Thread: beginners problem - trying to flip a string using pointers.

"flip a string" is not standard terminology. Do you mean to reverse a string?

If inve stands for invert, why not spend the extra two characters to make that explicit. Or call it str_reverse.

You need to fix your indentation. It's all over the place.

main should be defined to return an int and should have a "return 0;" statement at the end.

Since you're not using conio.h (and should avoid it), why not remove the include for it.

If you want a for-loop to have an empty body, make it more obvious by putting the semicolon, indented, on the next line.

Please fix those things and re-post.

The second for loop makes no sense.

Unlike searching forwards, where there is a \0 to mark the end of the string, there is no beginning of string marker.

Keep the input parameter constant. If you want to chase back and forth over the length of the string, then use a separate copy of the input pointer.

Thanks for the help and yes - I do mean to reverse a string. I tried to change some of it according to what you said. Salem, didn't understand your last remark though - you wanted me to make another copy of *ptr? isn't the input pointer copied in the memory when the function starts?

Anyway - this is the new code:

Code:

#include <stdio.h>


char str_invert(char *ptr);
 


char str_invert(char *ptr)                  /*a function that is supposed to be pointer - based, and to flip a string, and to return a pointer to the reversed string*/


{
char *str;
for(;*(ptr+1);ptr++)                        /*moving the pointer to its end*/
			; 
for(;*ptr;ptr--, str++)                     /*loop that moves the pointer from the end to the start, and moves the target pointer forward*/ 
	{str = ptr;}                            /*this doesn't work - wanted to change the value's of *str so it would be the same as *ptr but reversed*/ 


for(;*str;str--)                            /*loop that moves the target pointer from the end to the start*/
			;


return(*str);
}
 
 
     int main (void)
{
int i;


str_invert("hello");


return(0);




}

Assuming that str_invert() is supposed to reverse a string in place, it doesn't need to return a pointer, but if it does return a pointer it needs to return the original pointer.

The first loop is correct, you need to get a pointer to the end of the string.

The problem after this is do you understand how to reverse a string? You need to exchange characters in the string to move them around, and you'll need a temporary character variable to exchange characters, similar to temp = a, a = b, b = temp.

Try working out the logic with a small example string like "1234567". What characters need to be exchanged to reverse the string?

Originally Posted by rcgldr

Since the op hasn't replied to this thread in a while, here is an example program that I wanted to get archived in this thread

What if the string is the null string? (Not a NULL pointer, but a char * pointer to an ascii nul.) Your ++right will step right over it off into ... who knows where.

I wrote it similarly like so:

Code:

#include <stdio.h>

char *str_reverse(char *str)
{
    char *front, *back;

    if (!str || !*str)
        return str;
 
    for (back = str; *back; back++)
        ;

    for (front = str, --back; front < back; ++front, --back)
    {
        char t = *front;
        *front = *back;
        *back  = t;
    }

    return str;
}

int main(void)
{
    char str[] = "hello world";
    puts(str_reverse(str));
    return 0;
}

Originally Posted by oogabooga

What if the string is the null string?

I back up these example programs, but in this case I backed up in the wrong direction and copied an old file over the new file that checked for both. With visual studio, printf("%s\n", NULL) will not abort the program but instead display <null>. I'm not sure about other compilers. The "refixed" version is basically the same as yours, but your for loop method is more compact that the expanded while loop I used.

Code:

#include <stdio.h>

char * str_reverse(char *ptr)
{
char *right, *left;
char tmp;

    if(!ptr || !*ptr)           /* return if null ptr or string */
        return(ptr);
    right = left = ptr;         /* set left and right ptrs */
    while(*++right);            /* set right to last character */
    right--;
    while(right > left){        /* swap left / right side of string */
        tmp    = *left;
        *left  = *right;
        *right = tmp;
        left++;
        right--;
    }
    return(ptr);                /* return original ptr */
}

int main ()
{
char str[] = "this is a test";
    printf("%s\n", str_reverse(str));
    return(0);
}