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Java Arrays

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Transcript

  • 1. Java Arrays Jussi Pohjolainen Tampere University of Applied Sciences
  • 2. Types
    • Java has two kind of types
      • Primitive Types
        • int, byte, short, long, double, float, boolean, char
      • Class Types
        • Everything else, for example
          • String, Scanner, Arrays , Vector, JButton, JCheckBox
  • 3. Problem?
    • Implement an application that will calculate 100 students exam average.
    • Variables needed?
      • int studentA;
      • int studentB;
      • int studentC;
      • int studentD;
      • ...
  • 4. Several Variables at Once?
    • Array comes to the rescue!
    • Just a list of variables
    • Declare the array
      • int [] array;
    • Initialize the array and set it's size
      • array = new array[3];
    • Store values into array
      • array[0] = 2;
      • array[1] = 3;
      • array[2] = 7;
  • 5. Array
    • int [] myarray = new int[3];
    • myarray[0] = 1;
    • myarray[1] = 12;
    • myarray[2] = 88;
    • System.out.println(myarray[0]);
    • System.out.println(myarray[1]);
    • System.out.println(myarray[2]);
  • 6. myarray index length = 3 0 1 2 1 12 88
  • 7. Indexes
    • The values can be retrieved from the array using index.
    • The first value is found from index 0 and the last from length – 1
      • int [] myarray = new int[LENGTH];
      • myarray[0] = 1; // first one
      • myarray[LENGTH-1] = 22; // last one
  • 8. Initializing the Array with {}
    • int [] myarray = new int[3];
    • myarray[0] = 2;
    • myarray[1] = 8;
    • myarray[2] = 12;
    • <=>
    • int [] myarray = {2,8,12};
  • 9. Common Problems
    • int [] myarray1 = {2,8,12};
    • int [] myarray2 = new int[3];
    • // Output?
    • System.out.println(myarray1[3]);
    • // Output?
    • System.out.println(myarray2[0]));
    • // Output?
    • System.out.println(myarray1);
  • 10. Example 1
    • int [] myarray = {2,8,12};
    • int i = 0;
    • while(i < 3)
    • {
    • System.out.println(myarray[i]);
    • i++;
    • }
  • 11. Example 2
    • int [] myarray = {2,8,12};
    • int i = 0;
    • while(i < myarray.length )
    • {
    • System.out.println(myarray[i]);
    • i++;
    • }
  • 12. Example 3
    • int [] myarray = {2,8,12};
    • for(int i = 0; i < myarray.length; i++ )
    • {
    • System.out.println(myarray[i]);
    • }
  • 13. Example 3
    • int [] myarray = new int[3];
    • for(int i = 0; i < myarray.length ; i++ )
    • {
    • myarray[i] = myscanner.nextInt();
    • }
    • for(int i = myarray.length-1; i >= 0 ; i++ )
    • {
    • System.out.println(myarray[i]);
    • }
  • 14. COMMAND LINE ARGUMENTS
  • 15. Command Line Argument?
    • public class CommandLine {
    • public static void main( String [] args ) {
    • }
    • }
    Declaration of an array!
  • 16. Command Line Argument?
    • > java CommandLine Hello World
    • Hello
    • World
    • public class CommandLine {
    • public static void main( String [] args ) {
        • System.out.println(args[0]);
        • System.out.println(args[1]);
    • }
    • }
  • 17. About Command Line Argument
    • Command line argument is user input from the command line
    • Argument array is initialized automatically for you (size and content)
  • 18. What can go wrong?
    • > java CommandLine 10 5
    • 10 / 5 = 2
    • public class CommandLine {
    • public static void main( String [] args ) {
    • int a = Integer.parseInt(args[0]);
    • int b = Integer.parseInt(args[1]);
    • int result = a / b;
    • System.out.print(&quot;&quot; + a + &quot; / &quot; + b + &quot; = &quot;);
    • System.out.println(result);
    • }
    • }