It's part of a test and I'm stumped. There is a function
void foo(char **x)
The function signature is given and cannot be changed, so no passing
of other values. The test case involves defining this variable:
char *y[] = { /* bunch of stuff */ }
and calling
foo(y)
In the above, "bunch of stuff" is a series of triplets, two strings
followed by a null string (""). However, the last triplet ends with an
integer 0. This seems that it's supposed to signify the end of the
array. However, it appears to me that 0 is the same binary value as
for the empty string (NUL, \0, whatever). So in effect, one cannot
test for it as a sentry value because it's actually the same as the
preceding triplets.
How then, can the function foo() determine the bounds of the array?
Knowing the bounds of the particular test case is not sufficient since
the actual test suite may have arrays of varying size.
Oct 8 '08
30  3620 
"Bartc" <bc@freeuk.comw rote:
"Keith Thompson" <ks***@mib.orgw rote in message
"Bartc" <bc@freeuk.comw rites:
However NULL to represent an empty string can be a useful technique
(although not recognised by standard functions), hence my comment.
Sure, you can use a null pointer to represent an empty string. You
can also use the number 42, or the string "this is an empty string",
to represent an empty string. You can use anything you like to
represent anything you like, as long as you're consistent about it.
I quite like this other quote in this thread:
<la************ @siemens.comwro te in message
No. An empty string is like an empty box; a null pointer is no box at
all.
If you go to your drawer to get some socks, the absence of a drawer will
tell you you have no socks, just as well as an empty drawer. And you don't
have to open the drawer to find out.
I take it, then, that you've never worked with databases much. I assure
you, there is a world of difference between "We have no record of this
person" and "We have a record of this person, and his phone number field
is blank".
Basically, it's like asking how many legs a fish has, and how many legs
a null string has. In the first case, the answer is "none"; in the
second case, it's "What have you been smoking?".
But a null pointer *is not* an empty string, and it doesn't point to
an empty string. And, in my opinion, using a null pointer to
represent an empty string is a bad idea; it's too easy to accidentally
pass it to a function that doesn't know about your convention, and it
loses you the ability to distinguish between an empty string and the
absence of a string.
Well I used null pointers for empty strings before, outside of C. In C, that
would be problematic because standard functions tend to crash when you pass
null pointers. But I do still sometimes use null pointers this way, it just
needs some care.
No, it needs a situation where there is no difference between "doesn't
exist" and "is blank". Those situations are rarer than most people, even
most programmers with no database experience, believe.
Richard
Nate Eldredge wrote:
CBFalconer <cb********@yah oo.comwrites:
>"gg******@gmai l.com" wrote:
>>>
It's part of a test and I'm stumped. There is a function
void foo(char **x)
The function signature is given and cannot be changed, so no passing
of other values. The test case involves defining this variable:
char *y[] = { /* bunch of stuff */ }
and calling
foo(y)
In the above, "bunch of stuff" is a series of triplets, two
strings followed by a null string (""). However, the last
triplet ends with an integer 0. This seems that it's supposed
to signify the end of the array. However, it appears to me that
0 is the same binary value as for the empty string (NUL, \0,
whatever). So in effect, one cannot test for it as a sentry
value because it's actually the same as the preceding triplets.
Maybe "bunch of stuff" is that, but y is not. y is an array of
pointers to char, and those pointers also point to char arrays
that are not writeable. The arrays are strings, with a '\0'
terminal char. The last string is an empty string, and is used
to mark the end of the array.
Now consider what foo is. It appears, from your definition, to
be a function that returns void (i.e. nothing). It also
requires a parameter, x, which is a pointer to a char pointer.
Combining those things, the call to foo(y) cannot possibly do
anything.
Um? Here are some possible definitions for foo that do things.
void foo(char **x) { x[3] = "Hi mom!"; }
void foo(char **x) { while (*x) puts(*x++); }
void foo(char **x) { errno = EDOOFUS; }
void foo(char **x) { longjmp(somewhe re); }
void foo(char **x) { abort(); }
In other words you want to use globals and side-effects. I should
have specified further.
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.
On Oct 9, 12:31 pm, Nate Eldredge <n...@vulcan.la nwrote:
<snip>
Um? Here are some possible definitions for foo that do things.
void foo(char **x) { x[3] = "Hi mom!"; }
void foo(char **x) { while (*x) puts(*x++); }
void foo(char **x) { errno = EDOOFUS; }
void foo(char **x) { longjmp(somewhe re); }
Well, longjmp takes two arguments...
On Oct 9, 6:25 pm, vipps...@gmail. com wrote:
On Oct 9, 12:31 pm, Nate Eldredge <n...@vulcan.la nwrote:
<snip>
Um? Here are some possible definitions for foo that do things.
void foo(char **x) { x[3] = "Hi mom!"; }
void foo(char **x) { while (*x) puts(*x++); }
void foo(char **x) { errno = EDOOFUS; }
void foo(char **x) { longjmp(somewhe re); }
Well, longjmp takes two arguments...
Also, foo belongs in the programmers namespace, but EDOOFUS in the
implementations namespace, so the penultimate function can't work.
CBFalconer <cb********@yah oo.comwrites:
Nate Eldredge wrote:
>CBFalconer <cb********@yah oo.comwrites:
<snip>
>>Now consider what foo is. It appears, from your definition, to
be a function that returns void (i.e. nothing). It also
requires a parameter, x, which is a pointer to a char pointer.
Combining those things, the call to foo(y) cannot possibly do
anything.
Um? Here are some possible definitions for foo that do things.
void foo(char **x) { x[3] = "Hi mom!"; }
void foo(char **x) { while (*x) puts(*x++); }
void foo(char **x) { errno = EDOOFUS; }
void foo(char **x) { longjmp(somewhe re); }
void foo(char **x) { abort(); }
In other words you want to use globals and side-effects. I should
have specified further.
Programming in C without side effects is almost impossible. Even if
we add the extra condition that all the x[i] point to unmodifiable
arrays (a restriction that Nate Eldredge's examples comply with) then
foo can still be useful. A clearer example might be
void sort_strings(ch ar **x)
{
qsort(x, count_strings(x ), sizeof *x, sptr_cmp);
}
where count_strings is the function the OP was originally asking
about and sptr_cmp compares strings when passed void *s that point at
pointers to these strings.
There are lots of useful functions with the same prototype as foo.
--
Ben.
On Oct 9, 11:26*am, vipps...@gmail. com wrote:
On Oct 9, 6:25 pm, vipps...@gmail. com wrote:
On Oct 9, 12:31 pm, Nate Eldredge <n...@vulcan.la nwrote:
<snip>
Um? *Here are some possible definitions for foo that do things.
void foo(char **x) { x[3] = "Hi mom!"; }
void foo(char **x) { while (*x) puts(*x++); }
void foo(char **x) { errno = EDOOFUS; }
void foo(char **x) { longjmp(somewhe re); }
Well, longjmp takes two arguments...
Also, foo belongs in the programmers namespace, but EDOOFUS in the
implementations namespace, so the penultimate function can't work.
I think this is wrong. It can work. Either the implementation can
define EDOOFUS, which is just fine, or user code can define it if it
is not defined, also fine. It is generally a good idea NOT to start
your macros E[digit|uppercase] as it conflicts with a "future
direction" of a header. But the standard doesn't have language like
"reserved for any use", like leading __. But it is as far as I can
see legal and portable to put this in your code...
#include <errno.h>
#ifndef EDOOFUS
#define EDOOFUS 42
#endif
-David
Richard Bos <rl*@hoekstra-uitgeverij.nlwr ote:
>
No, it needs a situation where there is no difference between "doesn't
exist" and "is blank". Those situations are rarer than most people, even
most programmers with no database experience, believe.
Even programmers *with* database experience. Oracle, for example,
continues to treat empty string values and nulls identically, although
they have warned (forever) that that could change some time in the
future. I've run into way too much code that depends on that
equivalence and doesn't work when ported to a different db that does
distinguish.
--
Larry Jones
They can make me do it, but they can't make me do it with dignity. -- Calvin
On Thu, 9 Oct 2008 08:50:24 -0700 (PDT),
David Resnick <ln********@gma il.comwrote:
On Oct 9, 11:26Â*am, vipps...@gmail. com wrote:
>On Oct 9, 6:25 pm, vipps...@gmail. com wrote:
On Oct 9, 12:31 pm, Nate Eldredge <n...@vulcan.la nwrote:
[snip]
void foo(char **x) { errno = EDOOFUS; }
void foo(char **x) { longjmp(somewhe re); }
Well, longjmp takes two arguments...
Also, foo belongs in the programmers namespace, but EDOOFUS in the
implementation s namespace, so the penultimate function can't work.
I think this is wrong. It can work. Either the implementation can
define EDOOFUS, which is just fine,
And, in fact, there are implementations that do exactly that.
Martien
--
|
Martien Verbruggen | Quick! Hire a teenager while they still know
| everything.
| la************@ siemens.com wrote:
Richard Bos <rl*@hoekstra-uitgeverij.nlwr ote:
No, it needs a situation where there is no difference between "doesn't
exist" and "is blank". Those situations are rarer than most people, even
most programmers with no database experience, believe.
Even programmers *with* database experience. Oracle, for example,
continues to treat empty string values and nulls identically, although
they have warned (forever) that that could change some time in the
future.
Now, I'm not much of an SQL expert, certainly not compared to the people
at Oracle, but... isn't that illegal?
Richard
Richard Bos wrote:
la************@ siemens.com wrote:
.... snip ...
>
>Even programmers *with* database experience. Oracle, for
example, continues to treat empty string values and nulls
identically, although they have warned (forever) that that
could change some time in the future.
Now, I'm not much of an SQL expert, certainly not compared to
the people at Oracle, but... isn't that illegal?
No, it isn't. First, they can certainly design their own functions
in any fashion they wish. Second, even if implementing C standard
functions, this is simply imposing a definition for 'undefined
behavior' on THEIR system.
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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