为什么mathGPA不能正常工作？

Question

I'm working to make a GPA Calculator. One of the first things I was trying to tackle was the individual GPA for each class, which after calculating, would be added up to the total Grade Point Average. I defined two variables (mathGPA and historyGPA) mathGPA was working fine until I added the historyGPA. I'm now worried that as I progress through the project the previously coded individual GPA scores won't be recognized by Java. Here is the code:

package problemset.leveltwo;

import java.util.*;

public class gpaCalculator {

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);

    // A+
    double a = 4.00;

    // A-
    double b = 3.70;

    // B+
    double c = 3.30;

    // B
    double d = 3.00;

    // B-
    double e = 2.70; 

    // C+
    double f = 2.30;

    // C
    double g = 2.00;

    // C-
    double h = 1.70;

    // D+
    double i = 1.30;

    // D
    double j = 1.00;

    // D-
    double k = 0.70;

    // F
    double l = 0;

    System.out.println("Enter Your Math Grade (percentage)");
    int mathGrade = scanner.nextInt();

    System.out.println("Enter Your History Grade (percentage)");
    int historyGrade = scanner.nextInt();

    System.out.println("Enter Your English Grade (percentage)");
    int englishGrade = scanner.nextInt();

    System.out.println("Enter Your Science Grade (percentage)");
    int scienceGrade = scanner.nextInt();

    System.out.println("Enter Your Language Grade (percentage)");
    int languageGrade = scanner.nextInt();


    // MATH GRADE

    // Math = A
    if (mathGrade >= 96) {
        System.out.println("Math GPA = " + a);
        double mathGPA = 4.00;
        }

    // Math = A-
    if (mathGrade < 96) {
        if (mathGrade >= 90) {
            System.out.println("Math GPA = " + b);
            double mathGPA = 3.70;
        }
    }

    // Math B+
    if (mathGrade < 90) {
        if (mathGrade >= 86) {
            System.out.println("Math GPA = " + c);
            double mathGPA = 3.30;
        }
    }

    // Math B
    if (mathGrade < 86) {
        if (mathGrade >= 84) {
            System.out.println("Math GPA = " + d);
            double mathGPA = 3.00;
        }
    }

    // Math B-
    if (mathGrade < 84) {
        if (mathGrade >= 80) {
            System.out.println("Math GPA = " + e);
            double mathGPA = 2.70;
        }
    }

    // Math C+
    if (mathGrade < 80) {
        if (mathGrade >= 76) {
            System.out.println("Math GPA = " + f);
            double mathGPA = 2.30;
        }
    }

    // Math C
    if (mathGrade < 76) {
        if (mathGrade >= 73) {
            System.out.println("Math GPA = " + g);
            double mathGPA = 2.00;
        }
    }

    // Math C-
    if (mathGrade < 73) {
        if (mathGrade >= 70) {
            System.out.println("Math GPA = " + h);
            double mathGPA = 1.70;
        }
    }

    // Math D+
    if (mathGrade < 70) {
        if (mathGrade >= 66) {
            System.out.println("Math GPA = " + i);
            double mathGPA = 1.30;
        }
    }

    // Math D
    if (mathGrade < 66) {
        if (mathGrade >= 63) {
            System.out.println("Math GPA = " + j);
            double mathGPA = 1.00;
        }
    }

    // Math D-
    if (mathGrade < 63) {
        if (mathGrade >= 50) {
            System.out.println("Math GPA = " + k);
            double mathGPA = 0.70;
        }
    }

    // Math F
    if (mathGrade < 50) {
        System.out.println("Math GPA = " + 0);
        double mathGPA = 0.00;
    }



    // HISTORY GRADE

    // History = A
    if (historyGrade >= 96) {
        System.out.println("History GPA = " + a);
        double historyGPA = 4.00;
        }

    // History = A-
    if (historyGrade < 96) {
        if (historyGrade >= 90) {
            System.out.println("History GPA = " + b);
            double historyGPA = 3.70;
        }
    }

    // History B+
    if (historyGrade < 90) {
        if (historyGrade >= 86) {
            System.out.println("History GPA = " + c);
            double historyGPA = 3.30;
        }
    }

    // History B
    if (historyGrade < 86) {
        if (historyGrade >= 84) {
            System.out.println("History GPA = " + d);
            double historyGPA = 3.00;
        }
    }

    // History B-
    if (historyGrade < 84) {
        if (historyGrade >= 80) {
            System.out.println("History GPA = " + e);
            double historyGPA = 2.70;
        }
    }

    // History C+
    if (historyGrade < 80) {
        if (historyGrade >= 76) {
            System.out.println("History GPA = " + f);
            double historyGPA = 2.30;
        }
    }

    // History C
    if (historyGrade < 76) {
        if (historyGrade >= 73) {
            System.out.println("History GPA = " + g);
            double historyGPA = 2.00;
        }
    }

    // History C-
    if (historyGrade < 73) {
        if (historyGrade >= 70) {
            System.out.println("History GPA = " + h);
            double historyGPA = 1.70;
        }
    }

    // History D+
    if (historyGrade < 70) {
        if (historyGrade >= 66) {
            System.out.println("History GPA = " + i);
            double historyGPA = 1.30;
        }
    }

    // History D
    if (historyGrade < 66) {
        if (historyGrade >= 63) {
            System.out.println("History GPA = " + j);
            double historyGPA = 1.00;
        }
    }

    // History D-
    if (historyGrade < 63) {
        if (historyGrade >= 50) {
            System.out.println("History GPA = " + k);
            double historyGPA = 0.70;
        }
    }

    // History F
    if (historyGrade < 50) {
        System.out.println("History GPA = " + 0);
        double historyGPA = 0;


double total = mathGPA + historyGPA;
System.out.println("Total GPA = " + total);


        }

        }

}

The Error message Java Eclipse is giving me is "mathGPA cannot be resolved to a variable." I don't know what I'm writing incorrectly but I hope somebody can give me some insight to correct my code.

Answer 1

As was pointed out you define mathgpa inside the if statement so it only exists there.

Define one double for it before the if statements and get rid of the redundancies.

Next learn to use if..then..else to make life better

If you look at the last example in this tutorial it shoukd help

http://docs.oracle.com/javase/tutorial/java/nutsandbolts/if.html

Basically do an if then if it doesn't match use an else if until you get to the final else which will match

if (grade > 90) {
   // A
} else if (grade > 80) {
   // B
} else {
  // F
}

So a B is if less than 90 and greater than 80 and anything less than 80 is an F.