Loop won't end despite the use of a sentinel value.

Question

This is an example straight out of my text book, and I can't get it to end the loop.

import java.util.Scanner;

public class Play {

    System.out.println("Enter scores for all students.");
    System.out.println("Enter a negative number after");
    System.out.println("you have entered all the scores.");

    Scanner keyboard = new Scanner(System.in);
    double max = keyboard.nextDouble();
    double min = max;
    double next = keyboard.nextDouble();
    while(next >= 0)
    {
        if(next > max)
            max = next;
        else if (next < min)
            min = next;
        next = keyboard.nextDouble();

    }
    System.out.println("The highest score is " + max);
    System.out.println("The lowest score is " + min);
}

Answer 1

Your condition says next >= 0, it should be next > 0. Are you supposed to enter a negative value to finish entering values?

Answer 2

I fixed the demo for you (see below).

He was using while when he should use do while.

The demo states that you use a negative number to exit the loop (below zero).

 public static void main(String[] args) {
    System.out.println("Student score entry demo:");
    System.out.println("NB: Enter a negative number to finish.\n");
    Scanner keyboard = new Scanner(System.in);
    double max = -1;
    double min = max;
    double score;
    do {
      System.out.println("Enter student score: ");
      score = keyboard.nextDouble();
      if (score > max) {
        max = score;
      } else if (min < 0 || score < min && score >= 0) {
        min = score;
      }
    } while (score >= 0);
    // Output results
    System.out.println("The highest score was: " + max);
    System.out.println("The lowest score was: " + min);
  }

Output:

Student score entry demo:

NB: Enter a negative number after you have entered all the scores to finish.

Enter student score:
22

Enter student score:
33

Enter student score:
44

Enter student score:
55

Enter student score:
11

Enter student score: -3

The highest score was: 55.0 The lowest score was: 11.0

Answer 3

Here are my 0.02$. The logic is explained in the comments.

public static void main(String[] args) {

// Print instructions
System.out.println("Enter scores for all students.");
System.out.println("Enter a negative number after");
System.out.println("you have entered all the scores.");

Scanner keyboard = new Scanner(System.in);

// Init min and max
// Any grade will be lower than the current min grade 
// (which is the higest possible value a double can hold),
// and higher than the current max grade.
double max = Double.MIN_VALUE;
double min = Double.MAX_VALUE;

// Read first grade. Allow the user to terminate the loop
// without entering grades
double next = keyboard.nextDouble();
while(next >= 0)
{
    if(next > max)
        max = next;

    // Removed else-if. This ensures the first grade is
    // initialized correctly
    if (next < min)
        min = next;
    next = keyboard.nextDouble();

}

// Make sure we got some grades
if (max == Double.MIN_VALUE && min == Double.MAX_VALUE) {
    System.out.println("No scores entered. Your students are lazy.");
} else {
    System.out.println("The highest score is " + max);
    System.out.println("The lowest score is " + min);
}

}

Try to enter -1 as the only grade, and a list of grades followed by -1, see what you get.