Thread: palindromic in C

At first glance...

Code:

      scanf("%s",&str);

    //should be


      scanf("%s", str);

Don't cast malloc, and make sure you include the proper header for this function (stdlib.h).

You're also not freeing any of the memory you allocate.

Code:

/* warning: format '%s' expects type 'char *', but argument 2 has type 'char **'| */

scanf("%s",&str);

"scanf()" expects a pointer. "str" is already a pointer (you don't need the '&' operator).

I need to change the arrays in the function int palin to pointers.

You're not using any arrays in that function, you're using pointers. You're using array syntax, which is an acceptable way to accessing an offset of a pointer (as long as you don't attempt to access anything outside of the available memory pointed to).

Even though this is allowed, though, be aware that pointers and arrays are different things.

Question 6.2
Question 6.3
Question 6.4

It appears, though, that you're just using the array syntax to copy over the string using a loop. Why not just use the "strcpy()" function?

This leads to two important observations regarding your use of "strlen()". That function does not count the null character. Therefore:

(1) You aren't allocating enough memory for "normal" to completely hold the contents of "str" (you're one character too short); and

(2) Even if "normal" had enough space, the loop wouldn't copy the '\0' to "normal" (since '\0' is not counted for the length of the string). Using "strcpy()" instead would correct this problem (assuming you fixed the amount of memory allocated to "normal").

Originally Posted by jacobhammer

I need to change the arrays in the function int palin to pointers.

Strictly speakling, your function palin() has no arrays. It is using pointers as if they are arrays.

If you mean that you want to eliminate array syntax and use pointer syntax, then the general notion is that array[n] is equivalent to *(array + n).


As an exercise, you might want to try implementing palin() so it doesn't call any other functions. In other words, don't use malloc(), strlen(), strcmp() or your revstr(). The reason I suggest that is that you will learn more that way than by what you are doing. And you can do it without using array syntax too.