obsolete go to statements

[starlite79]

Hi,

I am trying to modify code that is using go to statements everywhere. I want to use something more standard and also easier to read, but I am giving myself a headache trying to sort this out. Here is what I have:

      IMPLICIT NONE

      INTEGER i,igap,iex,ik,imax,ipl,j,k,l,n
      DOUBLE PRECISION sv,x(n),k(n)
*      dimension x(n),k(n)
      igap = n

      DO 1 i = 1,n
 1    k(i) = i
* 5    if(igap.le.1) go to 25
      IF(igap.le.1) THEN

*  now sort k's (for identical values of x, if any)
         j = 1 
      ELSE
         igap = igap/2
         imax = n - igap
      END IF
 10   iex = 0
      do 20 i = 1,imax
      ipl = i + igap
      IF(x(i).LE.x(ipl)) GO TO 20
      sv = x(i)
      ik = k(i)
      x(i) = x(ipl)
      k(i) = k(ipl)
      x(ipl) = sv
      k(ipl) = ik
      iex = iex + 1
 20   CONTINUE
      if(iex.gt.0) go to 10
      go to 5
c
c  now sort k's (for identical values of x, if any)
c
 25   j = 1
 30   if(j.ge.n) return
      if(x(j).eq.x(j+1)) go to 33
      j = j + 1
      go to 30
c  have at least two x's with the same value - see how long this is true
 33   l = j
 35   if(x(l).ne.x(l+1)) go to 38
      l = l + 1
      if(l.lt.n) go to 35
c  j and l are the indices within which x(i) does not change - sort k
 38   igap = l - j + 1
 40   if(igap.le.1) j = l + 1
      if(igap.le.1) go to 30
      igap = igap/2
      imax = l-j+1 - igap
 45   iex = 0
      do 50 i=1,imax
      ipl = i + igap + j - 1
      if(k(i+j-1).le.k(ipl)) go to 50
      ik = k(i+j-1)
      k(i+j-1) = k(ipl)
      k(ipl) = ik
      iex = iex + 1
 50   continue
      if(iex.gt.0) go to 45
      go to 40

I started to rewrite using IF ELSE statements, but I am not sure when one if-else-if should end and another begins. Code someone offer help to get me started?

 

[mikrom]

It's nor easy to translate the old spaghetti code into structured code. You need to know what the program should do.

However, formally you can replace e.g

do <label_nr> <index=from, to>
<do_loop_body>
<label_nr> continue

with this

do <label_nr> <index=from, to>
  <do_loop_body>
end do

or this

if(<condition>) go to <label_nr>
<if_body>
<label_nr> <statement>

with this

if (.not.(<condition>)) 
  <if_body>
end if
<statement>

but without understanding exactly what the program should do, it's IMHO dangerous task.

 

[xwb]

Easiest way is to draw an old fashioned flowchart, pretend the start and end are two ends of a bit of string and pull it straight. Once you see the structure in a flowchart, it is quite easy to rewrite it.

Looks like a Shell sort (Shell-Metzner if you have older books). Have a look at

http://rosettacode.org/wiki/Shell_sort.

The increment in the URL is the same as the gap variable in your cod.e

 

[starlite79]

xwb,

It is probably not a coincidence that the subroutine I'm trying to modify is named shells! I will see what I can do with the link you provided.