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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. place item
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. place item
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] = milkgroceryList [1] = breadgroceryList [2] = eggsgroceryList [3] = frozen pizzagroceryList [4] = juicegroceryList [5] = applesgroceryList [6] = orangesgroceryList [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 ] = 5list [ 1 ] = 10list [ 2 ] = -3list [ 3 ] = -7list [ 4 ] = 8list [ 5 ] = 1list [ 6 ] = 9
Declaring & initializing an array
• to declare and initialize an array:
• int list[] = {5,10,-3,-7,8,1,9};
list [ 0 ] = 5list [ 1 ] = 10list [ 2 ] = -3list [ 3 ] = -7list [ 4 ] = 8list [ 5 ] = 1list [ 6 ] = 9
initializing elements of an array
• to initialize an array element:
• list[6] = 9;
list [ 0 ] = 5list [ 1 ] = 10list [ 2 ] = -3list [ 3 ] = -7list [ 4 ] = 8list [ 5 ] = 1list [ 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
1st 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 methodsimport 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.
3 6 1 9
3 6 1 9
3 1 6 9
3 1 6 9
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.
3 6 1 9
3 6 1 9
3 1 6 9
3 1 6 9
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.
3 6 1 9
3 6 1 9
3 1 6 9
3 1 6 9
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.
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