6.4. Debugging Logic Errors

We can debug runtime and syntax errors using the error messages produced. Logic errors, however, do not always produce error messages. This sometimes makes them tougher to debug.

While we can’t provide a step-by-step approach that applies to every possible logic error, we can give you some solid strategies. Two such strategies—using debugger tools and writing tests—will be covered in future lessons. In this section, we start with a basic and effective way to debug logic errors.

6.4.1. Printing Values

This example will work with logic and runtime errors. These are commonly seen in the C# environment.

When your code runs but doesn’t produce the expected results, it is important to check the values of the variables being used.

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

This program is designed to take ask the user for the temperature in Celsius and then convert it into Kelvin.

Before we get into the code let’s think about all the parts: Get a number from the user then use it in a math equation.

Okay, let’s get building:

Console.WriteLine("Temp in degrees C:");
string degreesC = Console.ReadLine();
Console.WriteLine("Degrees K: " + degreesC + 273.15);

repl.it

This program asks the user for a temperature in degrees Celsius and attempts to convert it to degrees Kelvin. Degrees Kelvin differs from degrees Celsius by 273.15. So if we enter 100 (in Celsius) we should see a converted value of 373.15 (in Kelvin). However, running the program as-is and entering 100 gives the message:

Temp in degrees C: 100
Degrees K: 100273.15

This is clearly incorrect. But the program does not generate an error, so it is not immediately clear what the issue is. To figure it out, we’ll use Console.WriteLine to see what the values of key variables are when the program runs.

Looks like our variables were pushed together, or concatenated, instead of added. Let’s look at our data types and see if that can give us any insight. We declared degreesC to be a string, but what about the number 273.15? We can add a Console.WriteLine statement to check the data type.

Console.WriteLine("Temp in degrees C:");
string degreesC = Console.ReadLine();
Console.WriteLine(273.15.GetType());
Console.WriteLine("Degrees K: " + degreesC + 273.15);

Running this with an input of 100 gives the output:

Temp in degrees C:  100
System.Double
Degrees K: 100273.15

The value of 273.15 in Line 3 is a double, but degreesC is initialized as a string. We can join them with the +, but final answer is still incorrect, so we need to keep digging for more information.

Looking at Line 4 in which we try to add 273.15 to degreesC. All we are doing is concatenating the two togehter. In the last chapter we learned how to convert data types using the Double.Parse method. Let’s see what happens if we use that?

Currently the data type of degreesC is a string. Let’s create a new variable and use the Double.Parse method to convert the string into a double. Let’s also run a Console.WriteLine statement to check the data type after our Double.Parse.

Console.WriteLine("Temp in degrees C:");
string degreesC = Console.ReadLine();
double degreesCDouble = Double.Parse(degreesC);
Console.WriteLine(degreesCDouble.GetType() + "  --- testing degreesCDouble");
Console.WriteLine(273.15.GetType() + " --- testing 273.15");
Console.WriteLine("Degrees K: " + degreesC + 273.15);

Running this with an input of 100 gives the output:

Temp in degrees C:  100
System.Double --- testing degreesCDouble
System.Double --- testing 273.15
Degrees K: 100273.15

That’s it! The variable degreesCDouble has the value 100, and is of data type double. However, degreesC is still a string.

Note

What are those extra strings in Lines 4 and 5? It’s a tag. Adding tags to your Console.WriteLine statements can also help debug your code. It is basic string concatenation, but can provide valuable information. We checked the data types of multiple variables. Tags remind you which variable is which or what you are expecting can help you stay organized as you debug and build.

We can fix our program by using our new variable, degreesCDouble.

Console.WriteLine("Temp in degrees C:");
string degreesC = Console.ReadLine();
double degreesCDouble = Double.Parse(degreesC);
Console.WriteLine("Degrees K: " + degreesCDouble + 273.15);

Running this with an input of 100 gives the output:

Temp in degrees C:  100
Degrees K: 100273.15

Wait what? This is another sneaky logic error. In Line 4, we are performing concatenation with our new variable, rather than printing any results. If we want to print the results of the calculation, we should make a variable to become the results of our calculations

Console.WriteLine("Temp in degrees C:");
string degreesC = Console.ReadLine();
double degreesCDouble = Double.Parse(degreesC);
double degreesK = degreesCDouble + 273.15;
Console.WriteLine("Degrees K: " + degreesK);

Running this with an input of 100 gives the output:

Temp in degrees C: 100
Degrees K: 373.15

It worked! Finally, right? We can take in user input? Check. We can use that input in a mathematical formula? We can now thanks to some conversions and additional variables. Great job!

Note that after debugging we removed all of our testing Console.WriteLine statements. Be sure to do the same when using this debugging technique.