if (mod(n,2) == 0) then
write(stdout, *) "Even"
else
write(stdout, *) "Odd "
end if
I want to print Even Odd Even Odd.
Why can't I use the $ descriptor for this?
There is no $ descriptor in your code!
But no, there is no $ descriptor in standard Fortran. Some compilers have it as an extension and other do not. Don't use it if you want portable programs.
Fortran 90 standard conforming way is using the non-advancing O/O with advance='no'. Note that it requires a format, not list-directed input.
write(stdout, some_format, advance='no') "Even"
some_format can be '(a)' or '(g0)' or whatever applicable.
Also, for a caveat see http://www.cs.rpi.edu/~szymansk/OOF90/bugs.html#3 (summary: there can still be a record length limit).
Related
I have a project written in VS2010 with Intel Visual Fortran. I have a dump subroutine to write a 2D matrix into file:
subroutine Dump2D(Name,Nx,Ny,Res)
implicit none
integer i,j,Nx,Ny
real(8) :: Res(Nx,Ny)
character(len=30) name,Filename
logical alive
write(filename,*) trim(Name),".dat"
Write(*,*) "Saving ",trim(Name)," Please wait..."
open (10,file=filename)
do i=1,Ny
Write(10,FMt="(D21.13\)") (Res(j,i),j=1,Nx)
Write(10,*)
end do
close(10)
Write(*,*) "Save ",trim(Name),"Complete!"
return
end subroutine Dump2D
It is ok to compile and run. But when I compile in emacs using gfortran it gives me the error:
I think it's because the gfortran doesn't recognize \ in a format for a write command. How do I fix this problem?
Write(10,FMt="(D21.13\)") (Res(j,i),j=1,Nx)
1
Error: Unexpected element '\' in format string at (1)
The edit descriptor \ relates to backslash editing. This is a non-standard extension provided by the Intel compiler (and perhaps others). It is not supported by gfortran.
Such backslash editing is intended to affect carriage control. Much as in this answer such an effect can be handled with the (standard) non-advancing output.1
As you simply want to output each column of a matrix to a record/line you needn't bother with this effort.2 Instead (as you'll see in many other questions):
do i=1,Ny
write(10,fmt="(*(D21.13))") Res(:,i)
end do
There are also other approaches which a more general search will find.
1 The Intel compiler treats \ and $ in the same way.
2 There are subtle aspects of \, but I'll assume you don't care about those.
Another approach (although francescalus answer is better in your case) would be to build a format string that contains the number of elements to include in your row. One way of doing this is to use the following to build the format string (which uses an explicit space character to separate elements within a line in the file):
WRITE(fmtString, '(A,I0,A)') '(', Nx, '(D21.13,:,1X))' *
Then use the format string variable in your WRITE statement as so:
do i=1,Ny
Write(10,FMt=fmtString) (Res(j,i),j=1,Nx)
end do
This approach can also be very useful if you want to use something other than spaces to separate elements (e.g. commas or semicolons).
*As that's a little difficult to read, I will provide an example. For Nx = 3, this would be equivalent to:
fmtString = '(3(D21.13,:,1X))'
Which is 2 numbers formatted using D21.13, each followed by a space, and a final number formatted using D21.13, but without a space after it (as the ":" stops at the final item).
The backslash is not valid in Fortran 77 FORMAT statements. Gfortran will not compile it, unless you fix the code. There is no flag that will change that AFAIK (-fbackslash should not help here).
If I understand the intention correctly (and I may be wrong), the backslash does the same as the dollar sign in some other compilers and prevents terminating a record (line). In that case the advance="no" put in the write statement should help. It is Fortran 90, but you should not avoid it just for that reason.
I have a project written in VS2010 with Intel Visual Fortran. I have a dump subroutine to write a 2D matrix into file:
subroutine Dump2D(Name,Nx,Ny,Res)
implicit none
integer i,j,Nx,Ny
real(8) :: Res(Nx,Ny)
character(len=30) name,Filename
logical alive
write(filename,*) trim(Name),".dat"
Write(*,*) "Saving ",trim(Name)," Please wait..."
open (10,file=filename)
do i=1,Ny
Write(10,FMt="(D21.13\)") (Res(j,i),j=1,Nx)
Write(10,*)
end do
close(10)
Write(*,*) "Save ",trim(Name),"Complete!"
return
end subroutine Dump2D
It is ok to compile and run. But when I compile in emacs using gfortran it gives me the error:
I think it's because the gfortran doesn't recognize \ in a format for a write command. How do I fix this problem?
Write(10,FMt="(D21.13\)") (Res(j,i),j=1,Nx)
1
Error: Unexpected element '\' in format string at (1)
The edit descriptor \ relates to backslash editing. This is a non-standard extension provided by the Intel compiler (and perhaps others). It is not supported by gfortran.
Such backslash editing is intended to affect carriage control. Much as in this answer such an effect can be handled with the (standard) non-advancing output.1
As you simply want to output each column of a matrix to a record/line you needn't bother with this effort.2 Instead (as you'll see in many other questions):
do i=1,Ny
write(10,fmt="(*(D21.13))") Res(:,i)
end do
There are also other approaches which a more general search will find.
1 The Intel compiler treats \ and $ in the same way.
2 There are subtle aspects of \, but I'll assume you don't care about those.
Another approach (although francescalus answer is better in your case) would be to build a format string that contains the number of elements to include in your row. One way of doing this is to use the following to build the format string (which uses an explicit space character to separate elements within a line in the file):
WRITE(fmtString, '(A,I0,A)') '(', Nx, '(D21.13,:,1X))' *
Then use the format string variable in your WRITE statement as so:
do i=1,Ny
Write(10,FMt=fmtString) (Res(j,i),j=1,Nx)
end do
This approach can also be very useful if you want to use something other than spaces to separate elements (e.g. commas or semicolons).
*As that's a little difficult to read, I will provide an example. For Nx = 3, this would be equivalent to:
fmtString = '(3(D21.13,:,1X))'
Which is 2 numbers formatted using D21.13, each followed by a space, and a final number formatted using D21.13, but without a space after it (as the ":" stops at the final item).
The backslash is not valid in Fortran 77 FORMAT statements. Gfortran will not compile it, unless you fix the code. There is no flag that will change that AFAIK (-fbackslash should not help here).
If I understand the intention correctly (and I may be wrong), the backslash does the same as the dollar sign in some other compilers and prevents terminating a record (line). In that case the advance="no" put in the write statement should help. It is Fortran 90, but you should not avoid it just for that reason.
I am learning Fortran because well, yeah, thought I'd learn it. However, I've found utterly no information on what the purpose of * is in print, read, etc:
program main
print *, "Hello, world!"
end program main
What is the purpose of this *? I've done some research however I don't think I understand it properly, I don't want to carry on learning until I actually know the point of *.
From what I've managed to find I think that it's some sort of format specifier, however, I don't understand what that means and I have no idea if I'm even correct. All the tutorials I've found just tell me what to write to print to the console but not what * actually means.
You are correct in that it is a format specifier.
There's a page on Wikibooks to do with IO and that states:
The second * specifies the format the user wants the number read with
when talking about the read statement. This applies to the write and print statements too.
For fixed point reals: Fw.d; w is the total number of spaces alloted for the number, and d is the number of decimal places.
And the example they give is
REAL :: A
READ(*,'(F5.2)') A
which reads a real number with 2 digits before and after the decimal point. So if you were printing a real number you'd use:
PRINT '(F5.2)', A
to get the rounded output.
In your example you're just printing text so there's no special formatting to do. Also if you leave the format specifier as * it will apply the default formatting to reals etc.
The print statement does formatted output to a special place (which we assume is the screen). There are three available ways to specify the format used in this output. One of these, as in the question, is using *.
The * format specifier means that the formatted output will be so-called list-directed output. Conversely, for a read statement, it will be list-directed input. Now that the term has been introduced you will be able to find many other questions here: list-directed input/output has many potentially surprising consequences.
Generally, a format specification item says how to format an item in the input/output list. If we are writing out a real number we'd use a format item which may, say, state the precision of the output or whether it uses scientific notation.
As in the question, when writing out a character variable we may want to specify the width of the output field (so that there is blank padding, or partial output) or not. We could have
print '(A20)', "Hello, world!"
print '(A5)', "Hello, world!"
print '(A)', "Hello, world!"
which offers a selection of effects. [Using a literal character constant like '(A)' is one of the other ways of giving the format.]
But we couldn't have
print '(F12.5)', "Hello, world!" ! A numeric format for a character variable
as our format doesn't make sense with the output item.
List-directed output is different from the above cases. Instead, (quote here and later from the Fortran 2008 draft standard) this
allows data editing according to the type of the list item instead of by a format specification. It also allows data to be free-field, that is, separated by commas (or semicolons) or blanks.
The main effect, and why list-directed is such a convenience, is that no effort is required on the part of the programmer to craft a suitable format for the things to be sent out.
When an output list is processed under list-directed output and a real value appears it is as though the programmer gave a suitable format for that value. And the same for other types.
But the crucial thing here is "a suitable format". The end user doesn't get to say A5 or F12.5. Instead of that latter the compiler gets to choose "reasonable processor-dependent values" for widths, precisions and so on. [Leading to questions like "why so much white space?".] The constraints on the output are given in, for example, Fortran 2008 10.10.4.
So, in summary * is saying, print out the following things in a reasonable way. I won't complain how you choose to do this, even if you give me output like
5*0
1 2
rather than
0 0 0 0 0 1 2
and I certainly won't complain about having a leading space like
Hello, world!
rather than
Hello, world!
For, yes, with list-directed you will (except in cases beyond this answer) get an initial blank on the output:
Except for new records created by [a special case] or by continuation of delimited character sequences, each output record begins with a blank character.
I have noticed the results of list-directed output write(*,*) in Fortran is compiler dependent.
Indeed, with the code:
program one
real(8), dimension(5):: r1
do i=1,5
r1(i)=sqrt(i*10.0)
end do
write(*,*) (r1(i), i =1,5)
end program one
intel compiler ifort gives standard output broken by a newline:
3.16227769851685 4.47213602066040 5.47722530364990
6.32455539703369 7.07106781005859
while gfortran gives the equivalent one line result:
3.1622776601683795 4.4721359549995796 5.4772255750516612 6.3245553203367590 7.0710678118654755
I think that ifort is writing maximum 3 items per line (when floating real numbers).
Is there any way to make the ifort output be like gfrotran, i.e. avoid the newline?
Ideally, I would like to keep list-directed output (*,*) instructions, so I am looking for something like a compiler option or so, if any.
Since verson 14, intel fortran compiler has the wrap-margin function. By default, the record is wrapped after 80 characters. For disabling this restriction, you should specify:
on Linux: -no-wrap-margin
on WIndows: /wrap-margin-
See more on Intel Fortran's reference guide
No. List-directed (free-format) output provides convenience, but you give up control. Various aspects of the output are unspecified and allowed to be chosen to the compiler. If you want full control, you have to use formatted output.
Look into edit descriptors in your favorite Fortran book or online documentation. You can use fmt specifier in the write statement to specify edit descriptors. For example:
write(*,fmt='(5(F6.4,3X))') (r1(i), i =1,5)
should output something similar to:
3.1623 4.4721 5.4772 6.3246 7.0711
See https://software.intel.com/en-us/forums/topic/401555
Specify FORT_FMT_RECL or use
write (,"(G0,1X))"
I need a portable function/subroutine to locate the position of the last non-blank character in a string. I've found two options: LEN_TRIM and LNBLNK. However, different compilers seem to have different standards. The official documentation for the following compilers suggests that LEN_TRIM is part of the Fortran 95 standard on the following platforms:
IBM: LEN_TRIM
Intel: LNBLNK and LEN_TRIM
gfortran: LNBLNK and LEN_TRIM
PGI: LEN_TRIM
However, it appears that nothing is guaranteed on compilers released before the F95 standard. My experience has been that older compilers might specify either LEN_TRIM or LNBLNK, but not necessarily both. My solution has been to use preprocessor conditionals:
#ifdef USE_LEN_TRIM
iNameLen = LEN_TRIM(cabase)
iExtLen = LEN_TRIM(caext)
#else
iNameLen = LNBLNK(cabase)
iExtLen = LNBLNK(caext)
#endif
and then pass -DUSE_LEN_TRIM to the preprocessor. However, I am not a big fan of preprocessor conditionals & extra compile-time flags. Do you have any suggestions for a portable (before the Fortran 95 standard) function that locate the position of the last non-blank character in a string?
LEN_TRIM is part of the Fortran 95 standard. I am not aware of any simple combination of intrinsics in Fortran 77 that could tackle this (TRIM also appeared with Fortran 95, and wasn't available in Fortran 77). But you can use a simple user function to perform that task if you really want to be portable without relying on compiler intrinsics and preprocessor:
function lentrim(s)
implicit none
character(len=*) :: s
integer lentrim
do lentrim = len(s), 1, -1
if (s(lentrim:lentrim) .ne. ' ') return
end do
end function lentrim
(tested with g77 and gfortran; use option -ffree-form to compile free-form source like this).
Before the standard, I would say no. I've seen the len_trim function implemented by hand on F77 codes. If you want to be standard toward old compilers, then you will have to provide your own implementation as a failsafe, and use a preprocessor conditional to pick what you need.
I'm stuck trying to find if this is a pre-Fortran 95 or not, but does
len(trim(s))
work?