Foundations of Programming: Fundamentals
Illustration by Richard Downs

Introduction to file I/O


Foundations of Programming: Fundamentals

with Simon Allardice

Video: Introduction to file I/O

It's a very common requirement to have a program need to read files on the hard drive or save files to the hard drive to persist information there. But this will be a pretty short movie if I was trying to show you how to do this in JavaScript. And again, because of security, it just doesn't do that. Now before I get a few emails telling me about a non-standard JavaScript desktop implementation or the file API of HTML5, I'm talking here about typical standard JavaScript as used on a normal webpage.
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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
  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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Watch the Online Video Course Foundations of Programming: Fundamentals
4h 47m Beginner Sep 22, 2011

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This course provides the core knowledge to begin programming in any language. Simon Allardice uses JavaScript to explore the core syntax of a programming language, and shows how to write and execute your first application and understand what's going on under the hood. The course covers creating small programs to explore conditions, loops, variables, and expressions; working with different kinds of data and seeing how they affect memory; writing modular code; and how to debug, all using different approaches to constructing software applications.

Finally, the course compares how code is written in several different languages, the libraries and frameworks that have grown around them, and the reasons to choose each one.

Topics include:
  • Writing source code
  • Understanding compiled and interpreted languages
  • Requesting input
  • Working with numbers, characters, strings, and operators
  • Writing conditional code
  • Making the code modular
  • Writing loops
  • Finding patterns in strings
  • Working with arrays and collections
  • Adopting a programming style
  • Reading and writing to various locations
  • Debugging
  • Managing memory usage
  • Learning about other languages
Developer Web
Simon Allardice

Introduction to file I/O

It's a very common requirement to have a program need to read files on the hard drive or save files to the hard drive to persist information there. But this will be a pretty short movie if I was trying to show you how to do this in JavaScript. And again, because of security, it just doesn't do that. Now before I get a few emails telling me about a non-standard JavaScript desktop implementation or the file API of HTML5, I'm talking here about typical standard JavaScript as used on a normal webpage.

This does not let you write code that just reads files off your hard drive or saves files to your hard drive. But seeing as I believe this is an important subject to talk about in the Fundamentals course I'm going to talk a little bit about file input and output. I'm going to start with couple of pseudocode examples and then show what it might look like in another language, because here's the thing. File input/output, File I/O, can actually be a little tougher than it first sounds, because to read the most basic of files, we need to know the path, we need to open the file, we need to read the contents of the file, and we need to close it.

So if I just wrote this as simple pseudocode, we'd say open file at path myfile.txt. This is allowing us to grab hold of it, to treat it as ours. Then I could read the contents of the file into a string variable and then I do need to close the file. It's always a rule to close the file when you're done with it. But here is the issue. There is a whole bunch of ways this could go wrong. What if there is no file at that path? What if some other program has that file already open? What if they've already grabbed it? What if that file is huge? Can we just read it all into a string variable? And you'll often find that dealing with file input/output, there is a lot more conditional code going on just to check all the things that could possibly go wrong.

But let's say we're not going to worry about it. Let's say we just want to read in a small text file that we know exists. Well, here's an example using the programming language Ruby. And as usual, don't worry too much about the syntax. It's not what we're focusing on here. Just the overall approach. Can you scan this code and see where the file path is? Can you see where we're probably opening the file? Can you see where we close it? It's not about whether you could turn around and write this down yourself. It's about that you understand the general approach here.

So this will open the file called simpleexample.txt. Because I didn't put any other path information here, this file should be in the same folder as the program itself. Now the next line, we have using that filename. And we're using this r in quotes here. This r means open this file in read-only mode. If we wanted to open this file so we could write to it, we could use w, or for both reading and writing, we could say r+.

Now it's very common for different programming languages to support opening files in different modes. One mode for reading, another mode for writing, another mode for read and write. You might have a mode for overwrite this if the file exists already or another mode for append to it if the file exists already. These are all very common. And we choose modes because it's helpful. Opening a file in read-only mode has less impact on the system and it allows other people to open it too.

Then we've got a couple of curly braces being used. Yes, it's not exactly the same as a JavaScript block, but it does appear to be some kind of container. What this is doing is reading each line and using puts, p-u-t-s, which is put string in Ruby to display each individual line. Then on the last line we're saying f.close. We're actually closing the file that we opened on line 2. So here's an example in Ruby of writing a string out to a file. So we start off by creating a string called message.

Again, it doesn't look exactly the same as a variable in JavaScript, but close enough that we can understand what's going on. Same thing with the filename = 'myoutput.txt'. Then we call I'm using a slightly different format here. I can see that I've got the w, which is open in write mode, and the last line we have here is this end. What this is going to allow us to do is have an automatic close of the file that I opened a couple of lines beforehand. Again, don't worry about the syntax. just get the overall approach here.

We need the path, we need to open the file, we need to write the contents, we need to close the file. Now I will say here Ruby is one of the simplest languages for reading and writing files. If I'd shown you these examples in C++ or Java or C#, we would have had a lot more lines to deal with. Now there's a big difference between reading a typical text file and reading a file of binary data like audio or video. It's very common when you start working with files that you'll find yourself needing to break things up a bit.

In most languages you rarely read an entire file at once unless you know it's going to be small. Instead, you read the file one chunk at a time. You break it up into pieces. One example would be that rather than reading a text file all at once, you process it one line at a time. Some pseudocode here might be that we open the file at the path myfile.txt and then we're going to have a loop that says while the file has lines left, we will read one line, we'll display that, and then we'll end the loop.

We'll loop around again. Does it still have lines left? Yes, it does. We'll read another line, display that line, and just go piece by piece through the entire file. Then finally, we'll close the file. And if we're opening the file, we close the file. With many languages, even higher-level ones like C#, Java, and Objective-C, you can end up working with files by working with what are called streams. The idea of a stream is quite simple. It just means a stream of bytes like a conveyor belt of bytes. And it can sound complex, but it really isn't. When working with streams, we just get to take them one chunk at a time.

And they're great because they allow you to stop caring where this data actually is. You don't really mind anymore. Is it on a file on your hard drive? It could be. Maybe I'm reading it off a network, off a website. It doesn't matter. It's just a stream of data that I need to deal with. But a typical task for a programmer new to any language is finding out how that language likes to work with file input and output. You'll find that most languages deal with the same concepts in slightly different ways, but they all have substantial built-in functionality for reading and writing different kinds of files.

Find answers to the most frequently asked questions about Foundations of Programming: Fundamentals .

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Q: Using TextEdit with Mac OS 10.9 Mavericks? 
A: If you're using the built-in TextEdit program in Mavericks to write your first examples and your code doesn't seem to be working, here's one reason why: by default, "smart quotes" are now turned on in TextEdit Preferences.
This is where TextEdit will automatically change pairs of double quotes to "smart quotes" - where the opening and closing quote are different, like a 66 and 99.
While this is fine for human eyes, programming languages don't want this - when writing code, they need to be the plain, generic straight-up-and-down quotes.
So make sure that in TextEdit > Preferences, that "Smart quotes" are unchecked.
Important! Whenever you make a change to TextEdit preferences, make sure to then completely quit out of the program (Command-Q or using TextEdit > Quit TextEdit) and then re-open it, as changes won't take effect on documents you already have open.
However, we're not finished - just because you've changed the preferences, it does **not** change any *existing* smart quotes back to "regular" quotes - it just doesn't add new ones - so make sure to go through your files for any time you wrote quotes and TextEdit may have changed them to smart quotes - look in both the JavaScript, and your HTML too, and compare to the downloadable exercise files if necessary.
If that sounds like a bit of a chore, I recommend just downloading a code editor like Sublime Text ( or TextMate ( and using that instead of TextEdit - it's only a matter of time before you'd move away from TextEdit anyway - we only used it in the course because it was built-in and a quick way to get started, but it's now become more of a inconvenience than it was before.
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