply it into the result but it doesn’t. By changing i < n to i <= n, we fix this problem.

To confirm, we trace once more for i=4.

Debug a method

Debug the following method for which the expected input-output mappings are provided in the javadoc (comment above the method). CRITICAL STEP!!! Write down the actual input-output mappings after every iteration of debugging

Solution

Performing debugging in Eclipse

Most of the modern Integrated Development Environments (IDEs) have a comprehensive debugging feature that let’s you trace the variables as your program executes.

In the IDE we are using (Eclipse), the debugger relies of placing breakpoints that are like pitstops in car race. The program runs till the next breakpoint where you can see the values of all the variables and when you hit resume, it goes to the next breakpoint.