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Introduction to file I/O

From: Foundations of Programming: Fundamentals

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.

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 File.open 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 File.open. 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.

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This video is part of

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Foundations of Programming: Fundamentals

61 video lessons · 85679 viewers

Simon Allardice
Author

 
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  1. 4m 14s
    1. Welcome
      1m 16s
    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 49s
    1. Building with the if statement
      7m 35s
    2. Working with complex conditions
      4m 10s
    3. Setting comparison operators
      6m 59s
    4. Using the switch statement
      6m 5s
  6. 17m 56s
    1. Breaking your code apart
      4m 1s
    2. Creating and calling functions
      2m 57s
    3. Setting parameters and arguments
      6m 7s
    4. Understanding variable scope
      2m 23s
    5. Splitting code into different files
      2m 28s
  7. 13m 32s
    1. Introduction to iteration
      4m 28s
    2. Writing a while statement
      5m 24s
    3. Creating a for loop
      3m 40s
  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 59s
    1. Working with arrays
      5m 47s
    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 26s
    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 3s
  13. 14m 17s
    1. Introduction to object-oriented languages
      5m 18s
    2. Using classes and objects
      6m 29s
    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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