Debugging 2

Examples

Work through some examples and/or homework assignments together with the students.

Understanding an existing program

Let's look at the code in homework/train-stations-complete.js. It's a program that can calculate routes between the train stations of various cities in the Netherlands, and also provides us with the distance between these cities. Very useful if you want to know how to get from one place to the other. Unfortunately there's a bug in this program, and even more unfortunate for us this program is a lot bigger than our previous examples.

We need to start looking at this program from different angles. We can see that it has various functions. Let's examine them first and write down what they might do.

Starting from the top we see that there's a function to create a train station. Then there's a variable called trainStations that contains stations of four Dutch cities. Let's console.log them.

const trainStations = {
    "Amsterdam": 
        {// [latitude, longitude]
         coordinates: [52.3791283, 4.8980833], 
         connections: ["Rotterdam", "Utrecht"]},
    "Rotterdam": 
        {coordinates: [51.92235, 4.4661983], 
         connections: ["Amsterdam", "Utrecht"]},
    "Utrecht":     
        {coordinates: [52.0894444, 5.1077981], 
         connections: ["Amsterdam", "Rotterdam", "Arnhem", "Oberhausen"]},
    "Arnhem": 
        {coordinates: [51.984034, 5.8983483], 
         connections: ["Utrecht", "Oberhausen"]},
    "Oberhausen":
         {coordinates: [51.4983534, 6.8131958],
          connections: ["Arnhem", "Utrecht"]}
};

console.log(trainStations);

Question: What would the output of console.log(trainStations) look like?

Then there's some small functions.

Question: What do these small functions do?

Then we come accross a very complicated looking function called distanceInMeters.

Question: Can you guess what it might do, even if you don't (fully) understand it?

It's very well possible this function has a bug, but because we don't really understand the code that's hard for us to verify.

Question: Even if we don't understand it, how can we get some confidence in that it might be correct or incorrect?

Summary

You've learned how to examine an existing program to get an understanding of what it does. You've learned that examining a program can give you the following insights:

1. You can examine functions separately from the rest of the program.
2. You can guess what a function does by looking at the name and its parameters.
3. If you don't understand some functions you can still make sense of what the program does.
4. By testing individual functions you can verify if they have a bug.