CSC 142/Chapter 6

Chapter 6: File Processing

Sections:

6.1 File reading basics

6.2 Token based processing

6.3 Line based processing

6.4 Advanced file processing

6.5 Case study: zip code lookup

Chapter 3 focused on a scanner for user input. Chapter 6 focuses on a scanner for file reading.

Many intro programming classes see this as a complicated topic, and Java doesn't make it easy. It's awkward, but it's manageable.

We will also explore exceptions relate to file processing.

(Python makes this a dream.)

with open('data.txt','r') as f:
    lines = f.readlines()

Done.

Section 6.1: File Reading Basics

Definitions

Definitions;

• File
• File extension
• binary
• ASCII
• Checked exception
• Throws clause

Material

Examples of the deluge of data available:

• Landmark-project (earthquakes, pollution, baserball, history, weather, etc)
• Gutenberg - see ciphertexts
• ncbi.nlm.nih.gov - biological/genomic data
• IMDB
• Fedstats.gov
• US census
• World bank
• CIA world factbook

Files and file objects:

• Data stored on computer as files
• Files have extensions
• Files can be stored as text, or as binary
• To deal with a file, use a File object
• This provides various methods
• Java API lookup/reference
• Note: we aren't constructing a NEW FILE, we're constructing a new object to represent an existing file

Reading files with scanner:

• Useful methods of File objects: (see list)
• File object is like a pipe: doesn't care much about what kind of fluid flowing thru, or where it comes from
• File object is the delivery system
• You can then pas sthe File object into a scanner
• Again, scanner is like nozzle at end of pipe - does not care much about File type or details of File object, just like nozzle doesn't care about type of fluid
• Need to deal with potential problems; file not there
• Checked exception - like "check" in chess
• Must be dealt with (can't just say, ignore and keep going)
• To handle this exception, put the code that may cause the error into a throws clause

Throws clause: diapers for your code

More in throws/catch clauses:

• You're anticipating a particular kind of mess
• Like an if statement, for exceptions
• If we see this kind of exception, catch it this particular way

public static void main (String[] args) throws FileNotFoundException {
    ...
}

Other exceptions:

• If you reach the end of a file, then ask for more
• NoSuchElementException

A word on the correct way:

Scanner input = new Scanner(new File("hamlet.txt"))

versus the incorrect way:

Scanner input = new Scanner("hamlet.txt")

(Latter would be like saying, a file with the literal contents "hamlet.txt")

NOTE: This is overloading in action (Scanner can take multiple data types)

Section 6.2: Token-Based Processing

Definitions

Definitions:

• Token-baesd processing
• Input cursor
• Consuming input
• File path
• Current directory

Token - a single chunk of letters or character data

• Usually WORDS separated by SPACES
• But could also be NUMBERS separated by COMMAS
• Or, other stuff...

Example: file with 5 numbers

• Read in the first 5 numbers
• Cumulative sum of first 5 numbers
• don't forget the throws

Output:

• Program outputs sum as 337.19999999 instead of 337.2

Utilize scanner functions:

• Scanners have next() and nextDouble() and etc to read next values

Structure of files:

• Computer sees a one-dimensional stream of characters: everything else is our own invention (e.g., line breaks are ignored so computer doesn't even see lines)
• Scanner handles details of, e.g., what to do when it gets to a newline char or a number char

Exceptions from wrong data type:

• InputMismatchException
• Pay close attention to errors: not clear, but provide you with hints

Moving through a file:

• Comptuer sees 1D stream of text
• Can't jump around - like a VCR tape
• So, current location/position is important (input cursor)
• Cursor moves down one char at a time
• Scanner handles details:
  • nextFloat() knows what to look for
  • advances cursor to next word

Scanner object info:

• if we repeatedly call Scanner, it doesn't reset the cursor
• one scanner --> one File, one position
• processTokens(input,2) --> first 2 tokens
• processTokens(input,3) --> processes tokens 3, 4, and 5 (not 1, 2, 3)

etc.

Paths and directories: