Writing a Good Program 4. Basic Software Engineering

1. Writing a Good Program 4. Basic Software Engineering

2. 4.2 Debugging a Program

3. It is often NOT the case that a program can run correctly once it is developed. • Debugging is often a necessary procedure. • Debugging can be divided into the following two stages: • 1. Compile-time debugging (relatively easy) • Correct the errors owing to incorrect use of language syntax or unresolved references. • Run-time debugging (often far more difficult) • Correct the errors owing to incorrect program logic or resource usage. e.g. Memory fault

4. Compile-time Debugging • Compile-time errors can often be spotted out by the compiler. • The compiler is able to detect the approximate error location, but cannot be exact. • Also, the error messages generated are sometimes irrelevant. • When a series of error messages are found, always try to debug them starting from the first one. • Once the first one is corrected, the rest may be solved as well.

5. Run-time Debugging Can have no error message • Comparing with compile-time errors, run-time errors are more difficult to locate and correct. • Run-time errors are often caused by wrong programming logic or improper use of resources. • Run-time errors may sometimes lead to incorrect results or even system failure. • Even worst, some run-time errors give no observable problem but gradually damage the system or the database.

6. How to avoid Run-time Errors? • C++ has embedded many standard features to allow possible run-time errors be detected during compile-time (will be covered throughout this course) • A structured way in program development can help avoid run-time errors • Never develop your program in one step. • Always ensure the skeleton is correct before going forward. Warning Rule of thumb: Try your very best to enable the compiler to detect the errors for you (that is, to convert run-time errors to compile-time errors whenever allowable).

7. How to Debug Run-time Errors? • The problem of run-time errors can be observed only at run-time. • One has to run the program in order to debug run-time errors. • To debug run-time errors, the first thing that has to be done is to locate the errors. • Achieved by the divide-and-conquer approach. • The simplest way is to set some check-points in the program and to see how many check-points the program can correctly get through.

8. #include <iostream> using namespace std; int main() { int a, b, c; cout << "Enter 3 numbers: \n"; cin >> a; cout << a << endl; // Checkpoint cin >> b; cout << b << endl; // Checkpoint cin >> c; cout << c << endl; // Checkpoint if (c = (a+b)) { cout << a << endl << b << endl << c; // Checkpoint cout << "c = a+b\n"; } return 0; } • The program on the right was used in Exercise 3.3. • It has a run-time error in line 12. • To correctly locate that errors, four checkpoints are added to show the value of a, b, and c at different parts of the program. The output tells which statements have been executed.

9. Run-time Debugger of Visual Studio • To help in debugging more complicated run-time errors, Visual Studio has provided a run-time debugger • Allow line-by-line tracing of program codes. • Allow arbitrary breakpoint in a program. • Allow examination of the status of all resources at the breakpoints, such as • Data in memory and • Data value of variables. • Allow monitoring of a particular resource of interest.

10. Step 1: Build the program of interest Click on the lines of code where you would like to set breakpoints Right-click  Breakpoint  Insert Breakpoint Breakpoints: The moments that you would like to examine the status of the resource allocated to the project.

11. Step 2: Build it successfully. Start the debugger by clicking Debug  Start Debugging This line of code is to be executed Autos displays variables used in the current line of code and the preceding line of code. Status window: Give the value of all related variables.

12. Status window: One can also particularly watch the result of an expression by typing it in.

13. Step Out: Return to the calling function Stop debugging Continue debugging until the next breakpoint Step Over: Execute just the current line Step Into: Like Step Over, but if the current statement is a function, go into this function to debug Check View  Toolbars  Debug if you cannot see the debug toolbar.

14. Step 3: Step over the codes On stepping over each line of code, the current values of the variables are shown.

15. Step 4: Find the error. The user inputs 2, 3 and 4 for a, b and c respectively. But after executing the if statement, c changes to 5. Obviously there is a problem in that statement. Using Step Over

16. Step 5: Fix the error Stop Debugging. Go back to the source and fix the error.

17. Exercise 4.2 1 #include <iostream> 2 using namespace std; 3 int main() 4 { int CurrentX; 5 int x = 50; 6 CurrentX = x; 7 { 8 int x = 100; 9 CurrentX = x; 10 } 11 CurrentX = x; 12 return 0; 13 } • The Run-time Debugger of Visual Studio can also help us analyze the run-time behavior of our program. • For the program on the right, estimate the values of CurrentX before and after the 6th, 9th, and 11th lines are executed. • Verify your solutions by using the Run-time Debugger.