string reversal

roaan · 07-03-2009, 09:29 PM

Hi,

I have a doubt about string reversal. The code below reverses the string character by character but when i give it to print out the result there occurs an error. I am unable to figure out where am i going wrong.

insert

Code:

#include <stdio.h>
#include <stdlib.h>


void xstrrev(char *);

int main(){

	int i;
	char *s[] = {
		"To err is human...",
		"But to really mess things up...",
		"One needs to know C!!"
	};

	for(i=0;i<3;i++){
		xstrrev(s[i]);
	}
}


void xstrrev(char *a){


	int len,j;
	char *p;

	printf("\n\r%s", a);
	len = strlen(a);
	printf("\n\r%d", len);
	p = malloc(len+1);

	for(j = 0;j<=len;j++){	
		p[j] = a[len-j];
		printf("\n\r%c%c", a[len-j], p[j]);
		if(j == len)
			printf("\n\r%s", p);
	} 
}

whiteflags · 07-03-2009, 09:42 PM

When j == len in the last iteration of the loop, you will be accessing a out of bounds.

This is because strlen counts from one when computing the length but array access starts from zero. Normally to iterate over an array you would write something like

for ( i = 0; i < N; i++ ) ...

Keep statement blocks like if ( j == len ) outside of the loop. 1. it will confuse you and 2. an if should not be necessary here, as the conditions under which your loop exists are obvious.

BEN10 · 07-04-2009, 12:27 AM

I was also trying the same problem but with a different approach, but the result is a run time error. Here is my code:

Code:

#include <stdio.h>
#include <stdlib.h>
void xstrrev(char *a)
{
	char t;
	int x,i;
	x=strlen(a);
	for(i=0;i!=(x/2);i++)
	{
		x--;
		t=a[i];
		a[i]=a[x];
		a[x]=t;
	}
	printf("%s\n",a);
}
int main()
{

	int i;
	char *s[] = {
		"To err is human",
		"But to really mess things up",
		"One needs to know C!!"
	};

	for(i=0;i<3;i++)
		xstrrev(s[i]);
}

My IDE shows error in the colored line. I'm completely unaware of what's the problem in that line.
And yes as the problem was similar so I didn't find it necessary to create a new thread for it. Hope its fine.

tabstop · 07-04-2009, 01:13 AM

One can't change a string literal. "One needs to know C!!" is constant; any attempt to change it will (usually/often) result in a runtime error.

BEN10 · 07-04-2009, 01:45 AM

That means I can't do this problem by swapping the characters. Or is there a way to do it?

Adak · 07-04-2009, 01:46 AM

I wouldn't use that type of logic. An easy adaption might be:

Code:

#include <stdio.h>
#include <stdlib.h>
void xstrrev(char *a)
{
	char t;
	int i, j;
	j=strlen(a) - 1;
	for(i=0; i < j; j--,i++)
	{
                t=a[i];
        	a[i]=a[j];
                a[j]=t;
      
	}
	printf("%s\n",a);
}
int main()
{

	int i;
	char *s[] = {
		"To err is human",
		"But to really mess things up",
		"One needs to know C!!"
	};

	for(i=0;i<3;i++)
		xstrrev(s[i]);

   printf("\n\n\t\t\t    Press Enter When Ready ");
   i = getchar();
   return 0;

}

BEN10 · 07-04-2009, 01:51 AM

But that's the main problem even the code you gave is not running.

BEN10 · 07-04-2009, 01:54 AM

Quote:

Originally Posted by tabstop

One can't change a string literal. "One needs to know C!!" is constant; any attempt to change it will (usually/often) result in a runtime error.

So how is this code working without any problem.

Code:

#include <stdio.h>
int main()
{

	int i;
	char *s[] = {
		"To err is human",
		"But to really mess things up",
		"One needs to know C!!"
	};
char *a="BEN10";
	s[2]=a;
	printf("%s",s[2]);
}

roaan · 07-04-2009, 06:52 AM

insert

Code:

Quote:
#include <stdio.h>
int main()
{

	int i;
	char *s[] = {
		"To err is human",
		"But to really mess things up",
		"One needs to know C!!"
	};
char *a="BEN10";
	s[2]=a;
	printf("%s",s[2]);
}

A string literal cannot be changed is true. What you are trying to do above is that you are telling the compiler to give the address of string "BEN10" to char pointer a and then you are assigning that address to s[2] (array of pointers to strings). You are not at all touching or modifying the characters stored at that location. So you dont get any error !!!!!

But i am still unable to do the string reversal. I have no idea whatsoever of where i am going wrong in the first code that i posted above ????????????????

BEN10 · 07-04-2009, 06:58 AM

Quote:

Originally Posted by roaan

insert

Code:

Quote:
#include <stdio.h>
int main()
{

	int i;
	char *s[] = {
		"To err is human",
		"But to really mess things up",
		"One needs to know C!!"
	};
char *a="BEN10";
	s[2]=a;
	printf("%s",s[2]);
}

A string literal cannot be changed is true. What you are trying to do above is that you are telling the compiler to give the address of string "BEN10" to char pointer a and then you are assigning that address to s[2] (array of pointers to strings). You are not at all touching or modifying the characters stored at that location. So you dont get any error !!!!!

But i am still unable to do the string reversal. I have no idea whatsoever of where i am going wrong in the first code that i posted above ????????????????

This code below works fine, why? Here I'm changing the string literals.

Code:

#include<stdio.h>
#include<string.h>
int main(void)
{
	char s[20]="my name is ben10",t;
	//gets(s);
	int i;
	int len=strlen(s);
	int x=len/2;
	for(i=0;i!=x;i++)
	{
		len--;
		t=s[i];
		s[i]=s[len];
		s[len]=t;
	}
	puts(s);
return 0;
}

whiteflags · 07-04-2009, 07:18 AM

Quote:

Originally Posted by tabstop

One can't change a string literal. "One needs to know C!!" is constant; any attempt to change it will (usually/often) result in a runtime error.

Oh wow I didn't see that yesterday.

Quote:

Originally Posted by BEN10

This code below works fine, why? Here I'm changing the string literals.

Code:

#include<stdio.h>
#include<string.h>
int main(void)
{
	char s[20]="my name is ben10",t;
	//gets(s);
	int i;
	int len=strlen(s);
	int x=len/2;
	for(i=0;i!=x;i++)
	{
		len--;
		t=s[i];
		s[i]=s[len];
		s[len]=t;
	}
	puts(s);
return 0;
}

But you're not changing string literals.

Earlier you had written char * s[], which declared s an array of char pointers, each element storing the address of a string literal. So when you passed s[i] to xstrrev, you were passing the literal.

Now you've changed the code to char s[20], which is a typical string. but if you initialize s to something like "foobar" it's the same as this:

Code:

char s[20] = { 'f','o','o','b','a','r', '\0' }; /* not a string literal */

I'm sorry for my earlier oversight.

Hopefully you can extrapolate from this that you could use a matrix of char to get what you first posted, an array of strings, in a safe way.

gets is also problematic.

roaan · 07-04-2009, 08:06 AM

insert

Code:

#include <stdio.h>
#include <stdlib.h>


void xstrrev(char *);

int main(){

	int i;
	static char *s[] = {
		"To err is human...",
		"But to really mess things up...",
		"One needs to know C!!"
	};

	for(i=0;i<3;i++){
		xstrrev(s[i]);
		printf("\n\r%s", s[i]);
	}
}


void xstrrev(char *a){

	int len,j;
	char *p, temp;

	//printf("\n\r%c", *a);
	len = strlen(a);
	//printf("\n\r%d %d", len, len/2);
	p = a + len -1;
	//printf("\n\r%d", p);
	for(j = 1;j<= len/2;j++){
	
		temp = *a;
		printf("%c %c", temp , *p);
		*a = *p;
        *p = temp;
		a++;
		p--;
	}
}

I tried usign this approach of pointers but still no success when i try to run it gives me an unhandled exception (i thought only java had exceptions)

WatchTower · 07-04-2009, 08:38 AM

You need return 0 in your main function.

edit: your header file as well #include <string.h> for you to use strlen() function

whiteflags · 07-04-2009, 08:55 AM

Quote:

Originally Posted by roaan

I tried usign this approach of pointers but still no success when i try to run it gives me an unhandled exception (i thought only java had exceptions)

If you insist on using pointers then you will have to allocate some space with malloc for each element, and then store a copy of the string you want to reverse. As has been explained several times, you cannot edit a string literal. The context in which a string literal is used makes all the difference (see my previous post).

If you wish to avoid using malloc, the simplest fix for your code is to use a matrix of char instead.

Code:

char s[3][512] = {
    "To err is human...",
    "But to really mess things up...",
    "One needs to know C!!"
};

Now s[i] should be a writable string instead of a pointer to a string literal.

Adak · 07-04-2009, 09:41 AM

Quote:

Originally Posted by BEN10

But that's the main problem even the code you gave is not running.

Perhaps it doesn't run on your compiler, but it runs just fine on mine (Turbo C).

07-03-2009, 09:42 PM	#2
whiteflags Registered User Join Date: Apr 2006 Location: United States Posts: 3,201	When j == len in the last iteration of the loop, you will be accessing a out of bounds. This is because strlen counts from one when computing the length but array access starts from zero. Normally to iterate over an array you would write something like for ( i = 0; i < N; i++ ) ... Keep statement blocks like if ( j == len ) outside of the loop. 1. it will confuse you and 2. an if should not be necessary here, as the conditions under which your loop exists are obvious. __________________ Os iusti meditabitur sapientiam Et lingua eius loquetur indicium "There is nothing either good or bad, but thinking makes it so." (Shakespeare, Hamlet, Act II scene ii) http://www.myspace.com/whiteflags99 Last edited by whiteflags; 07-03-2009 at 09:46 PM. Reason: wrong variable
whiteflags is offline

07-04-2009, 12:27 AM	#3
BEN10 DESTINY Join Date: Jul 2008 Location: in front of my computer Posts: 656	I was also trying the same problem but with a different approach, but the result is a run time error. Here is my code: Code: #include <stdio.h> #include <stdlib.h> void xstrrev(char a) { char t; int x,i; x=strlen(a); for(i=0;i!=(x/2);i++) { x--; t=a[i]; a[i]=a[x]; a[x]=t; } printf("%s\n",a); } int main() { int i; char s[] = { "To err is human", "But to really mess things up", "One needs to know C!!" }; for(i=0;i<3;i++) xstrrev(s[i]); } My IDE shows error in the colored line. I'm completely unaware of what's the problem in that line. And yes as the problem was similar so I didn't find it necessary to create a new thread for it. Hope its fine. __________________ HOPE YOU UNDERSTAND....... for( ; ; ) printf("If you can't make it good, at least make it look good"); PC specifications- 512MB RAM, Windows XP sp3, 2.79 GHz pentium D. IDE- Microsoft Visual Studio 2008 Express Edition
BEN10 is offline

07-04-2009, 01:13 AM	#4
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	One can't change a string literal. "One needs to know C!!" is constant; any attempt to change it will (usually/often) result in a runtime error.
tabstop is offline

07-04-2009, 01:45 AM	#5
BEN10 DESTINY Join Date: Jul 2008 Location: in front of my computer Posts: 656	That means I can't do this problem by swapping the characters. Or is there a way to do it? __________________ HOPE YOU UNDERSTAND....... for( ; ; ) printf("If you can't make it good, at least make it look good"); PC specifications- 512MB RAM, Windows XP sp3, 2.79 GHz pentium D. IDE- Microsoft Visual Studio 2008 Express Edition
BEN10 is offline

07-04-2009, 01:51 AM	#7
BEN10 DESTINY Join Date: Jul 2008 Location: in front of my computer Posts: 656	But that's the main problem even the code you gave is not running. __________________ HOPE YOU UNDERSTAND....... for( ; ; ) printf("If you can't make it good, at least make it look good"); PC specifications- 512MB RAM, Windows XP sp3, 2.79 GHz pentium D. IDE- Microsoft Visual Studio 2008 Express Edition
BEN10 is offline

07-04-2009, 06:52 AM	#9
roaan Registered User Join Date: Jun 2009 Location: US of A Posts: 300	insert Code: Quote: #include <stdio.h> int main() { int i; char s[] = { "To err is human", "But to really mess things up", "One needs to know C!!" }; char a="BEN10"; s[2]=a; printf("%s",s[2]); } A string literal cannot be changed is true. What you are trying to do above is that you are telling the compiler to give the address of string "BEN10" to char pointer a and then you are assigning that address to s[2] (array of pointers to strings). You are not at all touching or modifying the characters stored at that location. So you dont get any error !!!!! But i am still unable to do the string reversal. I have no idea whatsoever of where i am going wrong in the first code that i posted above ????????????????
roaan is offline

07-04-2009, 08:38 AM	#13
WatchTower Registered User Join Date: Jul 2009 Posts: 40	You need return 0 in your main function. edit: your header file as well #include <string.h> for you to use strlen() function
WatchTower is offline

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