C中的回文递归程序

Question

I've figured out on how to work around the string but I can't seem to get it to work. Maybe it's because of the scanf that I'm using. Please advise :)

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

int do_palindrome(char *str, int offset){
int ok = 1;
int length = strlen(str);

if(length/2 > 0)
    ok = (str[0] == str[length - 1 - offset])?
            do_palindrome(++str, ++offset):0;

return ok;
}
int main(){
int i = 0;
int ok = 0;
char* str[1] ;

    scanf("%c", str[1]);
    ok = do_palindrome(str[0], 0);
    printf("%s is palindrome? : %d\n", str[0], ok);


printf("Finished!");
return 0;

}

Answer 1

What you're thinking about is the struct hack:

typedef struct {
    char s[1];
} String;

int main()
{
    /* allocate 15 extra bytes for the string */
    String *s = malloc(sizeof *s + 15);

This allows you to declare an array of size 1 and then use it as a variable length, but you still have to give it some memory (via malloc) to use it. Then you can access it via s .

If you want a variable length string, you should malloc the amount of data you need. If you want to do it a little hacky (and if you're going to compile with Gcc) you can do this:

char * str;
scanf("%ms", str);
ok = do_palindrome(str, 0);
printf("%s is palindrome? : %d\n", str, ok);
free(str);

Answer 2

char* str[1] ;

declares an array of one char pointer

scanf("%c", str[1]);

reads a single char but tries to place it beyond the end of your array (C arrays are zero-based).

I think you want to read a string (char array). You can do this using

char str[20]; /* change the array size as required */
scanf("%19s", str); /* read one fewer chars than your array size */

Answer 3

char* str[10] ;
scanf("%s", str);
ok = do_palindrome(str, 0);
printf("%s is palindrome? : %d\n", str, ok);

You are trying to read a string to char type which is wrong and should be of type string. Also the subroutine call to do_palindrome should contain base address of str than just first character of the string.

Answer 4

if(length/2 > 0)
    ok = (str[0] == str[length - 1 - offset]) ?
            do_palindrome(++str, ++offset):0;

This statement will run (length - 1) times. When we check for palindrome, we only need to do (length / 2) comparisons. So I suggest to change code to the following:

if (length >= 2 && (length / 2) >= offset) {
    ok = (str[0] == str[length - 1 - offset]) ?
            isPalindrome(++str, ++offset) : 0;