Why do I see a cast to signed integer on my unsigned array index?

We explain here why even if the index of an array is unsigned, Polyspace says that there is conversion to signed integer

Answer 1

[MathWorks Support Team]

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'

}