Debugging a computer program:

Debugging a computer program:

Design well from the beginning.

write out a design of the program or module first, using English or pseudocode to list the steps you need to take as your first pass at designing your algorithm

write your comments first, rather than last, so you know what you are doing
use descriptive variable names (does each variable name accurately describe the contents?)
create a variable list early, and add to it as you have a need to create new variables

2. When you start coding, write a small amount of code, compile it, correct any syntax errors, recompile it, continue until the syntax errors are corrected. Then add another small amount of code (approximately 8 to 10 lines), compile it, etc.

Continue entering code in this fashion until all of the code is entered. In cases where you are entering looping structures, you may want to just get the loop working first (printing the loop variable to check for correct operation). Once the loop is working you can go about including other operations to be performed within the loop.

3. Test modules separately, using driver programs.

pass the module dummy values designed to test the program
create data to test modules with thorough testing in mind
use data to check the edges of possible data, looking for off by one errors
use several examples of incorrect data
if 0 is a possible value, use it, looking for division by zero errors
if the module is processing a list, try entering an empty list to ensure that the module can handle empty lists

4. Test main module separately, using stubs for modules when necessary.

stubs are "empty" copies of unfinished modules, which pass back a dummy, syntactically legal, value for the purpose of testing execution of the main program
Example stub:

static boolean isDivisor(int dividend, int possibleDivisor)

{

return true; // return dummy value

}

5. Use output to show where your program is in execution (if necessary)

create a boolean variable that can be set to true or false (e.g. boolean debug = true)
example output statement:
if (debug) System.out.println("entered isDivisor module");
when your program seems to work, set debug to false and thoroughly test your program before deleting the print statements
some suggested places to put output statements:
first statement in a module, signaling arrival
last statement before returning from a module, signaling completion
anywhere where knowledge of the contents of a variable can aid in debugging the program (include the name of the variable in the output statement to clarify output, e.g. System.out.println("primeCount = " + primeCount);
within a loop that you suspect may be causing the program to hang, i.e. an infinite loop

6. Use the compilers error messages to determine approximately where a syntax error is located, then closely examine the offending code, looking for the error.

Hand execute the code, tracing the values of variables with the progression of the program.

Make a list of the variables on paper
Read through the code, making entries under each variable name containing the value of that variable as it would be if the computer set it. If the value in a variable changes, cross out the old value and write the new value under or beside the old value.
Reserve a special place on the paper to write any output

Get a programming partner who can scan your program, looking for errors in logic or syntax (and you, of course, will do the same for your partner when necessary).
Start early, and remember that "short cuts" can often extend the time spent programming by a considerable amount.