Trying to calculate GPA

Question

here's my code so far(it's been updated):

import java.util.Scanner; 

public class gpa {

   public static void main(String[] args) {
      double gpa=0;
      double input = 0;
      String grade= "";
      int classes =0;
      double fail =0;
      do
      {
         System.out.print("Enter Grade (n to quit): "); 
         Scanner sc = new Scanner(System.in); 
          grade = sc.nextLine(); 
          grade = grade.toLowerCase(); 
          if (grade.equals("a"))
          {
             input = 4.0;
          }
          if (grade.equals("b"))
          {
             input = 3.0;
          }
          if (grade.equals("c"))
          {
             input = 2.0;
          }
          if (grade.equals("d"))
          {
             input = 1.0;
          }
          if (grade.equals("f"))
          {
             fail = 1;
          }

          gpa+=input;
          classes++;
      } while(grade!="n");
      System.out.print("GPA: " + gpa + "   "); 

      gpa/=classes;
      if (gpa>=2 && classes >=4 && fail !=1)
      {
         System.out.print("Eligible");    
      }
      else if(classes <4)
      {
         System.out.print("Ineligible. less than 4 classes"); 
      }
      else if (gpa < 2.0)
      {
         System.out.print("Ineligible. GPA is less than 2.0"); 
      }
      else if (gpa >=2.0 && fail == 1)
      {
         System.out.print("Ineligible. GPA is above 2.0, but has an F");
      }
      else if (gpa <2.0 && fail ==1)
      {
         System.out.print("Ineligible. GPA is below 2.0 and has F");
      }
   }

}

Here's what I need to be able to output:

1. Eligible
2. Ineligible, taking less than 4 classes
3. Ineligible, gpa below 2.0
4. Ineligible, gpa above 2.0 but has F grade (note: gpa >= 2.0)
5. Ineligible, gpa below 2.0 and has F grade

It keeps asking for input. How do I stop this? I tried converting grade into a char but that ended horribly for me. I think it might be a compiler error or something. Any help would be appreciated. Though helpful help would keep my blood pressure down.

Answer 1

You're using a String (an object) not a char, so you need to use the .equals method. e.g. instead of grade == "a", you need grade.equals("a").

Answer 2

This trips a lot of new Java programmers up, it's a very common mistake. You need to use the .equals method, not the == operator. See Java String.equals versus ==.

Answer 3

In addition to the broken comparison (testing if two strings reference the same object rather than if their contents are logically equal), you also fail to set input to 0 for a grade of F. As a result, a single f counts towards the GPA as the same as the previous grade.

When the person finally does input n, you still do the math to add the grade and increment the number of grades. Also, you never actually compute the GPA (you just sum the grades).

Answer 4

For a start, you can't compare strings using the == operator. What that does is compare the actual object references, ie. for grade == "a" it's checking if grade and "a" are the same objects, which they're not.

In order to check the content of the strings for equality, use .equals(), ie. grade.equals("a").

Another thing, shouldn't GPA be the average of your total scores? ie. gpa /= classes after you finish your loop.

Also, rather than using a double to store fail, the flag variable for knowing if they failed a subject, use a boolean and set it to true if the grade is F. Speaking of that if statement, it doesn't modify input and the previous value of input will be added to gpa instead of a fail grade like 0.0.

And what about invalid input? That'll also cause gpa to be incremented by the previous value of input.

EDIT: And use some else ifs, instead of separate if statements, since every other condition will be checked even if one condition passes. ie. If grade.equals("a") is true, you'll still unnecessarily be checking if grade is also "b", "c", "d" and "f".

EDIT2: With that list of conditions you just added and the way you have your last block of if statements, a general rule is to have your more specific conditions above the more generalised ones so that they're not shadowed by the generalised ones. ie. Number 5 ("Ineligible, gpa below 2.0 and has F grade") is shadowed by number 3 ("Ineligible, gpa below 2.0") if you keep them in that order, as mentioned by animuson.

Answer 5

You fixed the misuse of the equality operator everywhere but in your "while" clause. It should be

while(!grade.equals("n")

Answer 6

Java, JavaScript, PHP or C++, the language is irrelevant as the methods are all the same. You need to think seriously about the implications of using the same variable in these two lines in your code:

else if (gpa <2.0 && fail ==1)