Deadline coming up in a few hours, can't figure out what I'm doing wrong

[Suane Mane]

The exercise:


My files:
ArrayStatistics class


TestStatistics class


Main class


Errors:

[Carey Brown]

none of your methods return anything.

Please don't post screen snapshots, they are very hard to read. Cut and paste the text into the post.

[Suane Mane]

What are they suposed to return?

Here is the code instead, sorry for the images.

ArrayStatistics class

package lab3;

public class ArrayStatistics {
    public static double sum(double[] x) {
        double sum = 0;
        for (int i = 0; i < x.length; i++)
            sum += x[i];
    }

    public static double mean(double[] x) {
        double mean = sum / x.length;
    }

    public static double min(double[] x) {
        double min = Double.NEGATIVE_INFINITY;
        for (int i = 0; i < x.length; i++)
            if (x[i] < min) min = x[i];
    }

    public static double max(double[] x) {
        double max = Double.NEGATIVE_INFINITY;
        for (int i = 0; i < x.length; i++)
            if (x[i] > max) max = x[i];
    }
}

TestStatistics class

package lab3;

import static lab3.ArrayStatistics.*;

import java.util.Arrays;

public class TestStatistics {
    public static void main(String[] args) {
        testStatistics(new double[]{1.0, -2.0, 3.0, -4.0, 5.0});
    }

    public static void testStatistics(double[] x) {
        System.out.println("Array = " + Arrays.toString(x));
        System.out.println("Sum = " + sum(x));
        System.out.println("Average = " + mean(x));
        System.out.println("Min = " + min(x));
        System.out.println("Max = " + max(x));
    }
}

Main class

package lab3;

public class Main {
    public static void main(String[] args) {
        ArrayStatistics.sum();
        ArrayStatistics.mean();
        ArrayStatistics.min();
        ArrayStatistics.max();
    }
}

[Carey Brown]

You already declare the type each method should be returning, you're just not doing it. I would presume that you are getting compile errors on this.

Example:
public static double sum(double[] x) {

[Suane Mane]

Isn't the return already stated in the other class (TestStatistics)? That's the one that actually runs the methods and prints the results.

[Carey Brown]

System.out.println("Sum = " + sum(x));

This says that it is expecting sum(x) to return something, though it doesn't say what.

public static double sum(double[] x) {

This says that sum() is going to return a double, but then doesn't do it.

    public static double sum(double[] x) {
        double sum = 0;
        for (int i = 0; i < x.length; i++)
            sum += x[i];
    }

Here you create a local variable named "sum" which you use to calculate the sum but you never do anything with it, so when the method ends, the local variable sum, disappears along with all the work done to compute it. Nothing is returned.

[Suane Mane]

I don't know how I'm supposed to write a return statement then. I thought the parts like:

for (int i = 0; i < x.length; i++)
            sum += x[i];

declare what needs to be done when the method is accessed and I thought x was the array? And thus it would go through all array items and just add them up.

[Carey Brown]

Suane Mane wrote:I don't know how I'm supposed to write a return statement then. I thought the parts like:

for (int i = 0; i < x.length; i++)
            sum += x[i];

declare what needs to be done when the method is accessed and I thought x was the array? And thus it would go through all array items and just add them up.

It certainly does sum them up but the sum is not being returned.

   public static double sum(double[] x) {
       double sum = 0;
       for (int i = 0; i < x.length; i++)
           sum += x[i];
       return sum; // <----
   }

[Suane Mane]

I've added that now and still getting errors.

[Carey Brown]

a) Did you fix it in all your methods that declare that they are returning a value?
b) When you say you're getting errors, you need to cut and paste the complete error message into a post.
c) Re-post your code after you've made changes so we can keep in sync and that the error messages are in sync with your line numbers.

[Suane Mane]

Ok, I think I have it now. The problem was having 2 main methods. I didn't know how to approach this problem so I used the same solution as in previous labs where we were told to make a separate Main class with a main method to call other methods. Apparently it's useless here, as there is already a main method declared in the class TestStatistics. So this is the code now:

package lab3;

public class ArrayStatistics {
    public static double sum(double[] x) {
        double sum = 0;
        for (int i = 0; i < x.length; i++)
            sum += x[i];
        return sum;
    }

    public static double mean(double[] x) {
        double mean = sum(x) / x.length;
        return mean;
    }

    public static double min(double[] x) {
        double min = Double.POSITIVE_INFINITY;
        for (int i = 0; i < x.length; i++)
            if (x[i] < min) min = x[i];
            return min;
    }

    public static double max(double[] x) {
        double max = Double.NEGATIVE_INFINITY;
        for (int i = 0; i < x.length; i++)
            if (x[i] > max) max = x[i];
            return max;
    }
}

package lab3;

import static lab3.ArrayStatistics.*;

import java.util.Arrays;

public class TestStatistics {
    public static void main(String[] args) {
        testStatistics(new double[]{1.0, -2.0, 3.0, -4.0, 5.0});
    }

    public static void testStatistics(double[] x) {
        System.out.println("Array = " + Arrays.toString(x));
        System.out.println("Sum = " + sum(x));
        System.out.println("Average = " + mean(x));
        System.out.println("Min = " + min(x));
        System.out.println("Max = " + max(x));
    }
}

Results
Array = [1.0, -2.0, 3.0, -4.0, 5.0]
Sum = 3.0
Average = 0.6
Min = -4.0
Max = 5.0

It actually worked! Thanks for the help!

[Carey Brown]

public class Main {
    public static void main(String[] args) {
        ArrayStatistics.sum();
        ArrayStatistics.mean();
        ArrayStatistics.min();
        ArrayStatistics.max();
    }
}

This is giving you some of the errors. ArrayStatistics.sum() requires a double[] as an argument. Ditto for the other calls. You are throwing away the returned values from all of these methods, this won't give you a compile error but is somewhat pointless.

It seems that your TestStatistics class is what this class should be. In other words:
TestStatistics class -- good
Main class -- compiles but pointless

[Carey Brown]

A note on your style

    public static double max(double[] x) {
        double max = Double.NEGATIVE_INFINITY;
        for (int i = 0; i < x.length; i++)
            if (x[i] > max) max = x[i];
            return max;
    }

It is recommended that conditional and loop statements always be enclosed in braces. This is especially true for beginners. So you should have

    public static double max(double[] x) {
        double max = Double.NEGATIVE_INFINITY;
        for (int i = 0; i < x.length; i++) {
            if (x[i] > max) {
                max = x[i];
            }
        }
        return max;
    }

[Kyle Prouty]

I would take a step back and think about what the exercise is trying to achieve.

Here are some questions they are asking...

We are given a static class that has a main, we know that this class will start the execution of our program. Do I know how to compile and run a java class?

We are told to create a class. Do I know how to create a class?

We are told to add 4 static methods to that class. We are given the complete function definition, nice! Do I know what methods of a class are?

Do I know what public and static mean? Do I know what the double following public static means? Do I know what sum(double[] x) means?

We keep seeing static. What does that mean when my class is static. How does that change how methods are called?

Do I know how to iterate through an array?

Do I know how to return a value from a method?

[Suane Mane]

Oooh I see. I forgot about that. Too many braces everywhere! At least it works even without, but I see it's cleaner to work with like this and easier to follow the logic when looking back at it. Thanks!

[Suane Mane]

Kyle Prouty wrote:
I would take a step back and think about what the exercise is trying to achieve.

Here are some questions they are asking...

We are given a static class that has a main, we know that this class will start the execution of our program. Do I know how to compile and run a java class?

We are told to create a class. Do I know how to create a class?

We are told to add 4 static methods to that class. We are given the complete function definition, nice! Do I know what methods of a class are?

Do I know what public and static mean? Do I know what the double following public static means? Do I know what sum(double[] x) means?

We keep seeing static. What does that mean when my class is static. How does that change how methods are called?

Do I know how to iterate through an array?

Do I know how to return a value from a method?

Yeah, this is what I am trying to do with each exercise. I've struggled with Java last year and now am getting back on track and trying to do everything properly, learn from scratch. I was trying to figure out what the question was asking, what each part of code they provided actually means and what is expected of me to know so far and add in. I may be slow or something because this was quite hard for me. Been at it for hours.
I've found that Java theory and actual definitions are MUCH harder to understand than actually working with code. I was so motivated to write down everything, grabbed my notebook, sat down and looked at first few slides talking about classes, packages, methods and source files. I got stuck immediately trying to make sense of the hierarchy and what goes where and is a class a branch of a package? Is a method a child of a class? Is a class same thing as a source file then? I was frankly more confused after reading the slides than after trying to write a simple program in lab. I don't know if I'll ever understand the actual theory of Java.

[Mark Ii]

You can also write the source codes like below:

package lab3;

public class ArrayStatistics {
	
	    public static double sum(double[] x) {
	        double sum = 0;
	        for (int i = 0; i < x.length; i++) {
	            sum += x[i];
	        }
	        
	        return sum;
	    }
	 
	    public static double mean(double[] x) {
	        double mean = sum(x) / x.length;
	        
	        return mean;
	    }
	 
	    public static double min(double[] x) {
	        double max = Double.POSITIVE_INFINITY;
	        for (int i = 0; i < x.length; i++) {
	            if (x[i] < max) {
	            	max = x[i];
	            }
	        }
	        
	        return max;
	    }
	 
	    public static double max(double[] x) {
	        double min = Double.NEGATIVE_INFINITY;
	        for (int i = 0; i < x.length; i++) {
	            if (x[i] > min) {
	            	min = x[i];
	            }
	        }
	        
	        return min;
	    }
	}

package lab3;
 
import static lab3.ArrayStatistics.*;
 
import java.util.Arrays;
 
public class TestStatistics {
    
 
    public static void testStatistics(double[] x) {
        System.out.println("Array = " + Arrays.toString(x));
        System.out.println("Sum = " + sum(x));
        System.out.println("Average = " + mean(x));
        System.out.println("Min = " + min(x));
        System.out.println("Max = " + max(x));
    }
    
    public static void main(String[] args) {
        testStatistics(new double[]{1.0, -2.0, 3.0, -4.0, 5.0});
    }
}

[Mark Ii]

If you look at it I used POSITIVE_INFINITY and NEGATIVE_INFINITY fields of Double class to show you, a program can be written in many ways. I hope it helps.

[Kyle Prouty]

Here is a fun Java 8 way to do it.

public static double sum(final double[] x) {
    return
        Arrays.stream(x).sum();
}

public static double mean(final double[] x) {
    return sum(x)/x.length;
}

public static double min(final double[] x) {
    return
        Arrays.stream(x)
            .min().getAsDouble();
}

public static double max(final double[] x) {
    return
        Arrays.stream(x)
            .max().getAsDouble();
}

[Nathan Milota]

This is why I stopped doing college courses on programming.  If you are asked to do an assignment of which you never learned how to write methods first, it's a bad course.  

Every method except for void has to have a return statement of the type of value you are writing.

[Campbell Ritchie]

Kyle Prouty wrote:Here is a fun Java 8 way to do it. . . .

Careful; that code is broken and will fall over horribly if you pass a 0‑element array to it.
Try

System.out.println(DoubleStream.of(new double[]{}).summaryStatistics());

[Suane Mane]

Kyle Prouty wrote:
Here is a fun Java 8 way to do it.

public static double sum(final double[] x) {
    return
        Arrays.stream(x).sum();
}

public static double mean(final double[] x) {
    return sum(x)/x.length;
}

public static double min(final double[] x) {
    return
        Arrays.stream(x)
            .min().getAsDouble();
}

public static double max(final double[] x) {
    return
        Arrays.stream(x)
            .max().getAsDouble();
}

I have no idea what final, stream or getasdouble mean. I don't even fully understand the format of it. Is it stuff you learn later on?

[Carey Brown]

Yes. Later on. Anytime someone mentions "Java 8" it is usually not a beginner topic, and typically involves streams and or functional programming. These features were introduced in Java 8.

[Campbell Ritchie]

Suane Mane wrote:. . . I have no idea what final, stream or getasdouble mean. . . .

Please look here; I only worked out the average.

A couple of weeks ago, we had a long discussion about whether you should or shouldn't teach Streams to beginners; I think you should. Please look here. It makes very interesting reading.