For-lups problem

Question

I've made a program which calculates studensts' math and science scores.
In the program, I put name, scores of math and science, and then the code calculates sum, average of the scores, and their standings.
But I had a problem of standings.
import java.io.*;
class Scores
     public static void main(String[] args) throws Exception
     BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
     int[] math = new int[3]; //an array of math score
     int[] science = new int[3]; // an array of science score
     int[] sum = new int[3]; //an array of sum
     double[] avg = new double[3]; //an array of average
     String[] name = new String[3]; //an array of students name
     int[][] student = new int[3][3]; // an array of students
     int[] num = new int[3]; // an array of number
     int[] standing = new int[3]; //an array of students' standing
     for (int i=0; i<student.length; i++)
          System.out.print("Your name: ");
          name=in.readLine(); // put your name here
          System.out.print("Your Math grade: ");
          math[i]=Integer.parseInt(in.readLine()); //put your math score here
          System.out.print("Your Science grade: ");
          science[i]=Integer.parseInt(in.readLine()); //put your science score here
          sum[i]=math[i]+science[i];
          avg[i]=sum[i]/2.0;
          num[i]=i+1; // in order to put number from 1 to 3 to each student
     for (int i=0; i<student.length; i++) // calculating students' standing
          int rank=student.length; //rank starting with 3
          for (int j=0; j<student[i].length; j++)
               if(avg[i]>avg[j])
                    standing[i]=rank-1;
               else if(avg[i]<=avg[j])
                    standing[i]=rank;
     System.out.println();
     for (int i=0; i<student.length; i++)
          System.out.println("Number\tName\tMath\tScience\tSum\tAvg\tStanding");
          System.out.println(num[i]+"\t"+name[i]+"\t"+math[i]+"\t"+science[i]+"\t"+sum[i]+"\t"+avg[i]+"\t"+standing[i]);
          System.out.println();

Answer 1

The style of your code gives me the feeling that you are somewhat new to programming in general, so I'm going to give you some general debugging tips before actually answering your question.
The best way to find an error is to take some test data and go through the code yourself as if you were the computer and look at what it's actually doing (which may or may not be what you intended for it to do) at each step. I usually make a table of all the relevant variables, and fill in a new line each time any of them changes.
Here's the relevant portion of your code, cleaned up a bit (the missing [ i ]'s replaced, and tabs inserted in appropriate places) and line numbers added for reference.
1  for (int i=0; i<student.length; i++) // calculating students' standing
2  {
3     int rank=student.length; //rank starting with 3
4     for (int j=0; j<student.length; j++)
5     {
6          if(avg>avg[j])
7          {
8               standing[i]=rank-1;
9          }
10          else if(avg[i]<=avg[j])
11          {
12               standing[i]=rank;
13          }
14     }
15 }
at each step in the code, the variables I'll want to keep track of are:
i, name[ i ], avg[ i ], standing[ i ], rank, j, name[j], avg[j] (I included name[ i ] and name[j] just to make it easier to see which students we are talking about in each case. Technically, i and j would be sufficient.)
Let's say our three students' averages are as follows:
[0] John = 75
[1] Jane = 70
[2] Jim = 65
Let's skip straight to line 4 on our first iteration through the outer loop, our values are:
i = 0
name[i] = John
avg[i] = 75
standing[i] = 0
rank = 3
j = 0
name[j] = John
avg[j] = 75Since 75 <= 75, we skip straight to line 12:
i = 0
name[i] = John
avg[i] = 75
standing[i] = 3
rank = 3
j = 0
name[j] = John
avg[j] = 75Then go back to line 4:
i = 0
name[i] = John
avg[i] = 75
standing[i] = 3
rank = 3
j = 1
name[j] = Jane
avg[j] = 70Since 75 > 70, we go to line 8:
i = 0
name[i] = John
avg[i] = 75
standing[i] = 2
rank = 3
j = 1
name[j] = Jane
avg[j] = 70Back to line 4:
i = 0
name[i] = John
avg[i] = 75
standing[i] = 2
rank = 3 // note, rank has not been altered
         // this is the root of the problem
j = 2
name[j] = Jim
avg[j] = 6575 > 65 so to line 8:
i = 0
name[i] = John
avg[i] = 75
standing[i] = 2 // since rank was never changed to reflect
                // the comparison to Jane, the comparison
                // to Jim is using the original value of 3,
                // not 2 as you probably intended
rank = 3
j = 2
name[j] = Jim
avg[j] = 65Better would be:
1  for (int i=0; i<student.length; i++) // calculating students' standing
2  {
3     int rank=student.length; //rank starting with 3
4     for (int j=0; j<student.length; j++)
5     {
6          if(avg>avg[j])
7          {
8               rank--; // decrement rank here
9          }
10     }
11     standing[i]=rank; // once your final rank is established, assign it to standing
12 }
Other comments that might clean up your code a bit:
* I wouldn't use a separate variable where you have num -- just use i+1 directly wherever you would use it. (I only see one place it's even used.)
* A better overall design might be to create a Student class, with instance variables such as name, math, science, etc. and then create and use an array of Student objects, rather than keeping several parallel arrays. This has the added advantage that if your number of students changes, you only have to change the size of one array. (Keeping track of parallel arrays can be a royal pain in larger applications. You're better off getting out of the habit of using them early, IMO.)
* Do you really need to store both the sum and the average of the two scores? That seems a bit redundant to me. Storing the average might make it easier when you go to calculate ranks so you don't have to calculate it each time, but what is the advantage of keeping the sum around? When you print the report, you can just put (math + science) there. The only reason I could see to actually store sum would be for readability & efficiency if it's used in several different calculations, but here it's used exactly twice -- to calculate the average and to display it. If you were to create a separate student object, you could simply store the individual scores, and write methods to return the sum and average, then it could be transparent to the program whether you are actually storing the numbers or calculating them each time.
* In your final report, do you really want to put your column headers [i]inside your for loop?  It makes more sense to me to print them once, before you enter the loop, rather than printing them for each student.
Lisa