Use of the "-=" operator with array element in C

Question

Following code deletes the 7th character and beyond from string I figured this out but can anyone clarify me the general logic of an array and operations like S1[6] -= S1[6];. I searched array arithmetic but it relates with address arithmetic and pointers. Thanks for the help.

#include <string.h>

int main()   
{ 
   char S1[]= "Hello World";   
   S1[6] -= S1[6];
   printf("%s",S1);
}

Answer 1

S1[6] -= S1[6];

is equal to

S1[6] = S1[6] - S1[6];

which is also equivalent to

S1[6] = 0;

or better in the context

S1[6] = '\0';

---

S1[6], the 7th element of array S1 with the value of the 'W' character is subtracted by itself and the result, actually 0, is assigned back to the 7th array element.

It makes the 7th element of the string a null character, which terminates the string, although all characters which come after the 7th element aren´t overwritten/erased.

This has the effect, that the output of your program will be "Hello " instead of "Hello World".