Possible Duplicate:
Which of these two for loops is more efficient in terms of time and cache performance

Below are two programs that are almost identical except that I switched the i and j variables around. They both run in different amounts of time. Could someone explain why this happens?

Version 1

#include <stdio.h>
#include <stdlib.h>

main () {
  int i,j;
  static int x[4000][4000];
  for (i = 0; i < 4000; i++) {
    for (j = 0; j < 4000; j++) {
      x[j][i] = i + j; }
  }
}

Version 2

#include <stdio.h>
#include <stdlib.h>

main () {
  int i,j;
  static int x[4000][4000];
  for (j = 0; j < 4000; j++) {
     for (i = 0; i < 4000; i++) {
       x[j][i] = i + j; }
   }
}

As others have said, the issue is the store to the memory location in the array: x[i][j]. Here's a bit of insight why:

You have a 2-dimensional array, but memory in the computer is inherently 1-dimensional. So while you imagine your array like this:

0,0 | 0,1 | 0,2 | 0,3
----+-----+-----+----
1,0 | 1,1 | 1,2 | 1,3
----+-----+-----+----
2,0 | 2,1 | 2,2 | 2,3

Your computer stores it in memory as a single line:

0,0 | 0,1 | 0,2 | 0,3 | 1,0 | 1,1 | 1,2 | 1,3 | 2,0 | 2,1 | 2,2 | 2,3

In the 2nd example, you access the array by looping over the 2nd number first, i.e.:

x[0][0] 
        x[0][1]
                x[0][2]
                        x[0][3]
                                x[1][0] etc...

Meaning that you're hitting them all in order. Now look at the 1st version. You're doing:

x[0][0]
                                x[1][0]
                                                                x[2][0]
        x[0][1]
                                        x[1][1] etc...

Because of the way C laid out the 2-d array in memory, you're asking it to jump all over the place. But now for the kicker: Why does this matter? All memory accesses are the same, right?

No: because of caches. Data from your memory gets brought over to the CPU in little chunks (called 'cache lines'), typically 64 bytes. If you have 4-byte integers, that means you're geting 16 consecutive integers in a neat little bundle. It's actually fairly slow to fetch these chunks of memory; your CPU can do a lot of work in the time it takes for a single cache line to load.

Now look back at the order of accesses: The first example is (1) grabbing a chunk of 16 ints, (2) modifying all of them, (3) repeat 4000*4000/16 times. That's nice and fast, and the CPU always has something to work on.

The second example is (1) grab a chunk of 16 ints, (2) modify only one of them, (3) repeat 4000*4000 times. That's going to require 16 times the number of "fetches" from memory. Your CPU will actually have to spend time sitting around waiting for that memory to show up, and while it's sitting around you're wasting valuable time.

Important Note:

Now that you have the answer, here's an interesting note: there's no inherent reason that your second example has to be the fast one. For instance, in Fortran, the first example would be fast and the second one slow. That's because instead of expanding things out into conceptual "rows" like C does, Fortran expands into "columns", i.e.:

0,0 | 1,0 | 2,0 | 0,1 | 1,1 | 2,1 | 0,2 | 1,2 | 2,2 | 0,3 | 1,3 | 2,3

The layout of C is called 'row-major' and Fortran's is called 'column-major'. As you can see, it's very important to know whether your programming language is row-major or column-major! Here's a link for more info: http://en.wikipedia.org/wiki/Row-major_order

Assuming a student take 6 courses in a semester. All those couses have coures units(int), and depending on the score in each course there are points..

 so a score >=70 will have a point of 5

 <70 and >=60 will have a ponit of 4

and so on. For each course unit and point are multipied together, down the column for each column. Now when the score of a course is not found the grade is 'AR'. Now what i want is for the loops to omit the occurence of AR..i.e not adding the course unit of the course having a grade of 'AR'. But when i run my queries above the units still add to the total course units.

Query4 is used to generate some rows of course_unit and Score

  $query4 = mysql_query("SELECT  c.course_unit, m.score
  FROM    maintable AS m
  INNER JOIN students AS s ON
  m.matric_no = s.matric_no
  INNER JOIN courses AS c ON
  m.course_code = c.course_code
  WHERE m.matric_no = '".$matric_no."'
  AND m.level = '".$level."'")
  or die (mysql_error());

Query3 is used for the summation of the course_units

 $query3 = mysql_query("SELECT  SUM(c.
 course_unit) AS 'TOTAL'
 FROM    maintable AS m
 INNER JOIN students AS s ON
 m.matric_no = s.matric_no
 INNER JOIN courses AS c ON
 m.course_code = c.course_code
 WHERE m.matric_no = '".$matric_no."'
 AND m.level = '".$level."'")
 or die (mysql_error());

Grades in Respect to Score

 while ($row8 = mysql_fetch_assoc
 ($query8)) {
            if ($row8['score'] >= 70) {
              $grade = 'A';
            }
            elseif ($row8['score'] >= 60) {
               $grade = 'B';
            }elseif ($row8['score'] >= 50) {
               $grade = 'C';
            }elseif ($row8['score'] >= 45) {
               $grade = 'D';
            }elseif($row8['score'] >= 40) {
               $grade = 'E';
            }elseif($row8['score'] >= 0) &&
            ($row8['score'] < 40){
               $grade = 'F';
            }else{
               $grade = 'AR';
            }   
     }   

Calculation of the Grade Point

      $grade_point = 0;
      while ($row4 = mysql_fetch_assoc($query4)) {
         if ($row4['score'] >= 70) {
            $score = 5;
          }
          elseif ($row4['score'] >= 60) {
             $score = 4;
          }elseif ($row4['score'] >= 50) {
             $score = 3;
          }elseif ($row4['score'] >= 45) {
             $score = 2;
          }elseif($row4['score'] >= 40) {
             $score = 1;
          }elseif($row4['score'] >= 0 AND                       $row4['score'] < 40) {
             $score = 0;
          }else{
             $score = 0;
          } 

          $grade_point += $score * $row4['course_unit'];

      }

I have added

  if ( $grade == 'AR' )
  {
       continue;
  }

But the calculations are still the same. It adds the course_unit value of any course having

$grade == 'AR' .

I'll be most delighted with you answers. Thanks very much.

UPDATE

I have being able to solve the grade piont part by adding

     elseif($row4['score'] >= 0 AND                       $row4['score'] < 40) {
             $score = 0;
          }else{
             $score = 0;
          }

This sets both the occurences of a score between 0 and 39 to zero and also the default score of <0 (i.e AR) to zero. But it still set's the value of the courses having a grade of AR and a score of -1 to the default respective values of the course_unit.

I think this problem is being cause due to the fact that the course_unit are preloaded from the database. Any help?

Courses Table Stucture
=================

course_id
course_code
course_title
course_unit

I'll be most delighted with your answers. Thank you in anticipation.

Is it as simple as adding "AND NOT 'AR'" to your SELECT SUM statement?

Or... if your DB values are coming in as AR, why can't you use PHP is_int() in your loop? That would allow you to still assign 0 for F, and just skip over any non integer values being sent from your DB.