lecture-notes.md

CSE 142

Week 7

Lecture Notes

Sample File-Processing Program

File Processing

• Scanner is the main tool to use for file processing
• When doing both token-based processing and line-based processing use two different Scanner objects

Token-based Processing

• Processes the data in tokens using nextInt(), nextDouble(), and next()
• Skips past any newline characters
• Should not be used if your input is line-based, because token-based processing ignores the line breaks and looks only at the tokens

Line-based Processing

• Process data by line using nextLine()

• When doing line-based processing you often use a while loop, because you are unsure of the number of lines you will read in; an example method is shown below. Note that the method below returns the empty string if nothing is found, this is a good 142 solution.

  // searches for and returns the next line of the given input that contains
  // the given phrase; returns an empty string if not found
  public static String find(Scanner input, String phrase) {
      while (input.hasNextLine()) {
          String line = input.nextLine();
          if (line.toLowerCase().contains(phrase)) {
              return line;
          }
      }
      return "";
  }

Hybrid Approach

• Use line-based processing on the file, but then use token-based processing for the individual lines of the file

• Do this by passing the line itself (a String) into a new Scanner object to use token-based processing

• An example of this approach is shown below

  public static void print(String line) {
      Scanner data = new Scanner(line);
      int rank = data.nextInt();
      int votes = data.nextInt();
      double rating = data.nextDouble();
      System.out.print(rank + "\t" + rating + "\t" + votes + "\t");
      while (data.hasNext()) {
          System.out.print(data.next() + " ");
      }
      System.out.println();
  }

Arrays

• array: an object that stores many values of the same type

• element: one value in an array

• index: a zero-based integer to access an element from an array

  index	0	1	2	3	4	5	6	7	8	9
  value	12	49	-2	26	5	17	-6	84	72	3

  In the array above, 12 is stored as element 0, 5 is stored as element 4, and 3 is stored as element 9

• To declare and initialize an array use type[] name = new type[length];

  int[] numbers = new int[10]; // creates a new int array to hold 10 values; all starting at zero

• Different array types have different default values; the default values for common types are shown in the table below

  Type	Default Value
  int	0
  double	0.0
  boolean	false
  String	null

• To access elements use name[index]

• To modify elements use name[index] = value

• Arrays have random access which means you can access any element in the array without having to loop through the entire array

• The legal indexes of an array are between 0 and the array's length - 1. If you read or write outside of this range you will generate an ArrayIndexOutOfBoundsException

• Arrays and for loops are best friends

• An array's length field stores the number of elements, name.length, notice that there are no parens after length because it is a field and not a method

Code Examples

// print out the elements of an array
for (int i = 0; i < numbers.length; i++) {
  System.out.print(numbers[i] + " ");
}
System.out.println();

// multiply every value in an array by 2
for (int i = 0; i < numbers.length; i++) {
  numbers[i] = 2 * i;
}

// count how many values are above 5
int count = 0;
for (int i = 0; i < numbers.length; i++) {
  if (numbers[i] > 5) {
    count++;
  }
}

// using an array to count the number of each digit in a number
int num = 229231007;
int[] counts = new int[10];
while (num > 0) {
  int digit = num % 10;
  counts[digit]++;
  num = num / 10;
}

// print a histogram for the values in an array
for (int i = 0; i < numbers.length; i++) {
  System.out.print(i + ": ");
  for (int j = 0; j < counts[i]; j++) {
    System.out.print("*");
  }
  System.out.println();
}

// histogram with DrawingPanel
// Uses a DrawingPanel to draw the histogram
public static void draw(int[] count) {
    DrawingPanel p = new DrawingPanel(count.length * 6 + 12, 500);
    Graphics g = p.getGraphics();
    g.setColor(Color.BLUE);
    for (int i = 0; i < count.length; i++) {
        g.drawLine(i * 6 + 6, 475, i * 6 + 6, 475 - 10 * count[i]);
    }
}