Palindrome function fun

Hello programmers,

My assignment is to create a function that checks whether or not the user inputted string is a palindrome.

I think I made a mistake in the palindrome function because it seems to be unable to check inputs with spaces in it. The error most likely lies in the section where I attempt to use the isalpha function.

Here is my code, any help is very much appreciated!

Code:

---

/*------------------Main Functions------------------
Purpose: Prompts user to input string
Returns: Singal to the OS
--------------------------------------------------*/

int palindrome (char *string);

int main(void) {

char input[81];
char *stop = "done";

printf("Enter a string: ");
gets(input);

while( strcmp ( input, stop ) != 0) {
  if( palindrome ( input ) ) {
      printf("\nIt is a palindrome\n");
      }
  else {
      printf("\nIt is not a palindrome\n");
      }
  printf("\nEnter a string: ");
  gets(input);
  }
printf("\n***** Program Terminated *****\n");
}

/*------------------Palindrome Functions------------------
Purpose: Checks whether input is a palindrome
Returns: TRUE or FALSE
--------------------------------------------------*/

int palindrome (char *string) {

char *string_end = string + strlen(string) - 1;

while(TRUE) {
  while( isalpha( *string ) ) {
      string ++;
      }
  while( isalpha( *string_end) ) {
      string--;
      }
  if( string_end <= string) {
      return TRUE;
      }
  if( tolower(*string) != tolower(*string_end)) {
      return FALSE;
      }
  string_end--;
  string++;
  }
}

---

What is it you think isalpha does, and how do you think that's checking for matches?


Quzah.

I was told isalpha checks whether or not something is a letter. I actually forgot to put the not operator in front of the two isalpha functions, which I have in my code.

my while loop is actually this:

Code:

---

while(TRUE) {
  while( !isalpha( *string ) ) {
      string ++;
      }
  while( !isalpha( *string_end) ) {
      string--;
      }
  if( string_end <= string) {
      return TRUE;
      }
  if( tolower(*string) != tolower(*string_end)) {
      return FALSE;
      }
  string_end--;
  string++;
  }

---

I managed to fix my code.

I was actually decrementing the wrong thing in my while loop. I also changed from !isalpha( ) to !isalnum( ). My while loop should have been:

Code:

---

while(TRUE) {

  while( !isalnum( *string ) ) //changed functions here{

      string ++;

      }

  while( !isalnum( *string_end) ) //and here {

      string_end --;  //fix here

      }

  if( string_end <= string) {
      return TRUE;
      }
  if( tolower(*string) != tolower(*string_end)) {
      return FALSE;
      }
  string_end--;
  string++;
  }

---

You need to be checking if the ends of the passed in string match as you move those end points towards each other. If you reach the middle of the string or the end points pass one another then you can return true. If at any point the end points do not match you can abort early and return false.