INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Help understanding code!

Help understanding code!

(OP)
Hi, I am new with Fortran and have some understanding of what this code does but would anyone be able to describe what is going on inside each loop? Thanks!

SUBROUTINE gaussj(a,n,np,b,m,mp)
INTEGER m,mp,n,np,NMAX
REAL a(np,np),b(np,mp)
PARAMETER (NMAX=50)
INTEGER i,icol,irow,j,k,l,ll,indxc(NMAX),indxr(NMAX),ipiv(NMAX)
REAL big,dum,pivinv
do 11 j=1,n
ipiv(j)=0
11 continue
do 22 i=1,n
big=0.
do 13 j=1,n
if(ipiv(j).ne.1)then
do 12 k=1,n
if (ipiv(k).eq.0) then
if (abs(a(j,k)).ge.big)then
big=abs(a(j,k))
irow=j
icol=k
endif
else if (ipiv(k).gt.1) then
pause 'singular matrix in gaussj'
endif
12 continue
endif
13 continue
ipiv(icol)=ipiv(icol)+1
if (irow.ne.icol) then
do 14 l=1,n
dum=a(irow,l)
a(irow,l)=a(icol,l)
a(icol,l)=dum
14 continue
do 15 l=1,m
dum=b(irow,l)
b(irow,l)=b(icol,l)
b(icol,l)=dum
15 continue
endif
indxr(i)=irow
indxc(i)=icol
if (a(icol,icol).eq.0.) pause 'singular matrix in gaussj'
pivinv=1./a(icol,icol)
a(icol,icol)=1.
do 16 l=1,n
a(icol,l)=a(icol,l)*pivinv
16 continue
do 17 l=1,m
b(icol,l)=b(icol,l)*pivinv
17 continue
do 21 ll=1,n
if(ll.ne.icol)then
dum=a(ll,icol)
a(ll,icol)=0.
do 18 l=1,n
a(ll,l)=a(ll,l)-a(icol,l)*dum
18 continue
do 19 l=1,m
b(ll,l)=b(ll,l)-b(icol,l)*dum
19 continue
endif
21 continue
22 continue
do 24 l=n,1,-1
if(indxr(l).ne.indxc(l))then
do 23 k=1,n
dum=a(k,indxr(l))
a(k,indxr(l))=a(k,indxc(l))
a(k,indxc(l))=dum
23 continue
endif
24 continue
return
END

RE: Help understanding code!

It is very old (possibly Fortran IV) code for possibly solving an equation using gauss jordan (the subroutine name is gaussj). Try this

1) find a simultaneous equation where you know the answer
2) create a program that calls this subroutine and prints the results
3) stick in the figures
4) check if the result is what you expect it to be
5) re-run it in a debugger and see what it does by yourself

That will help you understand the program

RE: Help understanding code!

Oh, please, do not exagerate; I don't think this is Fortran IV...not with "then - endif". Fortran IV only had logical and numerical 'if'

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close