READ and WRITE Statements READ statement reads information from one or more records on a file into a list of
variables, array elements, etc. The WRITE statement writes information from a list of
items which may include variables, arrays, and expressions and produces one or more
records on a file. Each READ or WRITE statement can only transfer one record on an
unformatted file but on formatted files, including internal files, more than
one record may be transferred, depending on the contents of the format
specification.
The two statements have the same general form:
READ( control-list ) data-list
WRITE( control-list ) data-list
The control-list must contain a unit identifier; the other items may be optional
depending on the type of file. The data-list is also optional: if it is absent the
statement transfers one record (or possibly more under the control of a format
specification).
This may have any of the following forms:
UNIT= integer-expression The value of the expression must be zero or greater
and must refer to a valid I/O unit.
UNIT=* For the standard pre-connected input or output file.
UNIT=internal-file The internal-file may be a variable, array-element, substring,
or array of type character, see section 10.3.UNIT= is optional if the unit identifier is the first item in the
control list.
A format identifier must be provided when using a formatted (or internal) file but not otherwise. It may have any of the following forms:
FMT=label The label of a FORMAT statement in the same program unit.
FMT=format The format may be a character expression or character array
containing a complete format specification (section 10.6).
FMT=* For list-directed formatting (section 10.10).FMT= is also optional if the format identifier is the second item
in the control list and the first item is a unit identifier specified without its
keyword.
A record number identifier must be provided for direct-access files but not
otherwise. It has the form:
REC=integer-expression
The record number must be greater than zero; for READ it must refer to a record
which exists.
Error and End-of-file Identifiers
These may be provided in any combination, but END=label is only valid
when reading a sequential or internal file. See 10.5 for more information.
END=label ERR=label IOSTAT=integer-variable |
READ statement may contain variables, array-elements,
character-substrings, or complete arrays of any data type. An array-name without
subscripts represents all the elements of the array; this is not permitted
for assumed-size dummy arguments in procedures (because the array size
is indeterminate). The list may also contain implied DO-loops (explained
below).
The data list of a WRITE statement may contain any of the items permitted in a
READ statement and in addition expressions of any data type. As in all I/O
statements, expressions must not themselves involve the execution of other I/O
statements.
The simplest and most efficient way to read or write all the elements of an array is to put its name, unsubscripted, in the data-transfer list. In the case of a multi-dimensional array the elements will be transferred in the normal storage sequence, with the first subscript varying most rapidly.
An implied-DO loop allows the elements to be transferred selectively or in some
non-standard order. The rules for an implied-DO are similar to that of an ordinary
DO-loop but the loop forms a single item in the data-transfer list and is enclosed by a
pair of parentheses rather than by DO and CONTINUE statements. For example:
READ(UNIT=*, FMT=*) (ARRAY(I), I= IMIN, IMAX) WRITE(UNIT=*, FMT=15) (M, X(M), Y(M), M=1,101,5) 15 FORMAT(1X, I6, 2F12.3) |
WRITE(UNIT=*, FMT=5) (I,I=1,5), $ ((L,X(L,I),I=1,5),L=1,100) 5 FORMAT(1X,'LINE', 5I10, 100(/,1X,I4, 5F10.2)) |
The implied DO-loop has the general form:
( data-list, loop-variable = start, limit, step )
where the rules for the start, limit, and step values are exactly as in an ordinary DO
statement. The loop-variable (normally an integer) may be used within the
data-list and this list may, in turn, include further complete implied-DO
lists.
If an error or end-of-file condition occurs in an implied DO-loop then the
loop-control variable will be undefined on exit; this means that an explicit DO-loop is
required to read an indefinite list of data records and exit with knowledge of how
many items were actually input.
