|  | K&R Exercise 6-2 | | |
| | | mdh |  |
|  Posted: Fri Aug 29, 2008 3:34 am Post subject: K&R Exercise 6-2 |  |
In most of the exercises that K&R write, there seems to be a
relationship to some library function or some aspect of C that is used
later. So, quick question to those more experienced programmers. Other
than this being an illustration of the use of structs, is this
illustrative of some aspects of C that you use all the time.
Thanks as usual.
Write a program that reads a C program and prints in alphabetical
order each group of
variable names that are identical in the first 6 characters, but
different somewhere thereafter. Don't count
words within strings and comments. Make 6 a parameter that can be set
from the command line.
<<<<<<< |
| |
| | | Richard Heathfield | |
| | Posted: Fri Aug 29, 2008 3:34 am Post subject: Re: K&R Exercise 6-2 | |
| |  | |
mdh said:
| Quote: | In most of the exercises that K&R write, there seems to be a
relationship to some library function or some aspect of C that is used
later.
|
It's worse than that. They seem to be guiding you towards writing a C
implementation!
| Quote: | So, quick question to those more experienced programmers. Other
than this being an illustration of the use of structs, is this
illustrative of some aspects of C that you use all the time.
Thanks as usual.
Exercise 6-2
Write a program that reads a C program and prints in alphabetical
order each group of
variable names that are identical in the first 6 characters, but
different somewhere thereafter. Don't count
words within strings and comments. Make 6 a parameter that can be set
from the command line.
|
In C90, an implementation must be able to distinguish between external
identifiers that are unique in the first 6 characters (without regard to
case!). A program such as the one required in Exercise 6-2 could be useful
for diagnosing source code that ignores this limit (as I suspect a vast
amount of source code does).
(Incidentally, this rather strange rule was a pragmatic recognition of the
fact that, at the time the Standard was written, there really were linkers
around that couldn't handle long identifiers. Rather than outlaw those
linkers, the Standard worked them in. In C99, the limit was revised to 31,
so, if that /was/ the intent of the question, its last sentence turns out
to have been rather prescient.
--
Richard Heathfield <http://www.cpax.org.uk>
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999 |
| |
| | | mdh | |
| | Posted: Fri Aug 29, 2008 4:01 am Post subject: Re: K&R Exercise 6-2 | |
| | | |
On Aug 28, 8:57 pm, Richard Heathfield <r...@see.sig.invalid> wrote:
| Quote: | mdh said:
In most of the exercises that K&R write, there seems to be a
relationship to some library function or some aspect of C that is used
later.
It's worse than that. They seem to be guiding you towards writing a C
implementation!
So, quick question to those more experienced programmers. Other
than this being an illustration of the use of structs, is this
illustrative of some aspects of C that you use all the time.
Thanks as usual.
Exercise 6-2
Write a program that reads a C program and prints in alphabetical
order each group of
variable names that are identical in the first 6 characters, but
different somewhere thereafter. Don't count
words within strings and comments. Make 6 a parameter that can be set
from the command line.
In C90, an implementation must be able to distinguish between external
identifiers that are unique in the first 6 characters (without regard to
case!). A program such as the one required in Exercise 6-2 could be useful
for diagnosing source code that ignores this limit (as I suspect a vast
amount of source code does).
|
Thanks Richard. |
| |
| | | Eric Sosman | |
| | Posted: Fri Aug 29, 2008 10:11 am Post subject: Re: K&R Exercise 6-2 | |
mdh wrote:
| Quote: | In most of the exercises that K&R write, there seems to be a
relationship to some library function or some aspect of C that is used
later. So, quick question to those more experienced programmers. Other
than this being an illustration of the use of structs, is this
illustrative of some aspects of C that you use all the time.
Thanks as usual.
Exercise 6-2
Write a program that reads a C program and prints in alphabetical
order each group of
variable names that are identical in the first 6 characters, but
different somewhere thereafter. Don't count
words within strings and comments. Make 6 a parameter that can be set
from the command line.
|
The exercise seems designed to point out the difference
between strcmp() and strncmp(), in the context of a comparison
function for qsort(). So there's certainly a library tie-in,
if that's what you felt was missing.
--
Eric Sosman
esosman@ieee-dot-org.invalid |
| |
| | | pete | |
| | Posted: Fri Aug 29, 2008 11:11 am Post subject: Re: K&R Exercise 6-2 | |
mdh wrote:
| Quote: | On Aug 29, 5:11 am, Eric Sosman <esos...@ieee-dot-org.invalid> wrote:
mdh wrote:
In most of the exercises that K&R write, there seems to be a
relationship to some library function .......
Exercise 6-2
The exercise seems designed to point out the difference
between strcmp() and strncmp(), in the context of a comparison
function for qsort(). So there's certainly a library tie-in,
if that's what you felt was missing.
Thank you Eric.
|
However, the definition of "alphabetical order" according to strcmp,
isn't exactly the same on different character sets.
In ASCII, a variable name like "A6A", comes before "ABA".
In EBCDIC, a variable name like "A6A", comes after "ABA".
Also uppercase letters come before lower case in ASCII,
but not in EBCDIC.
--
pete |
| |
| | | Doug Miller | |
| | Posted: Fri Aug 29, 2008 11:17 am Post subject: Re: K&R Exercise 6-2 | |
In article <16OdnWCI1MoDaCrVnZ2dnUVZ_q_inZ2d@earthlink.com>, pfiland@mindspring.com wrote:
| Quote: | However, the definition of "alphabetical order" according to strcmp,
isn't exactly the same on different character sets.
In ASCII, a variable name like "A6A", comes before "ABA".
In EBCDIC, a variable name like "A6A", comes after "ABA".
Also uppercase letters come before lower case in ASCII,
but not in EBCDIC.
More specifically, |
EBCDIC: a..z < A..Z < 0..9
ASCII: 0..9 < A..Z < a..z
--
Regards,
Doug Miller (alphageek-at-milmac-dot-com)
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| |
| | | mdh | |
| | Posted: Fri Aug 29, 2008 12:37 pm Post subject: Re: K&R Exercise 6-2 | |
On Aug 29, 5:11 am, Eric Sosman <esos...@ieee-dot-org.invalid> wrote:
| Quote: | mdh wrote:
In most of the exercises that K&R write, there seems to be a
relationship to some library function .......
Exercise 6-2
The exercise seems designed to point out the difference
between strcmp() and strncmp(), in the context of a comparison
function for qsort(). So there's certainly a library tie-in,
if that's what you felt was missing.
|
Thank you Eric. |
| |
| | | August Karlstrom | |
| | Posted: Fri Aug 29, 2008 12:51 pm Post subject: Re: K&R Exercise 6-2 | |
Richard Heathfield wrote:
[...]
| Quote: | In C90, an implementation must be able to distinguish between external
identifiers that are unique in the first 6 characters (without regard to
case!). A program such as the one required in Exercise 6-2 could be useful
for diagnosing source code that ignores this limit (as I suspect a vast
amount of source code does).
|
So a library which exports e.g. the identifiers string_prepend and
string_append are not really C90 compliant?
August |
| |
| | | Harald van Dijk | |
| | Posted: Fri Aug 29, 2008 1:05 pm Post subject: Re: K&R Exercise 6-2 | |
On Fri, 29 Aug 2008 16:51:56 +0200, August Karlstrom wrote:
| Quote: | Richard Heathfield wrote:
[...]
In C90, an implementation must be able to distinguish between external
identifiers that are unique in the first 6 characters (without regard
to case!). A program such as the one required in Exercise 6-2 could be
useful for diagnosing source code that ignores this limit (as I suspect
a vast amount of source code does).
So a library which exports e.g. the identifiers string_prepend and
string_append are not really C90 compliant?
|
Correct. However, they are already not C90 compliant -- and not C99
compliant either -- for another reason: external identifiers starting with
str and followed by a lowercase letter are reserved for the implementation. |
| |
| | | Richard Heathfield | |
| | Posted: Fri Aug 29, 2008 1:11 pm Post subject: Re: K&R Exercise 6-2 | |
| | | |
August Karlstrom said:
| Quote: | Richard Heathfield wrote:
[...]
In C90, an implementation must be able to distinguish between external
identifiers that are unique in the first 6 characters (without regard to
case!). A program such as the one required in Exercise 6-2 could be
useful for diagnosing source code that ignores this limit (as I suspect
a vast amount of source code does).
So a library which exports e.g. the identifiers string_prepend and
string_append are not really C90 compliant?
|
Right, for two reasons:
(1) the names invade implementation namespace;
(2) the names are a touch on the long side.
In practice, (1) is often ignored, and (2) is practically always ignored.
But strictly speaking, yes, you are correct.
Of the two reasons, (1) is probably the more important. Whilst it is
certainly possible that some people are still using 6-sig-char linkers
nowadays, it isn't terribly likely that terribly many people are. Even
twenty years ago (and probably more), people were ignoring the 6-character
thing, and it doesn't seem to have caused any major software crises.
--
Richard Heathfield <http://www.cpax.org.uk>
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999 |
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