reading characters using structures and pointers

[kidush]

I am reading characters using structures and pointers, i am not allowed to use string. I need to use something like this code below, that wraps the text in case the word is not finished, i don't know how to use this code without strings.Could someone please help?
Thank you for your time.

#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>

int main () {

    int i;

    char s[] = "This is only a test for wrapping lines";

    printf("%s\n", s);

    i = 20; // our wrap point

    while(i!=0) {

         if (isspace(s[i])) {

             s[i] = '\n';

             break;

         }

        i--;

    }

    printf("\n%s\n", s);


    return 0;

}