10.4. Fixing Logic Errors

We debug syntax and runtime mistakes by looking at syntax highlighting and reading any error messages produced. Unfortunately, logic errors do not generate error messages or show up in the code highlighting. This makes them tougher to debug.

If there is a logic error in our program, it will run without crashing. However, it will NOT do the right thing. Remember that programs do exactly what we tell them to. If we include a logic error in our code, then the program will do exactly the wrong thing.

While we can’t provide a step-by-step approach that works for every possible logic error, we can start with one basic and effective strategy.

10.4.1. Printing Values

When our code runs but doesn’t give the expected results, it is important to check the values of the variables as they get used. We must also feed the program a specific set of data, and then compare the output to a known, correct answer.

Let’s look at a program that has a logical bug.

hours = input('Enter a number of hours: ')
minutes = hours * 60

print('Number of minutes = ', minutes)

This program asks the user for a number of hours and tries to convert it into the matching number of minutes. Since there are 60 minutes in 1 hour, entering 2 for the hours should show us a result of 120 minutes. However, running the program as-is and entering 2 gives the output:

Enter a number of hours: 2
Number of minutes =  222222222222222222222222222222222222222222222222222222222222

This is clearly incorrect. The program runs without an error message, so the only way we see the mistake is to know that the answer should be 120.

Tip

After fixing syntax and runtime errors, ALWAYS test your code by using a set of data that should produce an answer you ALREADY know.

Without an error message, it is not immediately clear what went wrong. To figure it out, we’ll use print to check the values of key variables as the program runs.

Let’s first make sure that hours looks like it should by adding a print statement just after we create the variable.

hours = input('Enter a number of hours: ')
print(hours)
minutes = hours * 60

print('Number of minutes = ', minutes)

Running this with an input of 2 gives the output:

Enter a number of hours: 2
2
Number of minutes =  222222222222222222222222222222222222222222222222222222222222

The second line is the value of hours, which appears to be correct. Unfortunately, the final answer is still wrong, so we need to keep digging for more information.

Looking at the line where we assigned minutes, we see that we want to multiply hours by 60. We assume that hours is a numerical value.

Let’s check the data type for hours.

hours = input('Enter a number of hours: ')
print(hours, type(hours))
minutes = hours * 60

print('Number of minutes = ', minutes)

Running this with an input of 2 gives the output:

Enter a number of hours: 2
2 <class 'str'>
Number of minutes =  222222222222222222222222222222222222222222222222222222222222

That’s it! The variable hours has the value 2, but it is a string rather than a number. When we assigned minutes the formula hours * 60, we told Python to repeat the string 60 times: "2" * 60 is "222222222222222222222222222222222222222222222222222222222222".

Note

If you count the characters in minutes (or use len(minutes)), you will find 60 copies of the substring hours.

We fix the logic error by converting the user’s input to the int data type.

hours = int(input('Enter a number of hours: '))
print(hours, type(hours))
minutes = hours * 60

print('Number of minutes = ', minutes)

Running this with an input of 3 gives the output:

Enter a number of hours: 3
3 <class 'int'>
Number of minutes =  180

Success!

Now that we found and fixed the logic error, the next step is to remove the print statement on line 2. It is not necessary for the normal running of the program.

Warning

After debugging, be sure to remove ALL of the print statements used to find and fix logic errors.