Scala
File I/O
Types of files
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There are two kinds of files: Text files, and binary files
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Of course, it’s not that simple…
Text files
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Text files are “human readable,” which really means that a lot of different
programs can understand them
ASCII (American Standard Code for Information Interchange) has been in
widespread use since about 1967
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Unicode was designed to support “all” the world’s languages, and began to be
used in about 1987
UTF-8 (UCS Transformation Format—8-bit) combines ASCII and Unicode,
and has become the Web standard
Binary files
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ASCII is a set of 128 character, many of which are “control” characters
Binary files are not designed to be human readable, and are often ad hoc
Some familiar binary file types are jpg, gif, png, doc, rtf, class
We’ll only be concerned with text files
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How to refer to a file
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There are four ways to tell the computer where to find a file (for
reading) or put a file (for writing)
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Provide the absolute path to the file
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Provide a relative path to the file (starting from the location in which the
program is running)
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This is not too bad, if you provide a complete directory containing both the
program and its data
It does require the instructor to start the program from the same place you do
Tell the operating system to keep the file in a special hidden location
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For those of you on Windows, this usually means starting with C:
This is almost always a terrible idea!
It means that anyone you give your program to (like, the instructor) must have
the exact same directory structure as you do
This is good for default settings, but not for data
Ask the user where the file is (often the best approach)
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How to ask the user
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Do this:
import scala.io.Source, scala.io.Source._,
scala.swing.FileChooser
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If you want to read in a file:
val chooser = new FileChooser
val response = chooser.showOpenDialog(null)
(User selects a file)
if (response == FileChooser.Result.Approve) { read in file }
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If you want to write out a file:
val chooser = new FileChooser
val response = chooser.showSaveDialog(null)
(User selects a file)
if (response == FileChooser.Result.Approve) { write out file }
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Which file was chosen?
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The result of calling FileChooser is either
FileChooser.Result.Approve, or it’s something
else
If it’s Approve, you can ask it which file was chosen
with chooser.selectedFile
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Accessing files by path
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import java.io.File
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val file = new File("C:/Programming/p1.txt")
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This gives you a file object that represents a file
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There is no guarantee that there is such a file
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All the world, except Microsoft, uses forward slashes (/) as path delimiters
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Microsoft standard is backslashes, \
In a string, backslashes must be doubled: ("C:\\Programming\\p1.txt")
Microsoft has ”seen the light” and now allows forward slashes
All the world, except Microsoft, uses case-sensitive file names
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Your TAs aren’t using Windows
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Reading from a file
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Either Source.fromFile(file) or
Source.fromFile(filename) will give you an iterator for the
file
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The iterator will give you one character at a time
Thus, for (ch <- Source.fromFile(file)) { print } will print
out the entire file, one character at a time
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Newlines are included
(ch <- Source.fromFile(file).getLines) { print } will print
out the entire file, one line at a time
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Newlines are discarded
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Putting it all together
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scala> import scala.io.Source, scala.io.Source._
import scala.io.Source
import scala.io.Source._
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scala> import scala.swing.FileChooser, java.io.File
import scala.swing.FileChooser
import java.io.File
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scala> val chooser = new FileChooser
chooser: scala.swing.FileChooser = scala.swing.FileChooser@516331f
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scala> val response = chooser.showOpenDialog(null)
response: scala.swing.FileChooser.Result.Value = Approve
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scala> if (response == FileChooser.Result.Approve) {
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for (line <- Source.fromFile(chooser.selectedFile).getLines) {
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println("Processing line: " + line)
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}
| }
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The End
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