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Introduction to debugging

From: Foundations of Programming: Fundamentals

Video: Introduction to debugging

When you're new to programming, it's usually difficult to write even a single line of code that works correctly. Programming languages are so specific and it's very easy to use the wrong case, to use a colon instead of a semicolon, to miss a closing quote, to use the wrong operator, the wrong name. when you write programs that aren't trivial, once you get beyond hello, world, it's really tough to write more than a few instructions without tripping up over your own logic. So yes, you expect your code to work and it's natural to get frustrated when it doesn't, but here's the thing that experienced programmers know that non-programmers don't.

Introduction to debugging

When you're new to programming, it's usually difficult to write even a single line of code that works correctly. Programming languages are so specific and it's very easy to use the wrong case, to use a colon instead of a semicolon, to miss a closing quote, to use the wrong operator, the wrong name. when you write programs that aren't trivial, once you get beyond hello, world, it's really tough to write more than a few instructions without tripping up over your own logic. So yes, you expect your code to work and it's natural to get frustrated when it doesn't, but here's the thing that experienced programmers know that non-programmers don't.

We don't expect our code to work anymore. Seriously, we don't ever expect to just write a program that works correctly the first time through. Our code still breaks all the time. You see programs aren't just written. They are edited into existence, they are built slowly, piece by piece and every programmer expects to spend far more time debugging their code than actually writing their code. We write a few lines, we check it, we fix it, we then write a few more, we check them, we go back and fix the first ones we just broke, and we keep repeating the process.

The thing is yes, it is easy to make mistakes, so don't worry about making mistakes. Go make a thousand of them, and then fix them, then make a few more. Now, we're going to debug our code. We're going to find out what went wrong and we're going to fix it. I am talking here of everything from just getting your program to run in the first place to six months down the road, you thought it was working fine but you're getting some odd behavior. So how do we start to think about this? When we have errors in our code, they really break down into two main categories.

The first and most obvious are syntax errors and then we also have logic errors. So let's talk about the difference. The most obvious kind of problem is with syntax. Something is wrong with the actual format of what we wrote. You spelled a keyword with the wrong case letter, you used a colon instead of a semicolon, you forgot to close a quote on a string, and if it's that easy to do it on the simplest statement there is, just think about what it's like when you've got a lot more code, thousands of lines of this.

Even experienced programmers will from time to time just have to go through letter by letter figuring out, what did I miss here? It's quite common to count the opening and closing curly braces to make sure that they match up. Now, one of the benefits of using programmer's text editors and having things like syntax highlighting is it can point some of these things out. Let's say I am working in an editor and I know that green means a comment. So if I see a big block of code like this, I can immediately think, hang on, there's a problem here, and what I can see is I accidentally opened a multi-line comment, making everything after this an actual comment, whereas what I wanted was a single line comment that looks like this.

So color coding can really help when you're just scanning your code to find syntax problems. But these are the kind of problems you will do less of as you get familiar with any language. You will still make them but you will make less of them. One of the benefits of using a compiled language rather than an interpreted language is these syntax problems are found before you even try to run the program, because you have to run your code through a compiler. But with an interpreted language like JavaScript, you often have to attempt to run it to find out what's wrong.

Then we have logic issues. These are really the main problems for any program and this is where the code you wrote is correct in syntax and if you're working in a compiled language, it will even compile, but there is a flaw in the logic. The fencepost error that we talked about early in the course is one small micro-example of a logic error. You've got a loop that just isn't going round enough times. It's one time too many or one time too few. Another example would be creating a function and that might have a lot of code in it but somewhere in the middle, you've got a return statement.

Well the problem is, if you hit a return statement and a function, you'll immediately jump back out of the function into whoever called it, which means any lines after the return statement would never be executed. And these are just three very obvious examples of logic errors. The problem with most of them is that they can be extremely difficult to find. There are some logic errors that even fall into their own category like problems with arithmetic. What do we mean by this? These are also correct in syntax but they just don't make sense.

So if I create some code like this, a variable called a, set equal it to 100 and then var b = 0, var result = a/b. The syntax here is correct: the right case, the right operators, we have the semicolons. The problem is this doesn't make sense. a/b here is 100/0. Divide by 0 is not allowed. It logically doesn't make sense, so the computer can't do it. Now, in JavaScript this might just get you a weird message from the alert statement.

But in many other programming languages, the line that tries to divide by 0 will cause your program to crash completely. And yes, these are just a few examples of the things that can go wrong. The logic errors will be the most significant problem you'll ever have as a programmer. But in any debugging situation, the first step is always reproduce the problem. Does the same error occur the same way each time? Hopefully it does, because that's going to make it easier to find. After this, we need to figure out where in our code the problem actually is and that can be more difficult than it seems.

For that, we need to figure out if our program is actually running the way we think it is.

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This video is part of

Image for Foundations of Programming: Fundamentals
Foundations of Programming: Fundamentals

61 video lessons · 89966 viewers

Simon Allardice
Author

 
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  1. 4m 15s
    1. Welcome
      1m 17s
    2. Making the most of this course
      2m 8s
    3. Using the exercise files
      50s
  2. 22m 11s
    1. What is programming?
      5m 45s
    2. What is a programming language?
      4m 48s
    3. Writing source code
      5m 34s
    4. Compiled and interpreted languages
      6m 4s
  3. 16m 29s
    1. Why JavaScript?
      4m 45s
    2. Creating your first program in JavaScript
      6m 54s
    3. Requesting input
      4m 50s
  4. 31m 38s
    1. Introduction to variables and data types
      5m 16s
    2. Understanding strong, weak, and duck-typed languages
      3m 51s
    3. Working with numbers
      5m 4s
    4. Using characters and strings
      4m 5s
    5. Working with operators
      4m 47s
    6. Properly using white space
      6m 46s
    7. Adding comments to code for human understanding
      1m 49s
  5. 24m 48s
    1. Building with the if statement
      7m 35s
    2. Working with complex conditions
      4m 9s
    3. Setting comparison operators
      6m 59s
    4. Using the switch statement
      6m 5s
  6. 17m 54s
    1. Breaking your code apart
      4m 1s
    2. Creating and calling functions
      2m 56s
    3. Setting parameters and arguments
      6m 7s
    4. Understanding variable scope
      2m 23s
    5. Splitting code into different files
      2m 27s
  7. 13m 31s
    1. Introduction to iteration
      4m 28s
    2. Writing a while statement
      5m 24s
    3. Creating a for loop
      3m 39s
  8. 19m 28s
    1. Cleaning up with string concatenation
      4m 30s
    2. Finding patterns in strings
      8m 3s
    3. Introduction to regular expressions
      6m 55s
  9. 19m 58s
    1. Working with arrays
      5m 46s
    2. Array behavior
      5m 29s
    3. Iterating through collections
      5m 18s
    4. Collections in other languages
      3m 25s
  10. 10m 50s
    1. Programming style
      5m 55s
    2. Writing pseudocode
      4m 55s
  11. 25m 55s
    1. Input/output and persistence
      3m 6s
    2. Reading and writing from the DOM
      8m 11s
    3. Event driven programming
      7m 47s
    4. Introduction to file I/O
      6m 51s
  12. 24m 25s
    1. Introduction to debugging
      5m 57s
    2. Tracing through a section of code
      7m 5s
    3. Understanding error messages
      3m 21s
    4. Using debuggers
      8m 2s
  13. 14m 16s
    1. Introduction to object-oriented languages
      5m 18s
    2. Using classes and objects
      6m 28s
    3. Reviewing object-oriented languages
      2m 30s
  14. 11m 14s
    1. Memory management across languages
      5m 11s
    2. Introduction to algorithms
      4m 2s
    3. Introduction to multithreading
      2m 1s
  15. 29m 20s
    1. Introduction to languages
      1m 42s
    2. C-based languages
      4m 40s
    3. The Java world
      3m 13s
    4. .NET languages: C# and Visual Basic .NET
      6m 17s
    5. Ruby
      3m 4s
    6. Python
      2m 56s
    7. Objective-C
      4m 3s
    8. Libraries and frameworks
      3m 25s
  16. 1m 2s
    1. Where to go from here
      1m 2s

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