REVERSE_WORD in c using pointers - c

Question

i have try this code made by me but no output and there's no errors?
#include <stdio.h>
void reverse(char *p,char *v,int size){
for(int i=size-1,j=0;i<0;i--){
*(v+j) = *(p+i);
j++;
}
}
int main(){
char word[6] = {"hello"};
char re_word[6];
re_word[5]='\0';
reverse(word,re_word,6);
printf("%s",re_word);
}

Answer 1

Using pointers it can look like this:
void reverse(char *w, char *revw, int slen) {
revw += slen - 1; // forward to pos. of last letter
revw[1] = '\0'; // one further *(revw+1)
while (*w)
*revw-- = *w++;
}
This is clear and symmetric, once it works, while your i-- and j++ are far apart.
slen is meant to be the number of letters w/o termination. Here the call:
char word[] = {"hello"};
char re_word[sizeof word];
reverse(word, re_word, sizeof word - 1);
strlen() should be used, probably, but these lines show how you can and have to control not just the total size but especially the last byte of the char array.
Without the correct length, reverse() would have to do a strlen() first, because it has to know how far away to put the first letter.
This *(v+j) = *(p+i) is more or less v[j] = p[i] and does not really take advantage of pointers, on the contrary.
(revw
caller) in reverse()
| |
v v
-4 -3 -2 -1 revw +1
o l l e h \0
... revw-- revw[1]
So revw is maybe not the best name inside the function; revw_first_backwards is meant...or fill_start. But before I fill backwards I do the one additional write to the right side to terminate the string: array notation using a pointer: revw[1] = '\0'.

Answer 2

First of all, i < 0 will always be false, given i = size - 1 > 0.
What you want is i >= 0.
Also, given size = 6, size - 1 will be equal to 5, and that is the NULL terminator position since array indexing in C start from 0. Perhaps use a C function such as strlen() to calculate the length rather than hard coding it.
void reverse(char *p, char *v, size_t size)
{
for (int i = size - 1, j = 0; i >= 0; i--)
{
*(v + j) = *(p + i);
j++;
}
}
int main()
{
char word[6] = {"hello"};
char re_word[6];
re_word[5] = '\0';
reverse(word, re_word, 5); /* or reverse(word, re_word, strlen(word)) */
printf("%s", re_word);
}