CS 46B - Lecture 5

Cay S. Horstmann

Pre-class reading

• Sections 11.1 - 11.3

Homework 3

• Part C: If you aren't using a loop, it's wrong.

Text I/O

• Use PrintWriter for output, Scanner for input
• Construct with File parameter
  • new PrintWriter(new File("output.txt"))
  • new Scanner(new File("input.txt"), "UTF-8")
  • Be sure to specify the character encoding—Special Topic 11.3, Lab 2
• Can throw exception—add throws FileNotFoundException to method
• Choose between
  • number: in.nextInt()/in.nextDouble()
  • line: in.nextLine()
  • word: in.next()
  • actual word: Call in.useDelimiter("[^A-Za-z]+"), then in.next()
  • character: Call in.useDelimiter(""), then in.next()

Classifiying Characters

• nextLine, next return String
• For that matter, substring returns String
• Should you ever use char?
• In general, no. In almost all cases, people who use char do it wrong
• Instead, use strings of length 1
• Or length 2 for those Unicode code points that require two char values
• Like the cat with heart-shaped eyes from lab 2 😻
• char is useful in the rare case that you need to classify characters
  • isDigit, isLetter, isUpperCase, isLowerCase, isWhiteSpace
  • char ch = in.next().charAt(0); if (Character.isLetter(ch)) ...

Scanning Strings

• Can call nextLine and then analyze string
  
• Sometimes it is easier to make a secondary scanner

• Scanner in = new Scanner(new File("input.txt"));
  String line = in.nextLine();
  Scanner lineScanner = new Scanner(line);
  String countryName = lineScanner.next(); // Read first word
  // Add more words to countryName until number encountered
  while (!lineScanner.hasNextInt())
  {
     countryName = countryName + " " + lineScanner.next();
  }
  int populationValue = lineScanner.nextInt();
  

Converting to Numbers

• Integer.parseInt, Double.parseDouble
• Can apply to substrings of a line
• You may need to trim
  • Integer.parseInt(" 13") throws an exception
• Beware of currency symbols and decimal separators
  • Double.parseDouble("$10.95") throws an exception
  • Integer.parseInt("1,000,000") throws an exception

Lecture 5 Clicker Question 1

An input file contains lines such as

Fred 40
Wilma 28
Mary Ann 30

Consider this code  to process a line:

String line = in.nextLine();
int n = line.lastIndexOf(" ");
String name = line.substring(0, n);
int age = Integer.parseInt(line.substring(n));

What is the problem with this code?

1. The name includes the trailing space
2. The call to Integer.parseInt throws an exception
3. Double names (such as “Mary Ann”) are not stored in name
4. The code won't work with characters such as 😻 that require two char values

Split

• str.split(regex) splits the string into an array
• Example: line.split(" ") splits along spaces
  • If line is "Mary Ann 30", get an array ["Mary", "Ann", "30"]
• What if there is more than one space between tokens?
• Use regular expression line.split("\\s+")
  • \s matches any whitespace
  • + means one or more
• Or split along anything that's not a letter or number: "[^A-Za-z0-9]"
  • You've seen this with Scanner.useDelimiter
  • [A-Z] means all letters from A to Z
  • ^ means “not”
• This is helpful for one of your homeworks

Lecture 5 Clicker Question 2

Complete this program to find the sum of the numbers in the second to last column. Use split.

The input file looks like this:

Abraham Lincoln		6 ft 4 in	193 cm
Lyndon B. Johnson	6 ft 3 1⁄2 in	192 cm
Thomas Jefferson	6 ft 2 1⁄2 in	189 cm
...

What result do you get?

1. 179.81
2. 7732
3. 7842
4. Something else

Caution: Mixing next and nextLine

• Input file

  1729
  Mary Ann
  1730
  Wilma

• Initially, the input contains
  
• Call

  int studentID = in.nextInt(); 

• Now the input contains
  
• If you call nextLine, you don't read Harry reads an empty string!
• Remedy: a call to nextLine after reading the ID:

  int studentID = in.nextInt(); 
  in.nextLine(); // Consume the \n 
  String name = in.nextLine();

Lecture 5 Clicker Question 3

Suppose the input contains the characters 6,995.00 12. What is the value of price and quantity after these statements?

double price = in.nextDouble(); 
int quantity = in.nextInt();

1. price is 6995 and quantity is 12
2. price is "6,995.00" and quantity is 12
3. price is 6, then an exception is thrown because the comma is not a valid part of an integer
4. an exception is thrown because a comma is not a valid part of a floating-point number

Useful Format Specifiers

• out.printf and String.format are your friends
• Alignment
  
  • out.printf("%-10s%10.2f", items[i] + ":", prices[i]);

  • Clams:       19.95
    Lobsters:   109.95

• x prints hexadecimal: String.format("%4x", s.charAt(0));
• Exotic flags
  • %,.2f prints decimal separators 100,000.00
  • %04d prints leading zeroes 0001

Lecture 5 Clicker Question 4

Have another look at the example with the clams. Why can't you use the simpler form

out.printf("%-10s:%10.2f", items[i], prices[i]);

1. A colon is not a valid flag
2. The colon would be at the wrong place
3. You can't format items[i] as a string with %s
4. You can't have a negative width of -10

Command-Line Arguments

• Common to use the command line for automating tasks
• Consider Total.java from Section 11.1
• What if we have a whole bunch of input files?
• If we can specify the arguments on the command line, this would be easy:

  for f in *.txt ; do java Total $f ${f/.txt/.out} ; done

• Get arguments in args array
• Now you finally know what this means:

  public static void main(String[] args)

• Common to use flags that start with -
  • -d for decoding in CaesarCipher.java

Before Next Class

• Read How To 11.1, Worked Example 11.1, Sections 11.4 - 11.5
• Review Lab 3
• Take the quiz