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 */
}

[Lew Pitcher]

On Dec 6, 3:15 pm, raj...@thisisno tmyrealemail.co m wrote:

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.

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

for (end = string; *end ; ++end); --end;
/* end points to last character in string */

while (string end)
{
char temp;

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

[Lew Pitcher]

Oops... pardon my typo...

On Dec 6, 3:28 pm, Lew Pitcher <lpitc...@teksa vvy.comwrote:

On Dec 6, 3:15 pm, raj...@thisisno tmyrealemail.co m wrote:

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.

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

for (end = string; *end ; ++end); --end;
/* end points to last character in string */

while (string end)
{
char temp;

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

}

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

for (end = string; *end ; ++end); --end;
/* end points to last character in string */

while (string < end)
{
char temp;

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

[rajash]

Lew Pitcher wrote:

On Dec 6, 3:15 pm, raj...@thisisno tmyrealemail.co m wrote:

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.

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

for (end = string; *end ; ++end); --end;
/* end points to last character in string */

while (string end)
{
char temp;

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

Uhh yes! I should have said clearly that it's Exercise 4.13 - write a
**recursive** version of reverse(s).

[Willem]

ra****@thisisno tmyrealemail.co m wrote:
) Uhh yes! I should have said clearly that it's Exercise 4.13 - write a
) **recursive** version of reverse(s).

That's silly. Reversing a string is clearly an iterative operation.
However, any iteration can be easily transformed into a tail recursion.

Note, also, that it is perfectly valid to have two functions.
One that recurses, and one that makes the initial call to the other.

Anyways, Here's a stab at a very inefficient and silly solution, using
a single function:

void reverse(char *string)
{
size_t len = strlen(string);
if (len 1) {
char tmp = string[len-1];
string[len-1] = 0;
reverse(string + 1);
string[len-1] = string[0];
string[0] = tmp;
}
}
SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

[Ben Bacarisse]

ra****@thisisno tmyrealemail.co m writes:

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; }

I'd either just write this code out, or I'd make this a function. If
I really needed it to be a macro, I'd use the "do {} while (0)" idiom
so as to get syntactic unit that can be followed by a ; and remain a
single statement.

void reverse();

This is not a prototype for reverse, just a declaration. Tell the
full story by writing:

void reverse(char *s);

main(int argc, char**argv)

int main is better and required in C99.

{
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 */
}

Spaces have come right down in price recently after the discovery of
the Peruvian space deposits. Help yourself to a few more.

Reverse is not a natural candidate for recursion (even in functional
languages a little care is needed to stop it being very inefficient)
but in C we have some extra options because we can modify the string.
I'd write it like this:

void rev(char *start, char *end)
{
if (start + 1 >= end)
return;
else {
char t = end[-1];
end[-1] = *start;
*start = t;
rev(start + 1, end - 1);
}
}

void reverse(char *s)
{
rev(s, strchr(s, 0));
}

--
Ben.

[pete]

Lew Pitcher wrote:

>
Oops... pardon my typo...

On Dec 6, 3:28 pm, Lew Pitcher <lpitc...@teksa vvy.comwrote:

On Dec 6, 3:15 pm, raj...@thisisno tmyrealemail.co m wrote:

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.

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

for (end = string; *end ; ++end); --end;
/* end points to last character in string */

while (string end)
{
char temp;

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

}

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

for (end = string; *end ; ++end); --end;
/* end points to last character in string */

Unless the string is (""),
in which case (end) is undefined.

while (string < end)
{
char temp;

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

--
pete

[CBFalconer]

pete wrote:

Lew Pitcher wrote:

.... snip ...

>

>void reverse(char *string) {
char *end;
for (end = string; *end ; ++end); --end;
/* end points to last character in string */

Unless the string is (""), in which case (end) is undefined.

> while (string < end) {
char temp;

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

So try:

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

for (end = string; *end; end++) continue;
/* leaving *end == 0 */
while (string < end) {
temp = *string;
*string++ = *(--end);
*end = temp;
)
} /* reverse, untested */

--
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

[RoS]

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

>This version of reverse, does nothing:

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

for (end = string; *end ; ++end);
if (*end) --end; else return;
/* end points to last character in string */

while (string < end)
{
char temp;

temp = *string;
*string++ = *end;
*end-- = temp;
}
}
This version of reverse, reverses a string:

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

for (end = string; *end ; ++end);
if (*string) --end; else return;
/* end points to last character in string */

while (string < end)
{
char temp;

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

If you can't see that, then you can't read code anymore.

Nobody else is seeing that code the way that you do.

[RoS]

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

>>This version of reverse, does nothing:

void reverse(char *string)

but str* were not implementation reserved?

>>{
char *end;

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

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

> /* end points to last character in string */

while (string < end)
{
char temp;

temp = *string;
*string++ = *end;
*end-- = temp;
}
}
This version of reverse, reverses a string:

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

for (end = string; *end ; ++end);
if (*string) --end; else return;
/* end points to last character in string */

while (string < end)
{
char temp;

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

If you can't see that, then you can't read code anymore.

Nobody else is seeing that code the way that you do.