How to complete frequency table in java

Question

I am having trouble with my code in java. I am supposed to enter "grades" while in a loop (my exit condition is -1) and put it into a frequency table, the problem is I get this error:

java.lang.ArrayIndexOutOfBoundsException: -1
    at MakeUp.main(MakeUp.java:36)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at      edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:267)

Here is my code:

class MakeUp
{
public static void main (String[] args)
{
  int grade[];
  int freq[]=new int [50];
  grade= new int [50];
  int count=0,mark;

  System.out.println("Enter the grades of the class");
  System.out.println("Just enter -1 to end the input");


  do
      { 
    System.out.println("Grade " +(count+1)+":");
    mark=In.getInt();
    if (mark>=0)
    {
     grade[count]=mark; 
     count=count+1;
    }
  }
  while(mark!=-1);


     for (int i=0; i < grade.length;i++)   
      { 
     grade[i]=mark;
   }

  if (mark==-1)
  {
  for (int i=0; i<grade.length;i++)
  { 
   ++freq[grade[i]];
  }
  System.out.println("Results"); 

  for (int i=0; i<grade.length;i++)
  {
    System.out.println("Grade " +(i+1) + " = " +grade[i]);
  }

System.out.println("Face\tFrequency\"");
System.out.println("=====================");
  for (int i = 0; i<freq.length; i++)
  {
  System.out.println((i+1)+"\t"+freq[i]); 
  }
  }  
  }  
 }

I'm not sure what the problem is and I have tried to change line 35 which has the problem but to no avail. Any help will be much appreciated. Thanks

Answer 1

The following loop fills your grade array with -1 values (since mark = -1, which is the exit condition of the do-while loop that was executed before) :

for (int i = 0; i < grade.length; i++) {
            grade[i] = mark;
        }

Here any grade[i] = -1 , so you are attempting to access the index -1 from your freqs array :

++freq[grade[i]];