	CS 535 Object-Oriented Programming Fall Semester, 2003 Streams & Files
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Contents of Doc 10, Streams & Files

References VisualWorks Application Developer Guide, doc/vwadg.pdf in the VisualWorks installation. Chapter 20 Files.

http://www.iam.unibe.ch/~ducasse/WebPages/Smalltalk/ST00_01.pdf

Reading VisualWorks Application Developer Guide, doc/vwadg.pdf in the VisualWorks installation. Chapter 20 Files.


Doc 10, Streams & Files Slide # 2

Streams

Iterate or traverse over

Sequenceable Collections
File contents

Maintains pointer to current position in collection

Basic Streams

Stream
   PositionableStream
      ReadStream
      WriteStream
         ReadWriteStream


Doc 10, Streams & Files Slide # 3

Stream Methods

next	Returns the next element
next: n	Returns the n next elements
nextPut: anElement	Inserts anElement at next position
nextPutAll: aCollection	Inserts collection elements starting at the next position
contents	Returns all the elements
flush	Write any unwritten information
atEnd	true if at the end of the collection
cr space tab crtab	Write the specified white space
print: anObject	Print anObject on the stream

PeekableStream Methods

skip: n	Increases the position by n
skipTo: anElement	Increases the position to after anElement
upToSeparator	Return contents up to a separator, skip over separtor
reset	Set position to 0
peek	Return next element, position not changed
peekFor: anObject	Return true if next element = anObject


Doc 10, Streams & Files Slide # 4

Examples

WriteStream

Write streams must be created on a collection
The stream grows the underlying collection if needed

String Example

| x |
x := WriteStream on: String new.
x
   nextPut: $A;
   nextPutAll: ' Cat in the Hat';
   nextPutAll: ' Comes Back';
   contents

Result

'A Cat in the Hat Comes Back'

Array Example

x := WriteStream on: Array new.
x
   nextPut: 5;
   nextPut: 'cat';
   nextPut: $a.
x contents

Result

#(5 'cat' $a)
Doc 10, Streams & Files Slide # 5

Collection Backing the Stream
A stream backed by a String

   WriteStream on: String new.

A stream backed by an Array

   An Array backs the write stream

Backing collection may limit the type of elements added

x := WriteStream on: String new.
x nextPut: 56.         “Runtime error, must be character”
x nextPut: 56 printString.   “Error, string is not a character”
x print: 56               “OK”
x nextPutAll: 56 printString      “OK”


Doc 10, Streams & Files Slide # 6

nextPut: & nextPutAll:
nextPut: adds one element to the stream

nextPutAll:

Argument must be a collection
Elements of the argument are added one at a time to the collection


Doc 10, Streams & Files Slide # 7

Repositioning of the stream

(WriteStream on: String new)
   nextPutAll: 'Cat in the Hat';
   position: 4;
   nextPutAll: 'Comes Back';
   contents

Result

 'Cat Comes Back'


Doc 10, Streams & Files Slide # 8

WriteStream Examples Continued
You can start with a large collection to avoid the need to grow

But don't worry about it

(WriteStream on: (String new:40))
   nextPutAll: 'Cat in the Hat';
   nextPutAll: ' Comes Back';
   contents

Result

 'Cat in the Hat Comes Back'

Streams can be on any sequential collection

(WriteStream on: Array new)
   nextPut: $A;
   nextPutAll: 'Cat-in-the-Hat';
   nextPutAll: 'Comes-Back';
   contents

Result

 #($A $C $a $t $- $i $n $- $t $h $e $- $H $a $t $C $o $m $e $s $- $B $a $c $k)


Doc 10, Streams & Files Slide # 9

WriteStream Examples Continued
Note the difference between adding a string and an array containing a string

(WriteStream on: Array new)
   nextPut: $A;
   nextPutAll: #('Cat in the Hat');
   nextPutAll: #('Comes Back');
   contents

Result

 #($A 'Cat in the Hat' 'Comes Back')

(WriteStream on: Array new)
   nextPut: $A;
   nextPutAll: 'Cat-in-the-Hat';
   nextPutAll: 'Comes-Back';
   contents

Result

 #($A $C $a $t $- $i $n $- $t $h $e $- $H $a $t $C $o $m $e $s $- $B $a $c $k)


Doc 10, Streams & Files Slide # 10

ReadStream Examples

| x |
x := ReadStream on: 'Cat-in-the-Hat-Comes-Back'.

Transcript	"Result in Transcript"
print: x next; cr;	C
print: x peek; cr;	a
print: x next; cr;	a
show: (x upTo: $e); cr;	t-in-th
show: (x upToAll: 'Comes'); cr;	-Hat-
show: x upToEnd; cr;	Comes-Back
show: x contents	Cat-in-the-Hat-Comes-Back

upTo: and upToAll:

Return up to the indicated element (collection)
upTo: sets the position after the element ($e)
upToAll: sets the position at the start of the indicated collection
Doc 10, Streams & Files Slide # 11

ReadStream on an Array

| x |
x := ReadStream on: 
   #('Cat' 'in' 'the' 'Hat' 'Comes' 'Back' 'Again' 'by' 'Zeus').

Transcript	"Result in Transcript"
show: x next; cr;	Cat
show: x peek; cr;	in
show: x next; cr;	in
show: (x upTo: 'Comes'); cr;	#('the' 'Hat')
show: (x upToAll: #( 'Again' 'by')); cr;	#('Back')
show: x upToEnd	#('Again' 'by' 'Zeus')


Doc 10, Streams & Files Slide # 12

Comment on ReadStream Example
The elements returned by the stream are elements in the underlying collection

upTo: requires elements of the underlying collection

upToAll: requires a collection of elements of the underlying collection

next returns an element of the underlying stream

ReadStream on: 'Cat-in-the-Hat-Comes-Back'.

The read stream is on a string
next returns a character
upToAll: requires a collection of characters

ReadStream on:  #('Cat' 'in' 'the' 'Hat' 'Comes' 'Back' 'Again' ).

The read stream is on an array of strings
next returns strings
upToAll: requires a collection of strings


Doc 10, Streams & Files Slide # 13

Files

Filename

Main file class

Example

| name file fileWrite fileRead|
name := 'sampleFile'.
file := name asFilename.
fileWrite := file writeStream.
fileWrite 
   nextPutAll: 'Hello world';
   nextPutAll: 'How are you?';
   cr;
   close.
fileRead := file readStream.
Transcript show: fileRead contents.
fileRead close.
fileAppend := file appendStream.
fileAppend 
   nextPutAll: 'I am well';
   cr;
   close.
Transcript show: file contentsOfEntireFile
Doc 10, Streams & Files Slide # 14

Creating
Filename named: 'filename'

'filename' asFilename

Both create a Filename object on a file
The filename string is a file in the current directory or Full path to the file


Doc 10, Streams & Files Slide # 15

Writing to a File
Filename>>writeStream

Opens a write stream on the file
If file does not exits create the file
If file does exist erase current contents

Filename>>appendStream

Returns a write stream on the file
If file does not exits create the file
If file does exist the stream appends to the contents

Filename>>readStream

Returns a read stream on the file
File must exist
Stream reads from the beginning of the file


Doc 10, Streams & Files Slide # 16

Closing file streams
Always close streams on files

If you do not close the stream, the VM keeps the file open


Doc 10, Streams & Files Slide # 17

Exceptions and Closing files
What happens when an exception is raised before you close a file?

| name file fileWrite |
name := 'sampleFile'.
file := name asFilename.
fileWrite := file writeStream.
1 /0.
fileWrite 
   nextPutAll: 'Hello world';
   nextPutAll: 'How are you?';
   cr;
   close.


Doc 10, Streams & Files Slide # 18

Using Ensure to close files

| name file fileWrite |
name := 'sampleFile'.
file := name asFilename.
[fileWrite := file writeStream.
1 /0.
fileWrite 
   nextPutAll: 'Hello world';
   nextPutAll: 'How are you?';
   cr.]
   ensure: [fileWrite close].


Doc 10, Streams & Files Slide # 19

Some File Operations in Filename

isDirectory	Returns true if Filename object is a directory
fileSize	Returns size of the file represented by filename object
delete	Delete the file or directory represented by filename object
directoryContents	Returns the contents of a filename object that represents a directory
makeDirectory	Make the filename object a directory.

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