I tried using the string reversal routine that occurs in K & R 2nd
edition (Pg. 62) & the program core dumps.
#include <stdio.h>
#include <stdlib.h>
void reverse ( char s[] )
{
int c, i, j;
printf("\nInput string is %s",s);
for (i=0, j= strlen(s)-1;i<j;i++,j--)
{
c = s[i];
s[i] = s[j];
s[j] = c;
}
printf("\nRever sed string is %s",s);
}
int main()
{
reverse("abcd") ;
}
Apart from the two prints, this is an exact copy of the routine. I am
using gcc on Cygwin.
Can someone tell me whats going wrong?
Thanks,
Ashok 6 5369 as***********@g mail.com wrote: I tried using the string reversal routine that occurs in K & R 2nd edition (Pg. 62) & the program core dumps.
#include <stdio.h> #include <stdlib.h>
void reverse ( char s[] ) { int c, i, j;
printf("\nInput string is %s",s);
for (i=0, j= strlen(s)-1;i<j;i++,j--) { c = s[i]; s[i] = s[j]; s[j] = c; }
printf("\nRever sed string is %s",s); }
int main() { reverse("abcd") ; }
Apart from the two prints, this is an exact copy of the routine. I am using gcc on Cygwin.
Can someone tell me whats going wrong?
Thanks, Ashok
You are attempting to modify a string literal, can't do that.
try this:
int main(void)
{
char foo[] = "abcd";
reverse(foo);
return 0;
}
You should put a '\n' at the end of your printfs
rather than the beginning, the last one may
or may not be displayed.
You should check for a 0 length string too, not
if strlen(s) is 0;
-David
Thanks David. Indeed the error was that a string literal was attempted
to be modified.
Here is what I finally got.
#include <stdio.h>
#include <string.h>
void reverse ( char *str )
{
int i; int len = strlen(str);
if (!len) return;
printf("Input string is %s \n", str);
for ( i=0; i < len/2; i++)
{
*(str + len-1-i) ^= *(str+i) ^= *(str + len -1-i) ^=
*(str+i);
}
printf("Output string is %s \n", str);
}
int main ()
{
char mystr[] = "abcde";
reverse(mystr);
}
Is the strlen implemented here same as what you are referring to?
Ashok
In article <11************ **********@z14g 2000cwz.googleg roups.com>
<as***********@ gmail.com> wrote: Here is what I finally got.
[snippage]
*(str + len-1-i) ^= *(str+i) ^= *(str + len -1-i) ^= *(str+i);
"Don't do that":
Archive-name: C-faq/faq
Comp-lang-c-archive-name: C-FAQ-list
URL: http://www.eskimo.com/~scs/C-faq/top.html
[Last modified July 3, 2004 by scs.]
3.3b: Here's a slick expression:
a ^= b ^= a ^= b
It swaps a and b without using a temporary.
A: Not portably, it doesn't. It attempts to modify the variable a
twice between sequence points, so its behavior is undefined.
For example, it has been reported that when given the code
int a = 123, b = 7654;
a ^= b ^= a ^= b;
the SCO Optimizing C compiler (icc) sets b to 123 and a to 0.
See also questions 3.1, 3.8, 10.3, and 20.15c.
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers. as***********@g mail.com wrote: void reverse ( char *str ) { int i; int len = strlen(str); if (!len) return; printf("Input string is %s \n", str); for ( i=0; i < len/2; i++) { *(str + len-1-i) ^= *(str+i) ^= *(str + len -1-i) ^= *(str+i); }
printf("Output string is %s \n", str);
}
A simpler pointer solution. It appeared that you wanted something
optimized, and pointers can be better than indexes. But maybe
compilers are smarter now.
void reverse ( char *s )
{
char c ;
char *s1, *s2 ;
printf("Input string is %s\n",s);
for ( s1=s, s2=s+strlen(s)-1 ; s1 < s2 ; s1++, s2-- )
{
c = *s1 ;
*s1 = *s2 ;
*s2 = c ;
}
printf("Reverse d string is %s\n",s);
}
David Stevenson
David Stevenson wrote: void reverse ( char *s ) {
char *s1, *s2 ;
s2=s+strlen(s)-1 ;
That's no good for when s points to a zero length string.
--
pete
David Stevenson wrote: void reverse ( char *s ) { [...] for ( s1=s, s2=s+strlen(s)-1 ; s1 < s2 ; s1++, s2-- )
As pete already pointed out:
s2=s+strlem(s)-1; is bad when strlen(s) == 0.
So here is a way to fix this (based on David Stevenson's code):
#include <assert.h>
#include <string.h>
#include <stdio.h>
char* reverse ( char *s )
{
char c ;
char *s1, *s2 ;
assert(s != NULL);
if (*s == '\0')
return s;
for ( s1=s, s2=s+strlen(s)-1 ; s1 < s2 ; s1++, s2-- )
{
c = *s1 ;
*s1 = *s2 ;
*s2 = c ;
}
return s;
}
int main(void)
{
char s[]="abc";
printf("Origina l string: %s\n",s);
printf("Reverse d string: %s\n",reverse(s ));
return 0;
}
Note:
I've added a return type (char*) to reverse(), and added the assertion
that s shouldn't be NULL, as both of this additions didn't seem like a
bad idea for me.
--
Robert Bachmann <ne**@rbach.pri v.at>, PGP-KeyID: 0x8994A748 This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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<---- Code starts -->
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