11/15: Ch. 7: Arrays What is an array? Declaring & allocating arrays Sorting & searching arrays

Arrays: what are they? a series of elements; a list. grocery list could be an array: –milk –bread –eggs –frozen pizza –juice –apples –oranges –Ramen noodles

Arrays: what are the positions? every item has a place; a position number. placeitem 1. milk 2. bread 3. eggs 4. frozen pizza 5. juice 6. apples 7. oranges 8. Ramen noodles

Arrays: how are they numbered? Java begins numbering at 0. placeitem 0. milk 1. bread 2. eggs 3. frozen pizza 4. juice 5. apples 6. oranges 7. Ramen noodles

Arrays: how are they named? Java names the list and refers to the place of the items in square brackets after the list name. groceryList [0] =milk groceryList [1]=bread groceryList [2] =eggs groceryList [3] =frozen pizza groceryList [4] =juice groceryList [5] =apples groceryList [6] =oranges groceryList [7] =Ramen noodles

Arrays: what are the elements? The individual items are called elements. The reference numbers are position numbers,or subscripts. groceryList [0] =milk groceryList [1]=bread groceryList [2] =eggs groceryList [3] =frozen pizza groceryList [4] =juice groceryList [5] =apples groceryList [6] =oranges groceryList [7] =Ramen noodles elements subscripts

Arrays: about the position numbers Subscripts must be integers or integer expressions. groceryList [0] =milk groceryList [1]=bread groceryList [2] =eggs groceryList [3] =frozen pizza groceryList [4] =juice groceryList [5] =apples groceryList [6] =oranges groceryList [7] =Ramen noodles

Arrays: function examples in Java an array of integers: list [ 0 ] = 5 list [ 1 ] = 10 list [ 2 ] = -3 list [ 3 ] = -7 list [ 4 ] = 8 list [ 5 ] = 1 list [ 6 ] = 9 list.length = 7 list [0] + list [2] = 2 list [1 + 3] = 8

Declaring and allocating an array to declare and allocate an array: int list[]; list = new int[7]; or int list[] = new int[7]; Another example: String args[] = new String[5]; list [ 0 ] = 5 list [ 1 ] = 10 list [ 2 ] = -3 list [ 3 ] = -7 list [ 4 ] = 8 list [ 5 ] = 1 list [ 6 ] = 9

Declaring & initializing an array to declare and initialize an array: int list[] = {5,10,-3,-7,8,1,9}; list [ 0 ] = 5 list [ 1 ] = 10 list [ 2 ] = -3 list [ 3 ] = -7 list [ 4 ] = 8 list [ 5 ] = 1 list [ 6 ] = 9

initializing elements of an array to initialize an array element: list[6] = 9; list [ 0 ] = 5 list [ 1 ] = 10 list [ 2 ] = -3 list [ 3 ] = -7 list [ 4 ] = 8 list [ 5 ] = 1 list [ 6 ] = 9

StudentPoll.java: pt. 1 //fig. 7.7: StudentPoll.java import javax.swing.*; public class StudentPoll { public static void main ( String args[] ) { int responses[] = { 1, 2, 6, 4, 8, 5, 9, 7, 8,10, 1, 6, 3, 8, 6,10, 3, 8, 2, 7, 6, 5, 7, 6, 8, 6, 7, 5, 6, 6, 5, 6, 7, 5, 6, 4, 8, 6, 8,10}; int frequency[] = new int [ 11 ]; String output = "";

About StudentPoll.java: pt. 2 for ( int a = 0 ; a < responses.length; a++ ) ++frequency [ responses [ a ] ]; output += "Rating\tFrequency\n"; for ( int r = 1; r < frequency.length ; r++ ) output += r + "\t" + frequency[r] + "\n";

About StudentPoll.java: pt. 3 JTextArea outputArea = new JTextArea (11,10); outputArea.setText( output ); JOptionPane.showMessageDialog ( null, outputArea, "Student Poll Program", JOptionPane.INFORMATION_MESSAGE ); System.exit ( 0 ); }

About “.length” length is actually an instance variable, indicating the number of elements in the array. We don’t have to declare it separately – it’s an automatic part of an array that we declare.

Passing arrays to methods Two ways: –passing the entire array as a whole –passing individual elements of the array The method that modifies the array must have an appropriate input type (parameter). Accepting an array: public void x ( int a[] ) Accepting elements: public void y ( int c ) public void z ( String d )

Getting more experience with arrays Pg 273: InitArray.java – declaring & allocating Pg 274: InitArray.java – initializing to values Pg 275: InitArray.java – using a for loop to set values Pg 276: SumArray.java – using a for loop to add up elements of an array

1 st Programs of the day pg. 278: StudentPoll.java After getting the program to run, modify the program to allow the user to input the array elements. –use a JOptionPane.showInputDialog to get the data from the user, and assign the CONVERTED value you get from there into the next array position. Use a for loop to provide the repetition and movement in the array position. Pg. 284 PassArray.java

Sorting and Searching Arrays Look at PassArray.java Sorting arrays: the bubble sort method Searching arrays: the linear search Searching arrays: the binary search

PassArray.java -- pt. 1 //Fig 7.10: PassArray.java //Passing arrays and individual elements to methods import java.awt.Container; import javax.swing.*; public class PassArray extends JApplet { JTextArea outputArea; String output; public void init() { outputArea = new JTextArea(); Container c = getContentPane(); c.add( outputArea );

PassArray.java -- pt. 2 int a[] = { 1, 2, 3, 4, 5 }; output = "Effects of passing entire " + "array call-by-reference:\n" + "The values of the original array are:\n"; for ( int i = 0; i < a.length ; i++ ) output += " " + a[ i ]; modifyArray ( a ); //passing the whole array output+="\n\nValues of the modified array are:\n";

PassArray.java -- pt. 3 for ( int i = 0; i < a.length ; i++ ) output += " " + a[ i ]; output += "\n\nEffects of passing array " + "element call-by-value:\n" + "a[3] before modifyElement: " + a[ 3 ]; modifyElement ( a [ 3 ] ); output += "\na[3] after modifyElement: " + a [ 3 ]; outputArea.setText ( output ); }

PassArray.java -- pt. 4 public void modifyArray ( int b[] ) { for ( int j = 0 ; j < b.length; j++ ) b [ j ] *= 2; } public void modifyElement ( int e ) { e *= 2; }

Sorting Arrays using Bubble Sort Reorganizing an array in some order (low to high, etc.) Bubble Sort compares two values, switches them in the array positions if appropriate, and checks the next two values

Sorting Arrays using Bubble Sort In this case, 3 & 6 are compared, and NOT switched. Then, 6 & 1 are compared, then switched. Then 6 & 9 are compared and NOT switched. This is ONLY ONE PASS through the array

Sorting Arrays using Bubble Sort Core of sorting: an if structure. This is nested inside a for loop to look at each pair of values in the array. This loop in nested inside another loop to make multiple passes through the array. Look at SortThem & its source code.source code

Searching Arrays: Linear Search How do you find a particular element value in an array? One way: a linear search. The core structure: an if statement in a for loop. The for loop gives movement through the array The if structure looks for a matching value in the elements.

Searching Arrays: Linear Search The for loop gives movement through the array The if structure looks for a matching value in the elements. for ( int n = 0 ; n < array.length ; n++ ) { if ( array [ n ] == key ) return n ; }

Searching Arrays: Binary Sort Go to the middle of the array. See if the number you are looking for is higher or lower, and go to the middle of that side. Repeat until found. Note that the array must already be sorted!

Searching Arrays: Linear vs. Binary A linear search works well for smaller arrays, and is simpler to understand and troubleshoot. A binary search is better for a large array. It is more efficient in its searching.

Program of the Day: BinarySearch pg. 291 BinarySearch.java Once you get it to work, figure out how it works. Next time: review searching and learn about multiple-subscripted arrays.