The ability to debug a program is one of the most crucial skills that you will need to acquire in order to be an effective programmer. There are three principle ways you can debug a program:
However, sometimes you do not even have a clue as to where the bug is occurring. In this case you must start inserting print statements at important points in the code in an effort to localize the bug. Important points in the program include immediately after any housekeeping you perform to start the program and immediately after any important computation performed by the program. When a print statement reveals that the value of a variable is incorrect, you know that the bug in your program must lie in code that precedes the computation of this variable and you can start moving your print statements earlier in the program. You keep moving the print statements earlier until the value of your important variable(s) become correct. At this point you know that the bug must lie between the current location of your print statements and the location of the print statements that first revealed the incorrect value of the variable. Sometimes this section of code is small enough that you can start debugging it by reading the code. Other times you must continue to try to further narrow down the location of the bug by moving your current print statements later in the code, until a value of a variable becomes incorrect, or moving the initial print statements earlier in the code, until they become correct. In any event what you are trying to do is "catch" the bug between the print statements that print correct variable values and print statements that print incorrect variable values.
Let's try this in practice with a simple program. Take a look at sum.cpp which sums a series of numbers and outputs their sum:
#include <iostream>
using namespace std;
int main() {
int sum;
int value;
while (!cin.eof()) {
cin >> value;
sum += value;
}
cout << "sum = " << sum << endl;
}
This program has two bugs in it and is small enough that you might be able to debug it by simply reading the code. However, let's suppose that you can't figure out what is wrong by simply reading the code.
First compile this code and run it against numbers.txt:
UNIX> ./sum < numbers.txt
We are re-directing input from stdin so that it comes from the file
numbers.txt. Look at numbers.txt. It would appear that
the sum of the numbers if 100. However, the program is either printing
150 or is printing some extremely big number for sum.
There is a good chance that you have no idea what is happening and cannot generate a hypothesis as to what is happening. Thus you need to insert print statements into the program. Remember that good practice is to put print statements after any housekeeping has been done, and after the computation of important variables. There are two places where you should be putting print statements in this program. Think about where you should put the print statements and then click here when you think you know the answer.