Adding grade letter to my array in my grading calculator dont work

[miguel ortiz]

Hello, I am new to java and here I have an issue with this code having multiple methods. this is for a grading calculator and ask me how many students, how many grades, and prints the result in an array table...what I want to do is to not just print each mark, but calculate the average and display the average in the array DisplayvalueInArray. my output right now is the student with all the marks for each student.

this is my java code:

package bucky;

import java.util.Scanner;

public class arrays {
private static final Object[] studentScoreArray = null;
public static void main(String args []){
	//creating the Scanner object for input
	Scanner keyboard = new Scanner(System.in);
	System.out.print("Enter the number of Students: ");;
	int studentNum = keyboard.nextInt();
	System.out.println();
	System.out.print("Enter the number of Homework: ");
	int studentHomeworkNo = keyboard.nextInt();
	
	
	AssignValueToArray(studentNum, studentHomeworkNo);
}


//this method assigns student scores to a two dimensional array

public static void AssignValueToArray(int studentNumber, int numberOfHomeWork){
	
	int [][] studentScore = new int [studentNumber] [numberOfHomeWork];
	Scanner keyboardForArray = new Scanner(System.in);
	
	int studCount = 1;
	for (int index = 0; index < studentNumber; index++){
		
		System.out.println("Please type in student "+studCount+" 's grade: ");
		for(int indexOfHomeWork = 0; indexOfHomeWork<numberOfHomeWork;indexOfHomeWork++){
			studentScore[index][indexOfHomeWork]=keyboardForArray.nextInt();
			}
		studCount++;
	}
	keyboardForArray.close();
	DisplayvalueInArray(studentScore);
}
//another method that displays student score in the two dimensional array
public static void DisplayvalueInArray(int[][] studentScoreArray){
	System.out.println("The Student Scores are: "+"\n");
	int studentCount = 1;
	for (int index = 0; index<studentScoreArray.length; index++){
		System.out.print("Grades for student "+studentCount+": ");
		for (int indexOfHomeWork = 0; indexOfHomeWork < studentScoreArray[index].length; indexOfHomeWork++){
			System.out.print(studentScoreArray[index][indexOfHomeWork] + "\t");
		}
		System.out.println();
		studentCount++;
		calculateGrade(index, studentCount);
	}
}
public static String calculateGrade(int averageGrade, int indexOfHomeWork){
	averageGrade = studentScoreArray.length/indexOfHomeWork;
	String letterGrade;
	
	//below the conditions to be tested
	if (averageGrade < 60 && averageGrade >=0)
		letterGrade = "F";
		
	else if (averageGrade >=60 && averageGrade < 70)
		letterGrade = "D";

	else if (averageGrade >=70 && averageGrade < 80)
		letterGrade = "C";

	else if (averageGrade >= 80 && averageGrade < 90)
		letterGrade = "B";

	else if (averageGrade >=90 && averageGrade <= 100)
		letterGrade = "A";

	else
		letterGrade = "Error, Incorrect Input";
	
	//DisplayvalueInArray(indexOfHomeWork);;
	return letterGrade;
}
	
}

my output:

Enter the number of Students: 2

Enter the number of Homework: 3
Please type in student 1 's grade: 
77
89
95
Please type in student 2 's grade: 
59
88
99
The Student Scores are: 

Grades for student 1: 77	89	95	
Exception in thread "main" java.lang.NullPointerException
	at bucky.arrays.calculateGrade(arrays.java:55)
	at bucky.arrays.DisplayvalueInArray(arrays.java:51)
	at bucky.arrays.AssignValueToArray(arrays.java:38)
	at bucky.arrays.main(arrays.java:17)

[Stefan Evans]

Hi Miguel. Welcome to the ranch!

So you're getting a null pointer exception.
I think I've spotted your problem, but lets see if we can point you in the direction to solve it yourself:

Can you identify from the error message which line of code is causing the problem?
Once you have found that line, take a look at it. What is on that line that might possibly throw a null pointer exception? i.e. what variable/object might be null here?
For the things that might be null, where should they be getting their value from?

[miguel ortiz]

so at the end of the line I have to add the variables averageGrade to be calculated (or add the whole loop to the DisplayvalueInArray method) I am kind of lost on what to do.

[miguel ortiz]

Well I think I broke this even more, now it doesn't even do anything, LOL wow.

[Stefan Evans]

>so at the end of the line I have to add the variables averageGrade to be calculated (or add the whole loop to the DisplayvalueInArray method) I am kind of lost on what to do.

Huh? You've lost me here.
How did you go with the questions I posed?

[miguel ortiz]

well to implement my if statements for taking the array values of homework and taking the average, and then with that average put the value through the loop and assign a letter grade and print it together in the DisplayValueArray. That's what I wanted to accomplish, but there is a brake somewhere and I cant figue it out...I still working on it..and now I am starting another one from scratch. But it would be nice if I could figure that code out lol, but I still have a great deal of learning to do, and I understand.

[Carey Brown]

Lines 6 and 25 both declare studentScoreArray variables. Which one do you want used on line 55?

[miguel ortiz]

I want to use 25

[Knute Snortum]

Right now the studentScoreArray on line 55 is the static final one on line 6. It is never used anywhere else. How could you use the parameter studentScoreArray in line 41 in line 55?

[miguel ortiz]

I'm currently adding another method to calculate the grade average, but to link other methods with the new one is what is killing me. I erased line 55. but like I said earlier everything I add something new the more complex it gets and more errors pop up, and I don't have enough experience to figure all out at this point by myself...did I made it to complex to start with? what is supposed to do is use arrays to save multiple students and multiple grades and calculate and be able to assign a grade letter according to the scale.

[miguel ortiz]

Again here I have my newest code, Still have the same problem...the calculateGrade method in line 22I want to get that average and assign it to my GradeLetter method in line 63 and loop it and then the result would be the grade letter which I want to put in the DisplayvalueInArray line 47...but I cant display the grade letter and cant figure it out. below is also my output. thank you guys for your help so far.

package gradingScore;
import java.util.Scanner;
public class gradingProgram { 
public static void main(String [] args){
	 
    //creating the Scanner object for input
    Scanner keyboard = new Scanner(System.in);
    System.out.print("Enter the number of Students: ");;
    int studentNum = keyboard.nextInt();
    System.out.println();
    System.out.print("Enter the number of Scores: ");
    int studentHomeworkNo = keyboard.nextInt();
     
     
    AssignValueToArray(studentNum, studentHomeworkNo);
    calculateGrade(studentHomeworkNo, keyboard, studentHomeworkNo);
}
 
 
//this method assigns student scores to a two dimensional array
 
private static void calculateGrade(int studentHomeworkNo, Scanner keyboard, int keyboardForArray) {
	int scoreAverage = keyboardForArray/studentHomeworkNo;
	
}


public static void AssignValueToArray(int studentNumber, int numberOfHomeWork){
     
    int [][] studentScore = new int [studentNumber] [numberOfHomeWork];
    Scanner keyboardForArray = new Scanner(System.in);
     
    int studCount = 1;
    for (int index = 0; index < studentNumber; index++){
         
        System.out.println("Please type in student "+studCount+" 's Score(s): ");
        for(int indexOfHomeWork = 0; indexOfHomeWork<numberOfHomeWork;indexOfHomeWork++){
            studentScore[index][indexOfHomeWork]=keyboardForArray.nextInt();
        }
        studCount++;
    }
    keyboardForArray.close();
    DisplayvalueInArray(studentScore, null);
}
//another method that displays student score in the two dimensional array
public static void DisplayvalueInArray(int[][] studentScore, String letterGrade){
    System.out.println("The Student Scores are: "+"\n");
    int studentCount = 1;
    for (int index = 0; index<studentScore.length; index++){
        System.out.print("Grades for student "+studentCount+": "+ letterGrade);
    
        for (int indexOfHomeWork = 0; indexOfHomeWork < studentScore[index].length; indexOfHomeWork++)
            System.out.print(studentScore[index][indexOfHomeWork] + "\t");
         
        System.out.println();
        studentCount++;
        
        }
}


        
public char  GradeLetter(int scoreAverage, int indexOfHomeWork){
      char letterGrade;
    	 
        //Assign a grade to the appropriate numerical value 
        //A=90-100 B=80-89 C=70-79 D=60-69 F=Below 60
        if (scoreAverage <= 100 && scoreAverage >= 90) {
        	letterGrade = 'A';
        	System.out.println(letterGrade);
        }
        else if (scoreAverage < 90 && scoreAverage >= 80) {
            letterGrade = 'B';
            System.out.println(letterGrade);
        }
        else if (scoreAverage < 80 && scoreAverage >= 70) {
        	letterGrade = 'C';
        	System.out.println(letterGrade);
        }
        else if (scoreAverage < 70 && scoreAverage >= 60) {
        	letterGrade = 'D';
        	System.out.println(letterGrade);
        } 
        else {
        	letterGrade = 'F';
        	System.out.println(letterGrade);
        }
 
        //Return the appropriate grade 
        return letterGrade;

}
}

I also want to get rid of that null from my result in the scores

Enter the number of Students: 2

Enter the number of Scores: 2
Please type in student 1 's Score(s): 
55
56
Please type in student 2 's Score(s): 
77
62
The Student Scores are: 

Grades for student 1: null55	56	
Grades for student 2: null77	62

[Knute Snortum]

When you pass in

DisplayvalueInArray(studentScore, null);

the String will evaluate to "null". What you probably want is:

DisplayvalueInArray(studentScore, "");

To get local variables from one method to another, the best way is to use a return type from the method you want a value from and pass that into an argument of the method that wants the value:

private int getValue() {
  // stuff
  return value;
}

private void useValue(int value) {
  value++;
  // other stuff
}

private void run() {
  int value = getValue();
  useValue(value);
  // or even...
  useValue( getValue() );
}

[Campbell Ritchie]

Welcome to the Ranch

Why have you marked so many methods static?

[miguel ortiz]

Thank you, I mark my methods static because is the only way everything is working so far, but I still have the same issue that I cannot calculate the grade letter and print it out with the grades on the array table