asked on

How do I display the sum of all positive numbers in the collection and the sum of all negative numbers using iterators?

This is my code:

/**
* Using class with iterators
*/
import java.util.ArrayList;
import java.util.Iterator;

public class Numbers {
private ArrayList<Double> numbers;

public Numbers() {
numbers = new ArrayList<Double>();

}

/**
*Add a number to the collection in a client specified position or at the
* end of the collection
*/
public void addNumber(double number) {
numbers.add(number);
}

/**
* Display all numbers in the collection
*/
public void displayNumbers() {
Iterator<Double> it = numbers.iterator();
while (it.hasNext()) {
System.out.println(it.next());

}
}

/**
* Display all numbers in the collection organized in 2 categories, first
* the positive numbers, second the negative numbers
*/
public void displayNumbersInCategories() {
double number = 0;
System.out.println("Positive Numbers");

Iterator<Double> it = numbers.iterator();

while (it.hasNext())
{number = it.next();
if (number >= 0.0)
{

System.out.println(number);
}

}

System.out.println("Negative Numbers");

it = numbers.iterator();

while(it.hasNext()) {
number = it.next();
if (number < 0.0) {
System.out.println(number);
}
}
}

/**
* Display the sum of all numbers in the collection
*/
public void displaySum() {
double sum = 0.0;
Iterator<Double> it = numbers.iterator();

while (it.hasNext()) {
sum = sum + it.next();

}

System.out.println(sum);
}

/**
* Display 2 sums: the sum of all positive numbers in the collection and the
* sum of all negative numbers in the collection
*/
public void displaySumsInCategories() {
double number = 0;
double sum = 0.0;
Iterator<Double> it = numbers.iterator();

System.out.println("Positive Numbers");

while (it.hasNext()) {
number = it.next();
sum = sum + it.next();

}

System.out.println(sum);
// }
System.out.println("Negative Numbers");

while(it.hasNext()) {
number = it.next();
sum = sum + it.next();
{
System.out.println(sum);
}
}
}

cyberkiwi

You could use Math.max and min to "mask" the positive/negative numbers.


    /**
     * Display 2 sums: the sum of all positive numbers in the collection and the
     * sum of all negative numbers in the collection
     */
    public void displaySumsInCategories() {
        double number = 0;
        double sum = 0.0;
        Iterator<Double> it = numbers.iterator();
       
        System.out.println("Positive Numbers");

        while (it.hasNext()) {
            number = it.next();
            sum = sum + Math.max(it.next(),0);
           
        }

        System.out.println(sum);
        // }
        System.out.println("Negative Numbers");

        while(it.hasNext()) {
            number = it.next();
            sum = sum + Math.min(it.next(),0);
            {
                System.out.println(sum);
            }
        }

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cyberkiwi

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Mick Barry

I posted how in your earlier question (where you accepted a comment that was incorrect)

public void displaySumsInCategories()
{
double sum1 = 0;
double sum2 = 0;
for (double n : numbers) {
if (n>0.0) {
sum1 += n;
} else {
sum2 += n;
}
}
System.out.println("Sum of positives "+sum1);
System.out.println("Sum of negatives "+sum2);
}

ryanbecker24

ASKER

Is there any other way to do it because I don't know if my teacher will like me using Math.min or Math.max.

cyberkiwi

Because this is obviously a learning exercise, I will make some explanations, otherwise we would be unethically helping you to cheat. If you can understand it (and therefore reproduce), then it is a different thing.

Good thing you already have code, so I can tell you where you went wrong. Hopefully that will give you some insight.

Read here on using the iterator in a for-each loop.
http://download.oracle.com/javase/1.5.0/docs/guide/language/foreach.html
Your while loop is ok, but more commonly used is for( ; ; ) or for ( : ). Not sure which one you have been taught. The for( : ) for is more elegant, although it is useful to learn the for( ; ; ) form for general use and also if multiple iterators need to be used in sync; can't with for( : ).

        System.out.println("Positive Numbers");

        while (it.hasNext()) {
            number = it.next();   /// HERE we retrieve one number (a) from iterator
            sum = sum + it.next();   /// WHAT GIVES, we take another number (b)
                                    /// At this point, we took (a) and it has gone to the ether
        }

        System.out.println(sum);
        // }
        System.out.println("Negative Numbers");

//// You forgot to reset "sum" here.  It is carrying forward the previous sum
        while(it.hasNext()) {    /// WE have reached the end of Iterator "it" using previous loop.
                                /// the condition will never be true and this loop won't happen
            number = it.next();
            sum = sum + it.next();   // same issue, 1 number taken, discarded, another taken
            {
                System.out.println(sum);  // Why is this in the loop?
                                        /// It is printed for each pass through this loop
            }
        }

============================================

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ryanbecker24

ASKER

Thanks