[Fortran] Passing elements of array to subroutine

[VasanthG]

I have one main program where I have calculated some array, say A(i,j)
If I want ot use the values in subroutine, and calculate some more values and give back the result to main program, how my syntax should be?
I tried this way. Its not working.
program a
......
call sub(A,B,C)
end program a

subroutine (X,Y,Z)
allocated memory same as A,B,C to X,Y,Z
some formulae
return
end subroutine sub

Thank you.

 

[uart]

Hi Vasanth, it's a good idea to specify what programming language you are using when asking a question. But let me guess it's fortran.

By default Fortran passes all parameters by "reference". This means that when you pass the array to the subroutine it merely passes a reference to the actual same array in the main program. So any changes made in the subroutine are automatically reflected in the variable back in the main program. Also there is no need to allocate memory into it in the subroutine.

 

[uart]

double post

 

[VasanthG]

Thank you so much. Yeah I am using FORTRAN.. sorry I missed to mention.
say do 10 i=1,10
do 10 j=1,10
A(i,j)=i+j
10 continue

This 10 cross 10 values must go to subroutine for a formula which uses
say g(i,j)=A(i,j)+2 and my o/p should be values of g(i,j)

For this case, if I am not allocating the memory for dummy variable, its showing errors of rank mismatch.
I am new to subroutines. If I am doing basic mistakes, pls correct me.
Thanks again.

 

[uart]

Thank you so much. Yeah I am using FORTRAN.. sorry I missed to mention.
say do 10 i=1,10
do 10 j=1,10
A(i,j)=i+j
10 continue

This 10 cross 10 values must go to subroutine for a formula which uses
say g(i,j)=A(i,j)+2 and my o/p should be values of g(i,j)

For this case, if I am not allocating the memory for dummy variable, its showing errors of rank mismatch.
I am new to subroutines. If I am doing basic mistakes, pls correct me.
Thanks again.

Yes you still need to define the array in the subroutine, but any modifications to that array will automatically be reflected as changes to the original array in the main program.

program test
  implicit none
  integer a(20)
  ...
  call example(a)
  ...

contains
subroutine example(x)
 implicit none
 integer x(20)
 ...           ! Changes to x() here cause changes in a() in the main program
end subroutine example
end program test

BTW. Whether or not it actually passes by reference or just uses "copyback" is implementation dependent. But in any case the "side effect" on "x" in the subroutine will change the values of "a" in the main.

 

[VasanthG]

Thank you so much for your time and help. I'll implement it in my code and check.

 

[jtbell]

Thread title edited.