CODEWITHSUNDEEP
fortran
ShanZhengYang
ShanZhengYang

Reputation: 17641

Fortran READ(*,*), WRITE(*,*) arguments

This question has been covered somewhat in previous SO questions. However, previous discussions seem somewhat incomplete.

Fortran has several I/O statements. There is READ(*,*) and WRITE(*,*), etc. The first asterisk (*) is the standard asterisk designating an input or output from the keyboard to/from the screen. My question is about the second asterisk:

The second asterisk designates the format of the I/O elements, the data TYPE which is being used. If this asterisk is left unchanged, the fortran complier uses the default format (whatever that may be, based on the compiler). Users must use a number of format descriptors to designate the data type, precision, and so forth.

(1) Are these format descriptors universal for all Fortran compilers and for all versions of Fortran?

(2) Where can I find the standard list of these format descriptors? For example, F8.3 means that the number should be printed using fixed point notation with field width 8 and 3 decimal places.

EDIT: A reference for edit descriptors can be found here: http://fortranwiki.org/fortran/show/Edit+descriptors

Upvotes: 4

Views: 20097

Answers (3)

user24352826
user24352826

Reputation: 1

Format descriptors are not universal for all Fortran Compilers. They are slight different for all versions of Fortran.

To learn format descriptors in an easy way I would like to refer you the following: https://web.ics.purdue.edu/~cs154/IOFormatting.pdf

Upvotes: 0

everythingfunctional
everythingfunctional

Reputation: 871

First, as a clarification, the 1st asterisk in the READ/WRITE statement has a slightly different meaning than you state. For write, it means write to the default file unit (in linux world generally standard out), for read it means read from the default file unit (in linux world generally standard in), either of which may not necessarily be connected to a terminal screen or a keyboard.

The 2nd asterisk means use list directed IO. For read statements this is generally useful because you don't need a specified format for your input. It breaks up the line into fields separated by space or comma (maybe a couple others that aren't commonly used), and reads each field in turn into the variable associated with that field in the argument list, ignoring unread fields, and continuing onto the next line if not enough fields were read in (unless a line termination character \ is explicitly included).

For writes, it means the compiler is allowed to determine what format to write the variables out (I believe with no separator). I believe it is allowed to do this at run time, so that you are all but guaranteed that the value it is trying to write will fit into the format specifier used, so you can be assured that you won't get ******* written out. The down side is you have to manually include a separator character in your argument list, or all your numbers will run together.

In general, using list directed read is more of a convenience to the user, so they don't have to fit their inputs into rigidly defined fields, and list directed writes are a convenience to the programmer, in case they're not sure what the output will look like.

Upvotes: 6

francescalus
francescalus

Reputation: 32406

When you have a data transfer statement like read(*,*) ... it's helpful to understand exactly what this means. read(*,*) is equivalent to the more verbose read(unit=*, fmt=*). This second asterisk, as you have it, makes this read statement (or corresponding write statement) list-directed.

List-directed input/output, as described elsewhere, is a convenience for the programmer. The Fortran standards specify lots of constraints that the compiler must follow, but this language has things like "reasonable values", so allowing output to vary by compiler, settings, and so on.

Again, as described elsewhere, fine user control over the output (or input) comes with giving a format specification. Instead of read(*,fmt=*), something like read(*,fmt=1014) or read(*,fmt=format_variable_or_literal). I take it your question is: what is this format specification?

I won't go into details of all of the possible edit descriptors, but I will say in response to (2): you can find the list of those edit descriptors in the Fortran standard (Clause 10 of Fortran 2008 goes into all the detail) or a good reference book.

To answer (1): no, edit descriptors are not universal. Even across Fortran standards. Of note are:

  • The introduction of I0 (and other minimal-width specifiers) for output in Fortran 95;
  • The removal of the H edit descriptor in Fortran 95;
  • The introduction of the DT edit descriptor in Fortran 2003.

Upvotes: 2

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