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From: ark@alice.UUCP
Newsgroups: comp.lang.fortran
Subject: Re: Problem with substrings
Message-ID: <6944@alice.UUCP>
Date: Thu, 4-Jun-87 00:33:45 EDT
Article-I.D.: alice.6944
Posted: Thu Jun  4 00:33:45 1987
Date-Received: Sat, 6-Jun-87 06:28:05 EDT
References: <286@nikhefh.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 40

In article <286@nikhefh.UUCP>, i91@nikhefh.UUCP writes:
> Does somebody know what according to the FORTRAN 77 standard 
> should happen in the following substring expression:
> 
>         CHARACTER*10 CH, STR
>         
> 	STR = 'ABCDEFGHIJ'
>         I   = 6
>         NC  = 0
>         CH  = 'FOO'//STR(I:I+NC-1)//'BAR'

This is not a valid Fortran 77 program.  On page 5-9 of the
Fortran 77 specification, the form of a substring expression
is given as

	v ( e1 : e2 )

where v is a string variable and e1 and e2 are optional integer
expressions.  e1 gives the index of the first character of the
substring and e2 gives the index of the last character.

Where len is the length of v, e1 and e2 must be such that

	1 <= e1 <= e2 <= len

If e1 is omitted, 1 is assumed.  If e2 is omitted, len is assumed.

In the program fragment given as an example, I is 6 and
I+NC-1 is 5.  Thus the inequality is not satisfied and the
program is invalid.

The Fortran 77 language definition says elsewhere that it
(the language definition) is restrictive on programs and
permissive on implementations.  That is: the definition says
what is a valid program.  A valid implementation is one that
does the right thing when given a valid program as input.

A fortran 77 implementation is thus permitted to do whatever
it pleases with an invalid program, including crash or deliver
garbage results.