created 05/31/03; revised 07/07/07


Chapter 47 Programming Exercises


Exercise 1 — Sum of Even, Odd, and All Elements

Complete the following program so that it computes the sum of all the elements of the array, the sum of the even elements, and the sum of the odd elements. Assume that all the numbers are zero or positive. Even integers are those for which N%2 is zero.

import java.io.* ;

class ThreeSums
{

  public static void main ( String[] args ) 
  {
    int[] data = {3, 2, 5, 7, 9, 12, 97, 24, 54};
    
    // declare and initialize three sums
    
    
    // compute the sums
    for ( int index=0; index < data.length; index++)
    {
    }
      
    // write out the three sums
    System.out.println(  );

  }
}      

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Exercise 2 — Two Largest Elements

Complete the following program so that it computes and writes out the two largest elements in the array.

import java.io.* ;

class TwoLargest
{

  public static void main ( String[] args )  
  {
    int[] data = {3, 1, 5, 7, 4, 12, -3, 8, -2};
    
    // declare and initialize variables for the two largest
    
    
    // compute the two largest
    for ( int index= ; index < data.length; index++)
    {
    }
      
    // write out the two largest
    System.out.println(  );

  }
}      

Change the program to use an enhanced for loop, if you want.

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Exercise 3 — Closest to Zero

Complete the following program so that it computes and writes out the element in the array that is closest to zero.

import java.io.* ;

class NearlyZero
{

  public static void main ( String[] args )
  {
    int[] data = {3, 1, 5, 7, 4, 12, -3, 8, -2};
    
    // declare and initialize variables
    
    
    // find the element nearest to zero
    for (  )
    {
    }
      
    // write out the element nearest to zero
    System.out.println(  );

  }
}      

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Exercise 4 — Reversal of Elements

Complete the following program so that it reverses the order of the values in data, then prints it out.

In the first version of the program there is only one array and its values are reversed with some fairly tricky programming.

import java.io.* ;

class ReverserVersion1
{

  public static void main ( String[] args ) 
  {
    int[] data = {1,2,3,4,5,6,7,8,9,10,11,12,13,14};
    
    // reverse the data
    for ( int j=0; j < be careful here; j++)
    {

    }
      
    // write out the new data
    for ( int j=0; j < data.length; j++)
    {

    }

  }
}      

Now write another program that uses two arrays. The first array data is not changed. The second array result gets the elements of data in reversed order.

import java.io.* ;

class ReverserVersion2
{

  public static void main ( String[] args )  
  {
    int[] data = {1,2,3,4,5,6,7,8,9,10,11,12,13,14};
    int[] result =
    
    // copy the data in reversed order to result
    for ( int j=0; j < be careful here; j++)
    {

    }
      
    // write out the result
    for ( int j=0; j < result.length; j++)
    {

    }

  }
}      

This version of the program is much easier, but costs more in memory. Usually this is a cost you should be willing to pay. In the very old days of computing, programmers would pick the solution that used the least memory, and often the cost was difficult code.

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Exercise 5 — Smooth Operator

An audio signal is sometimes stored as a list of int values. The values represent the intensity of the signal at successive time intervals. Of course, in a program the signal is represented with an array.

Often a small amount of noise is included in the signal. Noise is usually small, momentary changes in the signal level. An example is the "static" that is heard in addition to the signal in AM radio.

Smoothing a signal removes some of the noise and improves the perceptual quality of the signal. This exercise is to smooth the values in an integer array.

Say that the original values are in the array "signal". Compute the smoothed array by doing this: Each value smooth[N] is the average of three values: signal[N-1], signal[N], and signal[N+1].

For the first element of smooth, average the first two elements of signal. For the last element of smooth, average the last two elements of signal.

Use integer arithmetic for this so that the values in smooth are integers.

import java.io.* ;

class Smooth
{

  public static void main ( String[] args )  
  {
    int[] signal  = {5, 5, 4, 5, 6, 6, 7, 6, 5, 4, 1, 4};
    int[] smooth
    
    // compute the smoothed value for each
    //  cell of the array smooth
    smooth[0]  =  
    smooth[ signal.length-1 ] = 
    for (  )
    {

    }
      
    // write out the input
    for ( int j=0; j < smooth.length; j++)
    {

    }
    
    // write out the result
    for ( int j=0; j < smooth.length; j++)
    {

    }

  }
}      

In interpretting the results, remember that integer division discards the remainder. It does not compute a rounded value. Here is a sample run of the program:

C:\>java Smooth
signal: 1 5 4 5 7 6 8 6 5 4 5 4
smooth: 3 3 4 5 6 7 6 6 5 4 4 4
C:\>

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Exercise 6 — Data Tweeker

Say that you are interested in computing the average acid level of coffee as served by coffee shops in your home town. You visit many coffee shops and dip your pH meter into samples of coffee. You record your results in a text file such as the following. The first line of the file gives the number of values that follow.

13
5.6
6.2
6.0
5.5
5.7
6.1
7.4
5.5
5.5
6.3
6.4
4.0
6.9

Unfortunately, your pH meter sometimes produces false readings. So you decide to disregard the reading that is most distant from the average.

Create a text file containing the above or similar data. Now write a program that reads the data into an array (use input redirection as explained in Chapter 22). Compute the average of all the data. Now scan through the array to find the value that is farthest (in either direction) from the average. Remember the index of this value, and now scan through the array again, computing an average that does not include this value. Print the new average.

Here is a run of the program:

C:\>java CoffeeAverage < CoffeeData.txt
data[ 0 ] = 5.6
data[ 1 ] = 6.2
data[ 2 ] = 6.0
data[ 3 ] = 5.5
data[ 4 ] = 5.7
data[ 5 ] = 6.1
data[ 6 ] = 7.4
data[ 7 ] = 5.5
data[ 8 ] = 5.5
data[ 9 ] = 6.3
data[ 10 ] = 6.4
data[ 11 ] = 4.0
data[ 12 ] = 6.9
average: 5.930769230769231
most distant value: 4.0
new average: 5.6230769230769235

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Exercise 7 — Further Tweeking

Your pH meter has become much less reliable (ever since you dropped it on the floor due to uncontrollable shaking after ten cups of coffee). You decide to compute several averages from your data:

Write a program that computes these averages for the above or similar data.

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Exercise 8 —Tweekers Delight

Your Brain has started shaking, you have a couple of double espressos, and you notice that with an array of N values you can compute N averages:

Write a program that computes these averages and prints them out.

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