Lower triangular matrix and upper triangular matrix give me wrong answer

Question

I'm working in LU Decomposition in C.My code is very simple Algorithm can be parallelized using two loops one for updating lower triangular matrix and one for updating upper triangular matrix ,but it seems I miss understand something :(

      for (i=0 ; i<N ; i++){
 // A[i][i]=1;
  for (j=i+1 ;j<N ;j++){
      A[j][i] = A[j][i]/A[i][i]; //*Update L*//
  }
  for (j=i+1;j<N;j++){
      for(k=i+1 ;k<N;k++){

          A[j][k] = A[j][k] - A[i][k] * A[j][i];//*Update U*//
       }
    }
 }

  printf("\n Matrix after U transformation: \n");
  print_matrix(); 

for (i=0 ; i<N ; i++){
   A[i][i]=1;
  for (j=i+1 ;j<N ;j++){
      A[j][i] = A[j][i]/A[i][i]; //*Update L*//
  }
  for (j=i+1;j<N;j++){
      for(k=i+1 ;k<N;k++){

          A[j][k] = A[j][k] - A[i][k] * A[j][i];//*Update U*//
         }
      }
     }

     printf("\n Matrix after L transformation: \n");
     print_matrix(); 

This is what I should to get ?! what I'm doing wrong

L =

1.0000         0         0         0         0
0.2000    1.0000         0         0         0
0.2000    0.1667    1.0000         0         0
0.2000    0.1667    0.1429    1.0000         0
0.2000    0.1667    0.1429    0.1250    1.0000


U =

 50.0000   10.0000   10.0000   10.0000   10.0000
     0   48.0000    8.0000    8.0000    8.0000
     0         0   46.6667    6.6667    6.6667
     0         0         0   45.7143    5.7143
     0         0         0         0   45.0000

but what I got is ,,,,L not should be like this ?!

Source Matrix :
50      10      10      10      10
10      50      10      10      10
10      10      50      10      10
10      10      10      50      10
10      10      10      10      50

Matrix after U transformation: 
 50      10      10      10      10
  0      48       8       8       8
  0       0      47       7       7
  0       0       0      46       6
  0       0       0       0      45

 Matrix after L transformation: 
   1      10      10      10      10
   0       1       6       6       6
   0      -2       1      16      16
   0      -2       9       1    -129
   0      -2       9    -134       1

Thanks

You posted your code and told us what you should be getting, but didn't tell us what you are getting (or how it differs from the expected answer). — LarsH, Apr 20 '12 at 04:56
What do you think? There are several things so obviously wrong with your code that I have to wonder how much effort you've actually put into this. — Jeffrey Sax, Apr 20 '12 at 05:48
I worked very hard on it ,if I submit one vesriob that doesn't mean I don't keep tying thanks for your comment. — Malak ALkhthlan, Apr 20 '12 at 13:01
Look at http://apps.nrbook.com/c/index.html page 44 for a reference on how to code LU decomposition. — John Alexiou, Apr 23 '12 at 13:12

score 0 · Answer 1 · edited May 23 '17 at 12:21

0

Your U matrix is correct, beside that these are integers and not floats. The diagonal of your L matrix also is correct (you're setting it's values), but not the rest. After checking the code against the answer of "LU Decomposition from Numerical Recipes not working; what am I doing wrong?", which is (changed it a bit, added some braces):

for (i = 0; i < N; i++) {
    // compute U
    for (j = i; j < N; j++) {
        for (k = 0; k < i-2; k++) {
            A[i,j] -= A[i,k] * A[k,j];
        }
    }

    // compute L
    for (j = i+1; j < N; j++) {
        for (k = 0; k < i-2; k++) {
            A[j,i] -= A[j,k] * A[k,i];
        }
    }
}

I noticed that you are missing a loop in your code, which should be the problem. Also take a look at the mentioned SO question which provides some more useful hints.

edited May 23 '17 at 12:21

Community

1
1

answered Apr 20 '12 at 16:12

Sebastian

8,046
2
34
58

Thanks for yor replay.I'm looking for parller code to do the iterations,similer to this one http://www.hipc.org/hipc2011/studsym-papers/1569512927.pdf ,but unfortunately it is not working as I expect ,I still get the L part in wrong way. – Malak ALkhthlan Apr 20 '12 at 22:43
Well then: Grab yourself pencil and paper, choose a small matrix and do it by yourself to see where the error is. – Sebastian Apr 21 '12 at 06:39

Lower triangular matrix and upper triangular matrix give me wrong answer

1 Answers1