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Web Cartoon Maker: a Fun Way to Learn C++ Contents Previous Next

What is Debugging?

Programming is a complex process, and since it is done by human beings, it often leads to errors. For whimsical reasons, programming errors are called bugs and the process of tracking them down and correcting them is called debugging.

Fun Note: The very early computers did not use transistors and integrated circuits but used vacuum tubes, which operated at very high voltages – or in some cases, even electromechanical relays. It was not unusual for a real bug to get electrocuted inside the machine and affect operation – thus hardware “bugs” were a real problem.

There are a few different kinds of errors that can occur in a program, and it is useful to distinguish between them in order to track them down more quickly.

  1. Compile-time errors. The compiler can only translate a program if t he program is syntactically correct; otherwise, the compilation fails and you will not be able to run your program. Syntax refers to the structure of your program and the rules about that structure. For example, in English, a sentence must begin with a capital letter and end with a period. This sentence contains a syntax error. So does this one For most readers, a few syntax errors are not a significant problem, which is why we can read the poetry of E. E. Cummings without spewing error messages. Compilers are not so forgiving. If there is a single syntax error anywhere in your program, the compiler will print an error message and quit, and you will not be able to run your program. To make matters worse, there are more syntax rules in C++ than there are in basic English, and the error messages you get from the compiler are often not very helpful. During the first few weeks of your programming experience, you will probably spend a lot of time tracking down syntax errors. As you gain experience, though, you will make fewer errors and find them faster.

  2. Run-time errors . The second type of error is a run-time error, so-called because the error does not appear until you run the program. For the simple sorts of programs we will be writing, run-time errors are rare, so it might be a little while before you encounter one. A good example of run-time error is a division by zero. At the moment Web Cartoon Maker does not have a debugger yet and in case of a runtime error you may receive a message like “unknown error”.

  3. The third type of error is the logical or semantic error. If there is a logical error in your program, it will compile and run successfully, in the sense that the computer will not generate any error messages, but it will not do the right thing. It will do something else. In our case events in animated cartoon may be different than we expected. Specifically, it will do what you told it to do. The problem is that the program you wrote is not the program you wanted to write. The meaning of the program (its semantics) is wrong. Identifying logical errors can be tricky, since it requires you to work backwards by looking at the output of the program and trying to figure out what it is doing. In these cases it may be necessary to add code to your program for the specific purpose of aiding in locating the logical error.

One of the most important skills you should acquire from working with this book is debugging. Although it can be frustrating, debugging is one of the most intellectually rich, challenging, and interesting parts of programming. In some ways debugging is like detective work. You are confronted with clues and you have to infer the processes and events that lead to the results you see.

Debugging is also like an experimental science. Once you have an idea what is going wrong, you modify your program and try again. If your hypothesis was correct, then you can predict the result of the modification, and you take a step closer to a working program. If your hypothesis was wrong, you have to come up with a new one. As Sherlock Holmes pointed out: “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” (from A. Conan Doyle’s The Sign of Four).

For some people, programming and debugging are the same thing s. That is, programming is the process of gradually debugging a program until it does what you want. The idea is that you should always start with a working program that does something, and make small modifications, debugging them as you go, so that you always have a working program. In our case you can start with a very simple animated cartoon and make small modifications to it one by one.

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