Beginning Java (Histogram)

Question

I'm taking a beginner Java class, with the assignment to create a histogram program with the following output: (100 and 10 are user inputs).

How many numbers? 100 How many intervals? 10

Histogram
--------------------------------------------------------
  1 ****(4)
  2 ******(6)
  3 ***********(11)
  4 *****************(17)
  5 **************************(26)
  6 *************************(25)
  7 *******(7)
  8 ***(3)
  9 (0)
 10 *(1)
--------------------------------------------------------

My code is giving the following output however, can anyone help me point out what is going wrong, thanks so much.

How Many Numbers? 10
How Many Intervals? 10

Histogram
--------------------------------------------------------
 1 **********(10)
 2 **********(10)
 3 **********(10)
 4 **********(10)
 5 **********(10)
 6 **********(10)
 7 **********(10)
 8 **********(10)
 9 **********(10)
 10 **********(10)

For the input, 100 and 10 I get the error message:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 10 at Lab6.main(Lab6.java:44) I marked Line 44 below.

Which links to this code;

             intervalValue[j]++;

I'm not sure how to attach the generator (.jar) file, it is supposed to just generate the random #'s for us. Thanks again.

mport java.util.Scanner;

public class Lab6 {

    public static void main(String[] args) {

        int numbers, intervals;
        double intervalWidth;

        double max, mins, range;

        Scanner keyboard = new Scanner(System.in);

        System.out.print("How Many Numbers? ");

        numbers = keyboard.nextInt();

        System.out.print("How Many Intervals? ");

        intervals = keyboard.nextInt();

        double [] generate = new double[numbers];

        generate = randomGenerator(numbers);

        max = maximum(generate);

        mins = minimum(generate);

        range = max - mins;

        intervalWidth = range / intervals;

        int [] intervalValue = new int[intervals];

        for (int i=0; i < generate.length; i++) {

            for (int j = 0; j<generate.length; j++){
                double imin = mins+j*intervalWidth;
                double imax = max +j*(intervalWidth);
                if(generate[i] >= imin && generate[i] < imax)
                    intervalValue[j]++;         //LINE 44
            }
        }

        System.out.println("Histogram");

        System.out.println("--------------------------------" +
                           "------------------------");

        for (int a=0; a < intervalValue.length; a++) {

            System.out.print(" " + (a+1) + " ");

            for (int b=0; b < intervalValue[a]; b++) {
                System.out.print("*"); 
            }

            System.out.println("(" + intervalValue[a] + ")");
        }
    }

    private static double [] randomGenerator(int number) {
        double [] generate;

        generate = Generator.getData(number);

        return generate; 
    }

    private static double maximum(double [] a) {

        double max = a[0];

        for (int i = 1; i < a.length; i++) {        
            if (a[i] > max) {
                max = a[i];
            }         
        }

        return max;
    }

    private static double minimum(double [] a) {

        double mins = a[0];

        for (int i = 1; i < a.length; i++) {
            if (a[i] < mins) {
                mins = a[i];
            }
        }

        return mins;
    }
}

Solution 1

You need to remove the semicolon from the end of line 43:

if(generate[i] >= imin && generate[i] < imax);

The semicolon causes your if block to be empty. As a consequence, the line resulting in the exception is executed unconditionally.

Once you address that issue, here are a few hints to get you started on the further debugging of your code:

---

Take a look at the termination condition of your inner loop:

for (int j = 0; j<generate.length; j++)

You are iterating over the generated numbers in your outer loop; so you should be iterating over the intervals in your inner loop to determine the interval to which each generated number belongs.

---

Take a look at the code you are using to determine the bounds of the current interval in your loop:

double imin = mins+j*intervalWidth;
double imax = max +j*(intervalWidth);

These lines need to be modified to yield the correct values. Do a bit of 'pencil-and-paper debugging' to determine why they are currently in error.

---

I will leave the rest as an exercise for the reader for now. I will check back in later to see if you need any further assistance.

Solution 2

Since I was feeling generous on a Saturday I tried it out and rewrote your loop.

for (int j=0; j < generate.length; j++) {
    for(int i = 0; i < intervals; i++) {
        double imin = mins + i * intervalWidth;
        double imax = mins + (intervalWidth) * (i + 1);
        if(i == intervals - 1) imax = Double.POSITIVE_INFINITY;
        if(i == 0) imin = Double.NEGATIVE_INFINITY;

        if (generate[j] >= imin && generate[j] < imax) {
            intervalValue[i]++;
            break;
        }
    }
}

The infinity stuff is to catch the min and maxes in the histogram.

Solution 3

This will give you a very nice (basic) looking histogram. Try it

import java.util.HashMap;
import java.util.Map;

public class Histogram {
    public static void main(String[] args) {
        int[] age = { 25, 26, 33, 26, 27, 21, 26, 33, 21, 33, 21, 38, 19, 19};


        HashMap<Integer, Integer> m = new HashMap<Integer, Integer>();

        for (int i = 0; i < age.length; i++) {
            int c = 0;

            for (int j = 0; j < age.length; j++) {
                if (age[i] == age[j]) {
                    c++;
                }
            }
            m.put(age[i], c);

        }

        System.out.println("Histogram\n------------");
        for (Map.Entry<Integer, Integer> entry : m.entrySet()) {
            int key = entry.getKey();
            int value = entry.getValue();
            System.out.printf("%3d | ", key);
            for (int i = 0; i < value; i++) {
                System.out.print("=");
            }
            System.out.print(" " + value);
            System.out.println();
        }

    }

Outout:

Histogram
------------
 33 | === 3
 19 | == 2
 21 | === 3
 38 | = 1
 25 | = 1
 26 | === 3
 27 | = 1

I'm using HashMap to keep two related values namely the an array element and its frequency (how frequently it appears).

Then a nested loops to loop by each element along the array and count its frequency by using c variable.

After that I print it with for-each loop and normal loop

Comments

[Kekoa]

Unfortunately I can't compile it without your generator class.

[Mohammad Taha]

and you might want to consider using a code formatter (eclipse has one built in) also don't try to post binary files' content. It doesn't help :)

[Benzle]

How can I make them the same, or at least not create the error? -Thanks,

[Benzle]

Thanks everyone so much, this has been a great first experience on stack overflow!

[Andrei]

Could you explain the code in a few words ? It will make your answer so much better and you could get more votes.

[Amjad]

I'm using HashMap to keep two related values namely the an array element and its frequency (how frequently it appears). Then a nested loops to loop by each element along the array and count its frequency by using "c" variable. After that I print it with enhanced for loop and normal loop

[Andrei]

Added your explanation to the answer. In the feature don't forget to explain your code :) Good job!