C :: Copying Characters From One String To Another With Pointers?

So I'm writing a function isPalindrome() that can accept a string as an argument, and copy from it only the alphabetic characters in the argument (original) string to another string named alpha_array which contains only the alphabetic characters. Then the function should be able to call the isPurePalindrome function to determine if alpha_array is an ordinary palindrome.

The problem is that when I call isPalindrome in main, the program crashes.

Here's the code I have for isPurePalindrome and isPalindrome:

Code:

/* 1 */
int isPurePalindrome( const char * sentence ) // Can accept strings, array and pointer arguments
{
// Declarations

[Code].....