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From: qsc on 31 Jan 2008 12:32
I found it hard to understand the concept of a record in Fortran I/O.

Here are some of the questions I have about it.
1. one record is equivalent to one line?

2. Each read, write or print statement deal transfer one record?

3. What if in one write statement I want output several values? Will
they be treated as one record, or splitted into several? For example:
write(12) U(:)
Notice that I want binary output, and the array U have many
components. So now, will U be written in one record, or many? Will the
output be on one line, or many?

Thanks in advance!
From: Gordon Sande on 31 Jan 2008 13:22
On 2008-01-30 13:32:26 -0400, qsc <qingshan.chen(a)gmail.com> said:

> I found it hard to understand the concept of a record in Fortran I/O.

It is a very old concept that can be thought of as the formalization
of punch cards. You can look at the beginning of a punch card and get
the next card without bothering to look at the end of the previous one.

> Here are some of the questions I have about it.
> 1. one record is equivalent to one line?

Yes, for the simple cases. Assuming some reasonable meaning for line.
See below for one possible trouble even with that.

> 2. Each read, write or print statement deal transfer one record?

Yes, for the simple cases. Some fancy formats or advance="no" are
not simple cases. There is "new record" format item and advance="no"
stays on the same record. Both are useful once you get beyond the
common simple cases.

> 3. What if in one write statement I want output several values? Will
> they be treated as one record,

Yes, A record can have many values in in it.

> or splitted into several? For example:
> write(12) U(:)

No. Unless you do something quite strange. No easy examples of what that
might be.

> Notice that I want binary output, and the array U have many
> components. So now, will U be written in one record, or many? Will the
> output be on one line, or many?

Unformatted output (what you probably mean by binary as every thing
on a binary computer is in binary) and lines are probaly a symptom
of some confusion. What you see in a text editor is often not a
record as "line wrapping" will turn a long line (often a record)
into many lines on the display. Using a text editor with unformatted
records is typically not useful except when the data is characters.

Formatted output is when you convert the internal values into a form
that is suitable for people to read. It may use either an explicit
format (what many folks call formatted output) or allow the system
to choose a format when you ask for list directed (a "*" usually)
output. Both are formatted within the meanings established for Fortran.
This technical usage is often used incorrectly and leads to repeated
confusion.

> Thanks in advance!


From: Klaus Wacker on 31 Jan 2008 13:30
qsc <qingshan.chen(a)gmail.com> wrote:
> I found it hard to understand the concept of a record in Fortran I/O.
>
> Here are some of the questions I have about it.

The answers to your questions depend on whether you have formatted or
unformatted I/O.

> 1. one record is equivalent to one line?

In formatted I/O, yes. In un formatted I/O the concept of a line
doesn't exist.

>
> 2. Each read, write or print statement deal transfer one record?

In unformatted I/O, yes (unless you use "advance=no", then you only
get part of a record). In formatted I/O, one I/O statement can deal
with several records. E.g., a "/" format item starts a new record. If
you run off of the end of the format string and you have still items
left in the I/O list, a new record will be started.

>
> 3. What if in one write statement I want output several values? Will
> they be treated as one record, or splitted into several? For example:
> write(12) U(:)
> Notice that I want binary output, and the array U have many
> components. So now, will U be written in one record, or many?

One record.

Will the
> output be on one line, or many?

There is no line, write(12) U(:) is unformatted I/O. You may be
confusing this with write(12,*) U(:). This is formatted, list-directed
I/O. Here, the compiler or run time system is free to produce several
records or lines. It will usually do so if the line gets too long
otherwise.


>
> Thanks in advance!

--
Klaus Wacker klaus.wacker(a)udo.edu
Experimentelle Physik V http://www.physik.uni-dortmund.de/~wacker
Universitaet Dortmund Tel.: +49 231 755 3587
D-44221 Dortmund Fax: +49 231 755 4547
From: qsc on 31 Jan 2008 14:37
After reading Gordon and Klaus's replies, I came to the conclusion
that
write(12) U(:) will write all the values in U(:) as one record. But
the concept
of "line" doesn't apply here.

If I am correct in the statement above, then what will be the
structure of
the data file? Or put in another way, after writing the array into an
external data file, I load the data file using fread of matlab. The
function
fread will produce a data matrix. What will be the shape of the
matrix?
Is it (1,N), or is it (N, 1), or something else?

I will soon find it out by experiments. But if someone can shed light
on
it beforehand, it is greatly appreciated.

Thank you!
From: glen herrmannsfeldt on 31 Jan 2008 14:57
qsc wrote:

> After reading Gordon and Klaus's replies, I came to the
> conclusion that write(12) U(:) will write all the values
> in U(:) as one record. But the concept of "line" doesn't
> apply here.


> If I am correct in the statement above, then what will be
> the structure of the data file? Or put in another way, after
> writing the array into an external data file, I load the
> data file using fread of matlab. The function fread will
> produce a data matrix. What will be the shape of the matrix?
> Is it (1,N), or is it (N, 1), or something else?

All the data from one UNFORMATTED WRITE statement goes into
one record. The shape, if any, does not.

REAL A,B,C,D,X(4)
DATA A,B,C,D/1.,2.,3.,4./
WRITE(1) A,B,C,D
WRITE(1) A,B,C,D
REWIND 1
READ(1) X(1:4)
WRITE(*,*) X
READ(1) X(3:4),X(1:2)

You can read or write with separate scalar variables, arrays, array
elements, implied DO over any of the above, all goes into one record.

Systems may have a record length limit, and even if they don't it
might require a buffer the size of the record to be written.
Small records may have excess overhead, very large ones might
have other types of overhead.

-- glen

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