Example of implicit cast from unsigned to signed:
unsigned int r;
int tab[3] = {1,2,3};
void f(void) {
int x;
x = tab[r];
// check details on 'r' says :
// global variable 'r' (unsigned int 32): full-range [0 .. 2^32 -1]
// conversion from unsigned int 32 to int 32
}
The C Standard says that array indexes are (signed) integers. The reason behind that is that pointers and arrays are close in C.
For example, tab[index] is strictly equivalent to *(tab + index).
You can use pointer arithmetic with negative values, hence an array index can be negative.
Exemple of valid array access with a negative index:
void f(void) {
int x;
int *p;
int tab[3] = {1,2,3};
p = &tab[2]; // points to the 3rd element
x = p[-1]; // x is assigned to the second element, '2'
}
