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Negative array indexes in C?
Can I use negative indices in arrays?
#include <stdio.h>
int main(void)
{
char a[] = "pascual";
char *p = a;
p += 3;
printf("%c\n", p[-1]); /* -1 is valid here? */
return 0;
}
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JavaScript arrays are collections of items, where each item is accessible through an index. These indexes are non-negative integers, and accessing a negative index will just return undefined .
Python programming language supports negative indexing of arrays, something which is not available in arrays in most other programming languages. This means that the index value of -1 gives the last element, and -2 gives the second last element of an array. The negative indexing starts from where the array ends.
That means that you can have a pointer to the middle element of an array, and use it with a positive or negative index, and it's simple arithmetic. That is, negative indexes here will be out of bounds of the array, and will lead to undefined behavior.
substr deals with a starting offset and a length. It makes sense to me that substring does not allow a negative index, because there really isn't a such thing as a negative index (the characters in a string are indexed from 0 to n, a "negative index" would be out of bounds).
Yes, -1 is valid in this context, because it points to a valid location in memory allocated to your char a[] array. p[-1] is equivalent to *(p-1). Following the chain of assignments in your example, it is the same as a+3-1, or a+2, which is valid.
EDIT : The general rule is that an addition / subtraction of an integer and a pointer (and by extension, the equivalent indexing operations on pointers) need to produce a result that points to the same array or one element beyond the end of the array in order to be valid. Thanks, Eric Postpischil for a great note.
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