Reverse a string in place

[rajash]

Thanks for the additional comments.

Here is a solution to an exercise I had problems with. I still don't
think it's really what's wanted as it uses a "state variable" n - but
I can't see how to do it without this (or changing the function to
take an extra argument). Thanks for any hints.

#include<stdio. h>

#define SWAP(a,b) { char x; x=(a); (a)=(b); (b)=x; }

void reverse();

main(int argc, char**argv)
{
if(argc>1) {
reverse(argv[1]);
printf("%s\n", argv[1]);
} else
printf("Unspeci fied error\n");
}

void reverse(char *s)
{
static int n=-1;
if(n==-1)
n=strlen(s)-1;
SWAP(*s, s[n]);
if((n-=2)>=0)
reverse(s+1);
else
n=-1; /* make sure function is reentrant */
}

[pete]

RoS wrote:

>
In data Fri, 14 Dec 2007 20:53:42 +0100, RoS scrisse:

In data Fri, 14 Dec 2007 12:56:08 -0500, pete scrisse:
what is the difference of the below 2 routines?

yes i have seen it ...

void reverse(char *aa)
{char *a=aa, *b, t;
if(a==0||*a==0) return;
for( ; *a; ++a);
for(--a, b=aa; b<a; ++b, --a)
{t=*b; *b=*a; *a=t;}
}

this is my version whithout compile it nor debug it

OK

>This version of reverse, does nothing:

void reverse(char *string)

but str* were not implementation reserved?

No.
The rules are more complicated than that.

N869
7.26.10 General utilities <stdlib.h>
[#1] Function names that begin with str and a lowercase
letter (possibly followed by any combination of digits,
letters, and underscore) may be added to the declarations in
the <stdlib.hheader .

7.26.11 String handling <string.h>
[#1] Function names that begin with str, mem, or wcs and a
lowercase letter (possibly followed by any combination of
digits, letters, and underscore) may be added to the
declarations in the <string.hheader .

>{
char *end;

for (end = string; *end ; ++end);
if (*end) --end; else return;

when this case "*end!=0" is verified here? (never)

You are correct!

/* end points to last character in string */

while (string < end)
{
char temp;

temp = *string;
*string++ = *end;
*end-- = temp;
}
}

--
pete

[pete]

CBFalconer wrote:

However, if *string is 0 the else
clause arises, and the function returns without doing anything.

That's what it's supposed to do.

No action is required to reverse a zero length string.

If (*string) is 0,
then you are reversing a zero length string.

void reverse(char *string)
{
char *end;

for (end = string ; *end ; ++end);
if (*string) --end; else return;

--
pete

[CBFalconer]

ozbear wrote:

CBFalconer <cb********@yah oo.comwrote:

.... snip ...

>>
If the string is non-empty the test (*string) is true, --end is
executed, and end no longer points to a zero, but to the last
character in the string.

I didn't miss that point, but it isn't what Lew originally wrote,
since he was testing *end, not *string, for the early return
in the case of an empty string (please look at his original
"correction " to cater for empty strings). I corrected it.

Do you dispute that:
1) The original addition of testing *end was incorrect.
2) My modification corrects that flaw.
3) Lew's original correction for empty strings will cause the
code to not reverse -any- string (just not have UB for
empty ones).

If you disagree with any of (1), (2) or (3), please tell me why.
If you don't, then I don't understand what you were objecting to
in the first place.

Here is the original quote. string is the input parameter, and end
is local to the function.

>>>>>> for (end = string ; *end ; ++end);
>> if (*string) --end; else return;
> ^^^^^^

The above code exits for an empty string input. Only. I was
objecting to the misinterpretati on of the code by others.

--
Chuck F (cbfalconer at maineline dot net)
<http://cbfalconer.home .att.net>
Try the download section.

--
Posted via a free Usenet account from http://www.teranews.com

[CBFalconer]

pete wrote:

CBFalconer wrote:

>However, if *string is 0 the else
clause arises, and the function returns without doing anything.

That's what it's supposed to do.

That's what I said, and you objected to.

--
Chuck F (cbfalconer at maineline dot net)
<http://cbfalconer.home .att.net>
Try the download section.

--
Posted via a free Usenet account from http://www.teranews.com

[Ben Bacarisse]

CBFalconer <cb********@yah oo.comwrites:

Here is the original quote. string is the input parameter, and end
is local to the function.

>>>>>>> for (end = string ; *end ; ++end);
>>> if (*string) --end; else return;
>> ^^^^^^

The above code exits for an empty string input. Only. I was
objecting to the misinterpretati on of the code by others.

Who misinterpreted it? You certainly seemed to, since when the
correction (from *end to *string) was suggested by ozbear, your reply
started with the word "No". It was:

| ozbear wrote:
| Shouldn't that be:
| >
| if (*string) --end; else return;
| ^^^^^^
|
| No, because that clause detects the original supply of string as
| "", when end was never advanced, and exits early.

What you say after "No" suggests you know what it does, but I just
can't see why you start "No, ...". It made me think you had missed
the point of the correction. If you had replied "Yes, that clause..."
then I think this whole sub-thread would never have happened.

--
Ben.

[pete]

CBFalconer wrote:

>
pete wrote:

CBFalconer wrote:

However, if *string is 0 the else
clause arises, and the function returns without doing anything.

That's what it's supposed to do.

That's what I said, and you objected to.

I like ozbear's correction to Lew Pitcher's code.
If you do too, then we're all agreed.

--
pete