First and last exchange

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

int main() {
char str[255];
int i = 0,j = 0,b;
printf("Input: ");
gets_s(str);
while (str[i] != '.') {
i++;
if ((str[i] == ' ') || (str[i] == '.')) {
b = str[i - 1];
str[i - 1] = str[j];
str[j] = b;
j = i + 1;
}
}
printf("%s\n", str);
return 0;
}