Walnut/Ordinary Programming/InputOutput

2007-02-26T06:04:45Z

Frantz:

[[Category:Walnut|2]]

===I/O===

E file objects are created with the <file:name> expression (which produces a [http://www.erights.org/javadoc/java/io/File.html "tamed" Java File Object], i.e., a File object that follows capability security discipline). If you can hard-code the name, you usually do not need to use quotes (unless there is a space or another invalid url character). If the name is contained in a variable or is retrieved with a function call, you must XXX place a space between the colon and the representation of the name. If the word "file" is replaced with a single letter like "c", the letter is assumed to be the drive letter on a Windows machine:

<pre>

<nowiki># E sample
#File objects for hardwired files:
def file1 := <file:myFile.txt>
def file2 := <file:/home/marcs/myFile.txt>

#Using a variable for a file name:
def filePath := "c:\\docs\\myFile.txt"
def file3 := <file> [filePath]

#Using a single character to specify a Windows drive
def file4 := <c:/docs/myFile.txt>
def file5 := <c:\docs\myFile.txt></nowiki>

</pre>

Note that in the file3 example, there is a space between the <file> and the left bracket.

Note that in the file4 example, we used slashes, not backslashes, to separate directories on a Windows drive. ''In ''E'', the slash always works as a directory separator no matter the underlying operating system. ''

When constructing the filePath string we used double backslashes because in strings the backslash must be escaped by a backslash. A double backslash was not required in the hardwired file (file5).

File objects can represent directories as well as simple files. Files inside the directory can be accessed by indexing using square brackets:

<pre>

<nowiki># E syntax
def dir := <file:/home/marcs/dataDir/>
def file6 := dir["myFile.txt"]</nowiki>

</pre>

Both text files and directories work in conjunction with the "for" loop. With a directory, the loop iterates through the files in the directory. With a text file, the loop iterates through the lines of the file; each line-object is terminated with a newline ('\n') character. The lines in the loop are terminated with newline regardless of the underlying operating system, regardless of the operating system's end-of-line designation.

Files respond to the following messages [http://www.erights.org/javadoc/java/io/File.html (see the Edoc for a complete list)].

* exists -- returns true if the file exists
* deepReadOnly -- returns a file object that can be read, but not written. If the file is a directory, the readOnly nature is transitive to all files and directories reached via this object.
* setText(textString) -- creates the file and/or replaces the file contents with the text. When written out, the end-of-line designator is automatically converted to the local end-of-line mark for the underlying operating system. On Windows, for example, each line is automatically terminated by a cr/lf pair.
* getText -- returns the text from the file. Regardless of underlying operating system, the returned text uses a single newline character as the end-of-line mark.
* getBytes() -- returns the bytes of the file in a byte ConstList
* setBytes(byteList)

stdout and stderr are both TextWriters in the E environment.

==== Example: recursing through a directory tree searching for a file ====

def findFile(targetFileName, dir) :void {

def iterator(filename, file) {
if (filename == targetFileName) {
println(`Found $targetFileName at ${file.getPath()}`)
}
if (file.isDirectory()) {
findFile(targetFileName, file)
}
}
dir.iterate(iterator)
}

def ehomeDir := <file>[interp.getProps()["e.home"]]
findFile("Substituter.class", ehomeDir)

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Walnut/Ordinary Programming/InputOutput