10.6 Format Specifications

Every READ or WRITE statement which uses a formatted external file or an internal file must include a format identifier. This may have any of the following forms:

FMT=*: This specifies a list-directed transfer (and is only permitted for external sequential files). Detailed rules are given in section 10.10 below.
FMT=label: The label must be attached to a FORMAT statement in the same program unit which provides the format specification.
FMT=char-exp: The value of the character expression is a complete format specification.
FMT=char-array: The elements of the character array contain the format specification, which may occupy as many elements of the array as are necessary.

Note that the characters FMT= may be omitted if it is the second item in the I/O control list and if the unit identifier with UNIT= omitted comes first.

A format specification consists a pair of parentheses enclosing a list of items called edit descriptors. Any blanks before the left parenthesis will be ignored and (except in a FORMAT statement) all characters after the matching right parenthesis are ignored.

In most cases the format can be chosen when the program is written and the simplest option is to use a character constant: WRITE(UNIT=LP, FMT='(1X,A,F10.5)') 'Frequency =', HERTZ Alternatively you can use a FORMAT statement:

WRITE(UNIT=LP, FMT=915) 'Frequency =', HERTZ 915 FORMAT(1X, A, F10.5)

This allows the same format to be used by more than one data-transfer statement. The FORMAT statement may also be the neater form if the specification is long and complicated, or if character-constant descriptors are involved, since the enclosing apostrophes have to be doubled up if the whole format is part of another character constant.

It is also possible to compute a format specification at run-time by using a suitable character expression. By this means you could, for example, arrange to read the format specification of a data file from the first record of the file. The program fragment below shows how to output a real number in fixed-point format (F10.2) when it is small, changing to exponential format (E18.6) when it is larger. A threshold of a million has been chosen here.

CHARACTER F1*(*), F2*12, F3*(*) *Items F1, F2, F3 hold the three parts of a format specification. *F1 and F3 are constants, F2 is a variable. PARAMETER (F1 = '(1X,''Peak size ='',') PARAMETER (F3 = ')') *... calculation of PEAK assumed to be in here IF(PEAK .LT. 1.0E6) THEN F2 = 'F10.2' ELSE F2 = 'E18.6' END IF WRITE(UNIT=*, FMT=F1//F2//F3) PEAK

Note that the apostrophes surrounding the character constant 'Peak size =' have been doubled in the PARAMETER statement because they are inside another character constant. Here are two examples of output records, with blanks shown explicitly:

Peaksize=12345.67 Peaksize=0.987654E+08

FORMAT statement

The FORMAT statement is classed as non-executable and can, in principle, go almost anywhere in the program unit. A FORMAT statement can, of course, be continued so its maximum length is 20 lines. The same FORMAT statement can be used by more than one data transfer statement and, unless it contains character constant descriptors, used for both input and output. Since it is very easy to make a mistake in matching the items in a data transfer list with the edit descriptors in the format specification, it makes sense to put the FORMAT statement as close as possible to the READ and WRITE statements which use it.