Introduction to array: why use
arrays ?
Motivational example
• Problem:
• Write a program that reads in and stores away 5 double
numbers
• After reading in the numbers, the program computes the
average of the numbers
Motivational example (cont.)
• Solution: (Java program)
import java.util.Scanner;
public class Avg1
{
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
double a, b, c, d, e; // Hold 5 numbers
double avg;
System.out.print("Enter a number: ");
a = in.nextDouble();
// Read in number
System.out.print("Enter a number: ");
b = in.nextDouble();
// Read in number
Motivational example (cont.)
System.out.print("Enter a number: ");
c = in.nextDouble();
// Read in number
System.out.print("Enter a number: ");
d = in.nextDouble();
// Read in number
System.out.print("Enter a number: ");
e = in.nextDouble();
// Read in number
avg = (a + b + c + d + e)/5;
System.out.println(avg);
// Print average
}
}
Motivational example (cont.)
import java.util.Scanner;
public class Avg1
See
http://www.tacomacc.edu/home/ebasham/published/
Motivational example (cont.)
• Problem with this solution:
• It is not scalable
The program becomes unmanageably large if the input
size is large
Motivational example (cont.)
• Solution:
• Organize the storage of the information in such a way
that you can access the information through an index:
This kind of organized data is called an array data
structure
Motivational example (cont.)
• Java program using an array:
import java.util.Scanner;
public class Avg2
{
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
double[] a ;
a = new double[5];
elements
double sum, avg;
int i;
// index
// Define an array of 5
Motivational example (cont.)
for ( i = 0; i <= 4; i++ )
{
System.out.print("Enter a number: ");
a[i] = in.nextDouble();
// Read in number
}
/* --------------------------------------------------Use the "running sum" algorithm to compute total
--------------------------------------------------- */
sum = 0.0;
for ( i = 0; i <= 4; i++ )
{
sum = sum + a[i];
}
avg = sum/5;
System.out.println(avg);
}
}
// Print average
Motivational example (cont.)
Example Program: Avg2.java
Motivational example (cont.)
• Note:
• The solution using an array is scalable
• If you want to process 1000000 numbers, just
change the program to:
import java.util.Scanner;
public class Avg2
{
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
double[] a = new double[1000000];
double sum, avg;
int i;
// index
Motivational example (cont.)
for ( i = 0; i <= 999999; i++ )
{
System.out.print("Enter a number: ");
a[i] = in.nextDouble();
// Read in number
}
/* --------------------------------------------------Use the "running sum" algorithm to compute total
--------------------------------------------------- */
sum = 0.0;
for ( i = 0; i <= 999999; i++ )
{
sum = sum + a[i];
}
avg = sum/1000000;
System.out.println(avg);
// Print average
}
}