Peter AndreaeComputer Science

Victoria University of Wellington

Copyright: Peter Andreae, Victoria University of Wellington

Dealing with FilesCOMP 102 # 13 2013

© Peter Andreae

COMP 102 13: 2

Menu• Guidelines for designing loops• Files• Scanners• Examples of methods with loops to process files.Reading:• L-D-C:

AdministrationDid you miss the test???Flu shots: free for students. see details on the Forum.

© Peter Andreae

COMP 102 13: 3

Designing loopsIs the number of steps determined at the beginning?

int i = 0;int i = 1;while ( i < number) { while ( i <= number) {

do actions ⟩ do actions ⟩i = i + 1; i++;

} }• Note, can count from 0 or from 1!

Otherwise⟨intialise ⟩while ( ⟨condition-to-do it again ⟩ ) {

⟨body ⟩⟨increment ⟩

}

Shorthand for i = i+1

© Peter Andreae

COMP 102 13: 4

Designing loopsDoes exiting the loop depend on the actions?

if so orset up for the test while ( true ) {while ( test ) { set up for the test

do actions if ( exit-test ) { set up for the next test break;

} }do actions

}• Write out the steps for a couple of iterations

• including the tests to determine when to quit/keep going• Identify the section that is repeated

• preferably starting with the test• Wrap it with a while ( ) { }• Identify the condition for repeating.

© Peter Andreae

COMP 102 13: 5Nested loops: while inside while

public void drawCircles(int rows, int cols) {int diam = 20;int row = 0;while (row < rows) {

int y = 20 +row*diam;int col = 0;while ( col < cols ) {

int x = 50 + col*diam;UI.fillOval(x, y, diam, diam);col = col + 1;

}row = row + 1;

}}

© Peter Andreae

COMP 102 13: 6

Files• The UI text pane window is transient:

• Typing large amounts of input into the text pane is a pain!

• It would be nice to be able to save the output of the program easily.

• Large amounts of text belong in files• How can you write to a file?• How can you read from a file?

• Easy!• Files behave very like the UI text pane:• next, nextInt etc read from a file, (via a Scanner object)

• print, println, printf write to a file (via a PrintStream object)

© Peter Andreae

COMP 102 13: 7

Files: An overview

My Program :int a =??.nextInt(); :??.println(a*5); }

Enter numbers: 40 60 30 50 done

UI Window

UI.nextInt();

UI.println();A real file: “myfile.txt”

File object

Scannerobject

next();

println();PrintStre

amobject

© Peter Andreae

COMP 102 13: 8

Scanner• Scanner: a class in the java that allows a program to

read user input from a source (console, a file, a String, …)

• New concept? Not really!• UI.next() and other next methods are actually a

Scanner working in the background

while(UI.hasNextInt()){double amt = UI.nextDouble()

}

• Most UI next methods work same as Scanner next methods, but a very few subtle differences…see Java documentation

while(scan.hasNextInt()){ double amt = scan.nextDouble();}

Scanner scan = new Scanner(myfile);

© Peter Andreae

COMP 102 13: 9

Reading using Scanner:/** Read lines from a file and print them to UI text pane. */

public void readFile(){File myfile = new File(“input.txt”);Scanner scan = new Scanner(myfile);UI.println(“----------- input.txt --------------”); while (scan.hasNext()){

String line= scan.nextLine();UI.println(line);

}UI.println(“----------- end of input.txt --------------”);

}

• Almost right, but compiler complains!!!• Dealing with files may “raise exceptions”

© Peter Andreae

COMP 102 13: 10

Files: handling exceptionsIf a piece of code might raise an exception:

• Have to enclose it in a try {

… } catch (IOException e) { … }

public void readFile(){File myfile = new File(“input.txt”);try {

Scanner scan = new Scanner(myfile);while (scan.hasNext()){

String line = scan.nextLine();UI.println(line);

}UI.println(“----------- end of input.txt --------------”);

} catch (IOException e) { UI.println(“File failure: ” + e); }

what to do

what to do if it goes wrong

© Peter Andreae

COMP 102 13: 11

Reading from files: example 2/** Finds oldest person in file of ages and names. */public void printOldest(){

try {Scanner scan = new Scanner(new File(“names.txt"));String oldest = "";int maxAge = 0; while (scan.hasNextInt()){

int age = scan.nextInt();String name = scan.nextLine();if (age > maxAge) {

maxAge = age;oldest = name;

}} UI.printf(“Oldest is %s (%d)%n”, oldest, maxAge);

} catch (IOException e) { UI.printf(“File failure: %s%n”, e); }}

no need to prompt user!

33 James Bond25 Helen Clerk73 John F Quay19 pondy

Copes with multi-token names

© Peter Andreae

COMP 102 13: 12

Testing end of fileUnlike UI text pane, can test for end of file:

/** Finds oldest person in file of names and ages. */public void printOldest(){

try {Scanner scan = new Scanner(new File(“names.txt"));String oldest = “”;int maxAge = 0; while (scan.hasNext()){

String name = scan.next();int age = scan.nextInt();if (age > maxAge) {

maxAge = age;oldest = name;

}}UI.println(“Oldest is %s (%d)%n”, oldest, maxAge);

} catch (IOException e) { UI.printf(“File failure: %s%n”, e); }}

False when at end of file

name first; age secondNote: name must be just one token!!

James 33 Helen 25 John73 pondy 19

© Peter Andreae

COMP 102 13: 13

A common simple pattern• File with one entity per line,

described by multiple values:

while (sc.hasNext() ){String type = sc.next();double cost = sc.nextDouble();int wheels = sc.nextInt();String colour = sc.next();String make = sc.next()if (wheels > 4) {

….}else {

…}

}

bicycle 1025 2 green Gianttruck 120000 18 black Isuzucar 26495 4 red Toyota…

Read all the valuesinto variables

process the values inthe variables