In Python:
def select(x):
y = []
for e in x:
if e!=0:
y.append(e)
return y
that works as:
x = [1,0,2,0,0,3] select(x) [1,2,3]
to be translated into Fortran:
function select(x,n) result(y)
implicit none
integer:: x(n),n,i,j,y(?)
j = 0
do i=1,n
if (x(i)/=0) then
j = j+1
y(j) = x(i)
endif
enddo
end function
The questions are in Fortran:
for 1 if it is defined as y(n) the output will be:
x = (/1,0,2,0,0,3/) print *,select(x,6) 1,2,3,0,0,0
which is not desired!
!-------------------------------
Comments:
1- All given answers are useful in this post. Specially M.S.B and eryksun's.
2- I tried to adapt the ideas for my problem and compile with F2Py
however it was not successful. I had already debugged them using GFortran and all were successful. It might be a bug in F2Py
or something that I don't know about using it properly. I will try to cover this issue in another post.
Update: A linked question could be found at here.
I hope a real Fortran programmer comes along, but in the absence of better advice, I would only specify the shape and not the size of x(:)
, use a temporary array temp(size(x))
, and make the output y allocatable
. Then after the first pass, allocate(y(j))
and copy the values from the temporary array. But I can't stress enough that I'm not a Fortran programmer, so I can't say if the language has a growable array or if a library exists for the latter.
program test
implicit none
integer:: x(10) = (/1,0,2,0,3,0,4,0,5,0/)
print "(10I2.1)", select(x)
contains
function select(x) result(y)
implicit none
integer, intent(in):: x(:)
integer:: i, j, temp(size(x))
integer, allocatable:: y(:)
j = 0
do i = 1, size(x)
if (x(i) /= 0) then
j = j + 1
temp(j) = x(i)
endif
enddo
allocate(y(j))
y = temp(:j)
end function select
end program test
Edit:
Based on M.S.B.'s answer, here's a revised version of the function that grows temp y
with over-allocation. As before it copies the result to y at the end. It turns out i's not necessary to explicitly allocate a new array at the final size. Instead it can be done automatically with assignment.
function select(x) result(y)
implicit none
integer, intent(in):: x(:)
integer:: i, j, dsize
integer, allocatable:: temp(:), y(:)
dsize = 0; allocate(y(0))
j = 0
do i = 1, size(x)
if (x(i) /= 0) then
j = j + 1
if (j >= dsize) then !grow y using temp
dsize = j + j / 8 + 8
allocate(temp(dsize))
temp(:size(y)) = y
call move_alloc(temp, y) !temp gets deallocated
endif
y(j) = x(i)
endif
enddo
y = y(:j)
end function select