Thread: string reversal

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.

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.

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

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;

}

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

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]);
}

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;
}

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.

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.