回文C程序混乱

Question

I am exploring working with strings and I have some questions about this particular program. Could someone help explain why and how the x < i/2 as well as the word[i - 1 - x] works.

Why does it have to be i/2 ?
Why does it have to be word[i - 1 - x] ?

#include <stdio.h>


int main()
{
     char word[15];
     int i, x;

    printf("Enter a word\n");
    scanf("%s", word);

    for(i = 0; word[i] != '\0';)
        i = i + 1;


    for(x = 0; x< i/2; x++)

        if(word[x] != word[i-1-x])
        {printf("Your word is not a palindrome\n");}
        else
        {
            printf("Your word is a palindrome\n");
        }


}

Answer 1

You're comparing half the string. So for i/2 you're looking at the first half of the string and for word[i - 1 - x] your looking ahead at the "mirror" position of the other half of the string.

Answer 2

Why does it have to be i/2 ?

It does not have to be this way: i would work too, but it would not be optimal . If you have checked all character pairs going at it from both ends, and the letters are the same up to the middle of the word, the rest of the checks will succeed as well.

Consider for example the word "alula" . You start by comparing the initial a to the last a , then you compare l in the second position to l in the second position from the back, then you compare u to itself. Now you've reached the middle of the word at i/2 . If you continue, you would compare the same pairs of characters, but you would pick them from different indexes. It is guaranteed that all these pairs are going to match, so you can save yourself some CPU cycles by skipping them altogether.

Why does it have to be word[i - 1 - x] ?

Because arrays are zero-based. i-1 is the index of the last character, hence i - 1 - x is the x -th index counting from the back.

Answer 3

I am assuming that you have stored the string in a variable named string in the below code, this is the main logic of the function

NOTE : you have iterate the for loop for legth/2 times also and it will give the same results

  #include<stdio.h>
  #include<string.h>
  int main()  
    {
        int i;
        int flag=0;
        for(i=0;i<strlen(string);i++)
        {
            if(string[i]==string[strlen(string)-i-1])
            flag=1;
            else
            break;
        }
        if(flag==1)
        printf("palindrome string");
        else
        printf("not palindrome string");

        return 0;
}