Debugging Merge Sort Program with Fortran 90

[Soff]

I'm trying to write a Merge Sort program with fortran 90. However, this is what I already could do:

program sorting

integer :: i,ialloc,error,n,d
integer, pointer :: a(:)

Print *,'How many numbers would you like to type in?'
Read(*,*)d

Print*,' Enter the values, separated by commas'

Read(*,*)(a(t),t=1, d)

!Result
  call Merge(a,b,c,m,n)
    print *,'Sorted list:'
    write(*,*) (a(i),i=0,n-1)
  print *,' '

end program sorting

!Merge Sorting of a() of dimension m and b() of dimension n
!into c() of size m+n=d
Subroutine Merge(a,b,c,m,n)

integer :: m,n,a(0:m-1),b(0:n-1),c(0:m+n-1)
integer :: i,j,k,d
  i=0; j=0; k=0 !Start
  do while (i<m.and.j<n)
    if (a(i)<b(j)) then
      c(k)=a(i)
      i=i+1
    else
      c(k)=b(j)
      j=j+1
    end if
    k=k+1
  end do
  do while (i<m)  !pick up any remainder
    c(k)=a(i)
    k=k+1; i=i+1
  end do
  do while (j<n)
    c(k)=b(j)
    k=k+1; j=j+1
  end do
  return
End

It starts as it was planed, but after typing in the numbers, I get an error message. So, can anyone help me? Where is the bug?

 

[Soff]

Okay, I started again. This is my improved version (but still not working!):

program sorting

integer :: d,length,center,t,i,n,m
integer :: a(128), b(128)

!List of numbers
Print *,'How many numbers would you like to type in?'
Read(*,*)d

Print*,' Enter the values, separated by commas'
Read(*,*)(a(t),t=1, d)


length=d

i=1
do while (length>=2)
	
	length=length/2
	i=i+1
end do

write(*,*)i,length
n=1


do while (n<=i)
	if (a(2*n)>a(2*n-1)) then
		b(2*n)=a(2*n)
		b(2*n-1)=a(2*n-1)
	else 	
		b(2*n)=a(2*n-1)
		b(2*n-1)=a(2*n)	
	end if
	n=n+1
end do

write(*,*)(b(t),t=1,d)

a(i)=b(i)

n=1
m=1
length=length*2

do while (n<=i/2)
	do while (m<=length-1.and.a(length*n)>=a(length*n-m))
		m=m+1
	end do
	b(n*length-length-1-m)=a(length*n)
	do while (m<=length-1.and.a(length*n-1)>=a(length*n-m))
		m=m+1
	end do
	b(n*length-length-1-m)=a(length*n-1)
	do while (m<=length-1.and.a(length*n-2)>=a(length*n-m))
		m=m+1
	end do
	b(n*length-length-1-m)=a(length*n-1)
	do while (m<=length-1.and.a(length*n-3)>=a(length*n-m))
		m=m+1
	end do
n=n+1
end do

write(*,*)(b(t),t=1,d)
end program

 

[rcgldr]

In the first example, you're not sorting but merging "a" and "b" to create "c". "b" is never initialized, and you're printing "a", instead of the output, "c".

Sort algorithms (plus links to zips of source code) were discussed in this thread:

efficient sorting

Here is a direct link to the sorts that sort an array (vector) of 64 bit integers, msort.cpp is a merge sort.

sortv.zip

My merge sort is pointer oriented, if I get the time, I'll make a duplicate that uses indexes instead.