Second real written to stdout with the P descriptor is wrong by factor 10

user_6168231 0
Viewed 69

Here's a minimal working example:

program test_stuff

  implicit none

  real :: b

  b = 10000.0

  write(*,'(A10,1PE12.4,F12.4)')  "b, b:     ", b, b

end program

which I simply compile with gfortran test_stuff.f90 -o test_stuff

However, running the program gives the following output:

$ ./test_stuff
b, b:       1.0000E+04 100000.0000

The second real written to the screen is wrong by a factor of 10.

This happens with gfotran 9.3.0 as well as 10.2.0, so I definitely must be doing something wrong, but I can't see what it is. Can anybody spot what I'm doing wrong?

gfortran fortran
2 Answers

The P control edit descriptor "temporarily changes" (Fortran 2018 13.8.5) the scale factor connection mode of the connection.

However, what is meant by temporary is until the mode is changed again or until the end of the data transfer statement: (Fortran 2018 12.5.2)

Edit descriptors take effect when they are encountered in format processing. When a data transfer statement terminates, the values for the modes are reset to the values in effect immediately before the data transfer statement was executed.

In the case of the question, both output values are thus processed with the scale factor having value 1.

This scale factor is responsible for the "wrong" second value: there is a difference in interpretation of the scale factor for E and F editing. For E editing the scale factor simply changes the representation, with the external and internal values the same (with the significand scaled up by 10 and the exponent reduced by 1), but for F editing the output value is itself scaled:

On output, with F output editing, the effect is that the externally represented number equals the internally represented number multiplied by 10k

So while 10000 would be represented by 0.1000E+05 with scale factor 0 and 1.0000E+04 with scale factor 1 under E12.4, under F12.4 the value 10000 is scaled to 100000 with the scale factor in place.

As a style note: although the comma is optional between 1P and E12.4 (and similar), many would regard it much better to include the comma, precisely to avoid this apparent tight coupling of the two descriptors (or looking like one descriptor). As the scale factor has a different effect for each of E and F, has no effect for EN and sometimes but not always has an effect with G, I'm not going to argue with anyone who calls P evil.

user_31570760

You are looking for section 12.5.2 of the Fortran 2018 standard.

A connection for formatted input/output has several changeable modes: these are ... and scale factor (13.8.5).

Values for the modes of a connection are established when the connection is initiated. If the connection is initiated by an OPEN statement, the values are as specified, either explicitly or implicitly, by the OPEN statement. If the connection is initiated other than by an OPEN statement (that is, if the file is an internal file or pre-connected file) the values established are those that would be implied by an initial OPEN statement without the corresponding keywords.

The scale factor cannot be explicitly specified in an OPEN statement; it is implicitly 0.

The modes of a connection can be temporarily changed by ... or by an edit descriptor. ... Edit descriptors take effect when they are encountered in format processing. When a data transfer statement terminates, the values for the modes are reset to the values in effect immediately before the data transfer statement was executed.

So when you used 1P in your format, you changed the mode for the connection. This applies to all output items after the 1P has been processed. When the write statement completes the scale factor is reset to 0.

user_119285270
Related