c - What is with the null character in reverse string program? -

i had make simple program reverse string. got code own understanding google since couldn't work. runs fine , outputs should , understand of except reverse[j] = '\0' statement. kept getting symbols in output when didn't state want know how works. can explain please?

#include<stdio.h>  int main(void) {     char original[20], reverse[20];     int length, i, j;      printf("enter string:\n");     gets(original);       length = strlen(original);      (i = length - 1, j= 0; >= 0; i--, j++)         reverse[j] = original[i];      reverse[j] = '\0';  //i don't know statement      printf("the string reversed is:\n %s\n", reverse);      return 0; } 

if want character array contain string has have terminating zero.

for example function strlen used in program counts characters in character array before terminating zero.

also function printf used format specifier %s outputs characters character array until terminating 0 encountered.

for example if have array this

char s[10] = "hello"; 

then call strlen( s ) returns 5 instead of 10. , call printf( "%s\n", s ); outputs 6 characters (including new line character).

consider demonstrative program

#include <stdio.h>  int main(void)  {     char s[10] = "hello";      printf( "%d\n", printf( "%s\n", s ) );      return 0; } 

its output is

hello 6 

this initialization

char s[10] = "hello"; 

is equivalent to

char s[10] = { 'h', 'e', 'l', 'l', 'o', '\0', '\0', '\0', '\0', '\0'}; 

if need reverse string stored in array obvious need reverse characters before first terminating zero. , if want copy string in reversed order character array need append destination array terminating zero.

this loop

for (i = length - 1, j= 0; >= 0; i--, j++)     reverse[j] = original[i]; 

copies in reversed order characters except terminating 0 original character array starting last character before terminating 0 destination character array. need append destination character array terminating zero

reverse[j] = '\0';