Negative array index in C

[vharry]

Hi all,

char *a;
char b[10]={"12345789"};
a=b;

Can any one tell what actually a[-1] would refer to.
What is the use of it /its common usage.

-
Harry

[donbock]

char *a;
char b[10]={"12345789"};
a=b;

Can any one tell what actually a[-1] would refer to.
What is the use of it /its common usage.

Your example is will yield either an intermittent bug or a segment fault; but there is a similar construction that does work. The following code excerpt comes from "The Standard C Library" by P.J. Plauger, copyright 1992.

#include <limits.h>
#if EOF != -1 || UCHAR_MAX != 255
#error WRONG TOLOWER TABLE
#endif
static const short tolow_tab[257] = {EOF,
  0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,   /* 0x00 - 0x07 */
  ...
  0x40,  'a',  'b',  'c',  'd',  'e',  'f',  'g',  /* 0x40 - 0x47 */
  ...
  };
const short *_Tolower = &tolow_tab[1];
 
#define tolower(c) _Tolower[(int)(c)]
 
int (tolower)(int c) {
  return (_Tolower[c]);
}

The difference between this example and yours is that the pointer is set to the address of the second element of the array. Thus there is something at offset -1 from the pointer.

For an array variable A with n elements, the index absolutely must not be outside the range from zero to n. Note that you can refer to the address of A[n], but you can't actually try to access that element. The compiler knows if you break these rules.

For a pointer variable, you can use pointer arithmetic to access locations on either side of the location pointed to. Pointer arithmetic can be indicated either by a literal arithmetic expression or through array index format. The compiler doesn't know if your pointer arithmetic expression is meaningful.

Cheers,
donbock

[donbock]

const short *_Tolower = &tolow_tab[1];

This line would be better as

const short * const _Tolower = &tolow_tab[1];

Cheers,
donbock

[vharry]

This line would be better as

const short * const _Tolower = &tolow_tab[1];

Cheers,
donbock

Hi donbock,

Thanks a lot .
got it .

Above code would be
char *a ;
char b[10]={"123456789"};

a=&b[strlen(b)-4];

a[0] = would be -> 6
a[-1] = 5
a[-2]= 4
a[-3]= 3
etc

It should accessed as above way .
Thanks
harry