Transcript Arrays - New York University

Introduction to Computers and
Programming
Lecture 16: Arrays (cont)
Professor: Evan Korth
New York University
Road Map
• More array examples.
• Passing arrays to methods
• Reading: Chapter 5, Section 5.4
• Reading: Chapter 6, Section 6.4
review
•
•
•
•
What is an array?
What is a subscript?
What is another term for a subscript?
Given an array of size n, what is the valid range of
subscripts?
• How can you obtain the size of an array in Java?
• What happens if you try to access an element
outside the valid range of subscripts?
• What are the default values given to array
elements in Java?
7.4
Examples Using Arrays (Cont.)
• Histogram
• Using the elements of an array as counters
– Use a series of counter variables to summarize data
 2003 Prentice Hall, Inc. All rights reserved.
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Outline
// Fig. 7.7: RollDie.java
// Roll a six-sided die 6000 times.
import javax.swing.*;
public class RollDie {
public static void main( String args[] )
{
int frequency[] = new int[ 7 ];
RollDie.java
Declare frequency
as
array of 7 ints
Line 9
Generate 6000 random
Declare frequency
integers in range 1-6
as array of 7 ints
// roll die 6000 times; use die value as frequency index
for ( int roll = 1; roll <= 6000; roll++ )
++frequency[ 1 + ( int ) ( Math.random() * 6 ) ];
String output = "Face\tFrequency";
Increment
frequency values at
index associated with random number
String output
// append frequencies to
for ( int face = 1; face < frequency.length; face++ )
output += "\n" + face + "\t" + frequency[ face ];
System.out.println(”Rolling a Die 6,00 times” );
Lines 12-13
Generate 6000
random integers in
range 1-6
Line 13
Increment
frequency values at
index associated with
random number
System.out.println ( output );
System.exit( 0 );
} // end main
} // end class RollDie
 2003 Prentice Hall, Inc.
All rights reserved.
7.4
Examples Using Arrays (Cont.)
• Using arrays to analyze survey results
– 40 students rate the quality of food
• 1-10 Rating scale: 1 mean awful, 10 means excellent
– Place 40 responses in array of integers
– Summarize results
 2003 Prentice Hall, Inc. All rights reserved.
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// Fig. 7.8: StudentPoll.java
// Student poll program.
import javax.swing.*;
public class StudentPoll {
Outline
Declare responses as
Declare
frequency as array of 11
array to store
40 responses
StudentPoll.jav
int and ignore the first
element
a
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 ];
Lines 9-11
Declare responses
as array to store 40
responses
Line 12
Declare frequency
as array of 11 int
For each response, increment
and ignore the first
frequency values at index
element
String output = "Rating\tFrequency\n";
associated with that response
// append frequencies to String output
Lines 16-17
for ( int rating = 1; rating < frequency.length; rating++ )
For each response,
output += rating + "\t" + frequency[ rating ] + "\n";
increment
JOpSystem.out.println( output );
frequency values at
System.exit( 0 );
index associated with
that response
// for each answer, select responses element and use that value
// as frequency index to determine element to increment
for ( int answer = 0; answer < responses.length; answer++ )
++frequency[ responses[ answer ] ];
} // end main
} // end class StudentPoll
 2003 Prentice Hall, Inc.
All rights reserved.
Outline
StudentPoll.jav
a
 2003 Prentice Hall, Inc.
All rights reserved.
7.4
Examples Using Arrays (Cont.)
• Some additional points
– When looping through an array
• Index should never go below 0
• Index should be less than total number of array elements
– When invalid array reference occurs
• Java generates ArrayIndexOutOfBoundsException
– Chapter 15 discusses exception handling
 2003 Prentice Hall, Inc. All rights reserved.
7.6
Passing Arrays to Methods
• To pass array argument to a method
– Specify array name without brackets
• Array hourlyTemperatures is declared as
int hourlyTemperatures [] = new int[24];
• The method call
modifyArray( hourlyTemperatures );
• Passes array hourlyTemperatures to method
modifyArray
 2003 Prentice Hall, Inc. All rights reserved.
Passing arrays to methods (cont)
• In the method header, use similar syntax as that
for array declaration:
public static void modifyArray(int array [] )
or
public static void modifyArray(int [] array
)
7.5
References and Reference Parameters
• Two ways to pass arguments to methods
– Pass-by-value
• Copy of argument’s value is passed to called method
• In Java, every primitive is pass-by-value
– Pass-by-reference
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•
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Caller gives called method direct access to caller’s data
Called method can manipulate this data
Improved performance over pass-by-value
In Java, every object is simulated pass-by-reference
– In Java, arrays are objects
• Therefore, arrays are passed to methods by reference
– Technically, we are using pass by value but the “value” we
are passing is a reference to the array.
 2003 Prentice Hall, Inc. All rights reserved.