回文 function，不在 C 中返回值

Question

When I run this function I don't get a return value of 1 or 0. Im not sure why, I'm new to pointers and any type of help/tips would be greatly appreciated.

int isPalindrome (char * str)
{
    char def[SIZE];
    int length = strlen(str);
    for(int count; count <= length; count++ ){
        def[count] = str[count];
    }

    int c;
    char *begin, *end, temp;

    begin  = str;
    end    = str;

    for (c = 0; c < length - 1; c++)
        end++;

    for (c = 0; c < length/2; c++)
    {        
        temp   = *end;
        *end   = *begin;
        *begin = temp;

        begin++;
        end--;
    }

    for(int count2; count2 <= length; count2++){
        if(str[count2] != def[count2]){
            return 0;
        }
        return 1;
    }
}

The function is called with..

 if(isPalindrome(arr) == 1) 
     printf ("\nIs a palindrome.\n\n");

Answer 1

To check if a string is a palindrome you create a reversed string, after first allocating enough room for all chars, by looping through the original one and changing the order. you can then use a strcmp to check if the reversed and original string are the same.

int isPalindrome (char * str)
{
  int length = strlen(str); 
  char* reversed = malloc(sizeof(char)*length); //we allocate enough space for length chars

  for(int i = 0; i < length; i++) {
    reversed[i] = str[length-1-i]; //populate reversed with the chars in str but in the reversed order
  }

  if(strcmp(str,reversed) == 0) //strcmo(a,b) return 0 if they are equal
  {
    free(reversed); //deallocate the space for reversed
    return 1;
  }
  free(reversed); //deallocate the space for reversed
  return 0;
}

Answer 2

In these loops

for(int count; count <= length; count++ ){
    def[count] = str[count];
}

and

for(int count2; count2 <= length; count2++){
    if(str[count2] != def[count2]){
        return 0;
    }
    return 1;
}

there are used uninitialized variables count and count2 . So the function has undefined behavior.

Pay attention the the function is declared and defined incorrectly. It is too complicated and uses a magic number SIZE .

Also you should use the type size_t instead of the type int because the return type of the function strlen is size_t and in general an object of the type int can not accommodate an object of the type size_t .

There is no need to create an auxiliary array and change the original string to check whether the given string is a palindrome. Moreover the parameter shall be defined with the qualifier const . Otherwise you will be unable to check whether a string literal is a palindrome because modifying a string literal invokes undefined behavior.

Also if you are trying to use pointers then there is no need to use also indices in loops.

The function can be defined much simpler.

Here you are.

#include <stdio.h>
#include <string.h>

_Bool isPalindrome( const char *s )
{
    const char *first = s, *last = s + strlen( s );

    if ( first != last )
    {
        while ( first < --last && *first == *last ) ++first;
    }

    return !( first < last );
}

int main(void) 
{
    char *s1 = "121";

    printf ( "\"%s\" is %s%s\n", s1, isPalindrome( s1 ) ? "a " : "not ", "palindrome." );

    char *s2 = "1221";

    printf ( "\"%s\" is %s%s\n", s2, isPalindrome( s2 ) ? "a " : "not ", "palindrome." );

    return 0;
}

The program output is

"121" is a palindrome.
"1221" is a palindrome.

As you can see the function uses only pointers and neither index.

Answer 3

Your program has multiple problems. Lets see them one by one:

for(int count; count <= length; count++ ){
            ^^^^
    def[count] = str[count];
}

You are not initialising count, you are just declaring it. Without any initial value count will just be some garbage. To fix this:

for(int count = 0; count <= length; count++ ){
    def[count] = str[count];
}

Please note that older C versions require you to declare all variables in the beginning of a function block as far as I remember. So this is anyways wrong.

Next, your palindrome logic is incorrect. You are just swapping characters between begin and end.

All you have to do is check while iterating HALF THE STRING is whether the characters from the beginning equal to the end.

for (c = 0; c < length/2; c++)
{
    // Check if "begin" equals to the current "end"
    if (*begin != *end) 
    {
        return 0;
    }

    // Move "begin" forward and "end" backwards
    begin++;
    end--;
}

Here is your complete function in working condition:

int isPalindrome(char* str)
{
    int length = strlen(str);

    int c;
    char *begin, *end;

    begin = str;

    // Why use the below for loop when you can directly move end to the end?
    end = str + length - 1;

    // for (c = 0; c < length - 1; c++)
    //    end++;

    for (c = 0; c < length / 2; c++) {
        if (*begin != *end) {
            return 0;
        }

        begin++;
        end--;
    }
    return 1;
}