CS 111A Lecture Notes - Arrays - Chapter 6

Arrays (One Dimensional)

• Array - a series of variables that share a name and are referred to using their name and a number. e.g.
    int[] score = new int[3];
  declares 3 integer variables: score[0], score[1], and score[2].
• Array declarations can be written with the square braces next to the datatype or next to the variable name. And the declaration of the array variable can be separate from the construction of the array:
    int score[]; // declares the array variable, allocating just 1 reference variable
  score = new int[3]; // constructs the actual array, allocating 3 integer variables
• Individual variables in an array can be called indexed variables, subscripted variables, or my favorite: elements
• Number in brackets of array construction is its size, and it can be any integer value greater than 0.
• Number in brackets when the array is used is the index (also called subscript). It can be any integer value greater than or equal 0 and less than the array size (length). When indexed in this way, we are referring to an individual variable (element) in the array.
• Arrays can be of any base type, such as
    char[] card;
  String entries[];
• Arrays are built into the Java language to act somewhat like a class, and we use reference variables to refer to them. But arrays don't have methods we use in the normal way. Arrays do have, however, fields such as length (length is the number of elements declared for the array).
• If you refer to an index value out of range, say score[3], that will cause a run-time error. Array accesses are bounds-checked at run-time in Java. So unlike in C++, the language won't let you get away with accessing an array out of bounds - it'll throw an exception and crash the program.
• You can initialize the values of an array at the same time that you declare and construct it, e.g.:
    int[] score = {100, 95, 97};
• After an array has been declared and constructed, array assignment must be done one element at a time. So you can't write
  "score = {100, 95, 97};" on a line by itself, or
  "score = 0;"
• Example: BasicArray.java
• In-class exercise: Use an array of strings to store names of several people, input by the user. For an extra challenge, output the longest name at the end.
• NameArray.java Solution to above in-class exercise

  Arrays and Methods

• Array elements can be passed as arguments to functions just like individual variables.
• Example: ArrayElementArguments.java - Demonstrates how to pass an element of an array into a method.
• An entire array (array parameter) can also be passed as an argument to a function, if that function is declared with a corresponding array parameter. e.g.
    void get_hours(int[] hours);
• We use reference variables to access arrays, so the variable you use to refer to the array is really just a pointer to the array's contents. Therefore, when a whole array is passed, its contents may be changed.
• Notice also that any size array may be passed in for "hours" above.
• ArrayParam.java - Demonstrates how array parameters work
• ReturnArray.java - shows how to create an array in a function and return it
• In-class exercise: write a method to input three numbers from the user and return them in an array.
• MethodToInputArray.java Solution to above in-class exercise
• SearchGroup.java - Sequential search example, searches an array of student IDs to see if user-entered ID is in the array.

  Selection Sort

• There are many different sorting algorithms, which you'll study more if you continue on to CS 111C, but today we'll look at one algorithm for sorting a sequence of values. The algorithm is called Selection Sort. It involves finding the smallest item in the sequence, then putting it at the beginning. Then finding the next smallest and putting it second, etc.
• ArrayExp.java - Inputs and sorts an array of ints.

  Command-Line Arguments (Section 8.3)

• Command-Line Arguments - information that is passed into the program as soon as the program starts running. This information is specified on the command-line when the program is executed, right after the name of the program itself.
• The command-line arguments are passed into the main method as an array of Strings. The first element of the array is the first string after the program name, the second element is the second string, etc. These strings are separated by spaces on the command-line. If there are no command-line arguments, then the array of Strings is null.
• SortCommandLine.java Command-line argument variation of ArrayExp.java

  For-Each Loop

• There is a variation on the for-loop, called the for-each loop. It allows you to loop through all the elements in an array directly, without using an index. Here's an example:
• At the end of our first array example, BasicArray.java, we had the following code:

          System.out.println("You entered:");
          for (i=0; i < size; i++)
            System.out.print(inputNums[i] + " ");
    

• If we write that using a for-each loop, it becomes:

          System.out.println("You entered:");
          for (double num : inputNums)
            System.out.print(num + " ");
    

• A for-each loop cannot be used to modify array elements, because the looping variable gets a copy of the array element. Just use a for-each loop if you want to use each element in an array, without changing it.
• In-class exercise: use command-line to specify a list of names, search that list to find a name. Use a for-each loop if you can.
• SearchName.java Solution to above in-class exercise

Multidimensional Arrays

• A muti-dimensional array is really an array of arrays. You can also think of it as a matrix of values.
• For example, you might use a two-dimensional array of ints to store the color value for each pixel on your screen (although this the real screen color is stored as three values: brightness of red, green, and blue) :
    int[][] pixels = new int[480][640];
  
• To set the pixel in the position (3, 5) (meaning row #3, column #5) to the value 7 you would type:
    pixels[3][5] = 7;
  
• To access the one-dimensional array of ints on row number 5, use: pixels[5]
• For a function to accept an entire multi-dimensional array as a parameter, that parameter must be a 2-dimensional array type. So to clear the screen:

      void clearScreen(int[][] p)
      {
        int x, y;
  
        for (y=0; y < p.length; y++)
        {
          for (x=0; x < p[y].length; x++)
            p[y][x] = 0;
        }
      }
    

  We would call the above function with the following function call:
    clearScreen(pixels);
  
• QuarterlySales.java - a 2-Dimensional array example
• Here is a complete program using the above pixel example
• JaggedArray.java - Shows how a 2-D array can be jagged: have different numbers of columns for each row. Inputs test scores for courses - each course can have a different number of tests.

ArrayLists

• ArrayList is a pre-defined Java class you can use like an array. It has built-in features to make it easier to work with arrays.

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