"Array subscript is not an integer" in C program

The `deviation` function throws me the following error: `"Array subscript is not an integer"`. If you can help me locate the cause of error I'll appreciate it.

float average(float data[], int n) {
float total = 0;
float *p = data;

while (p < (data + n)) {
total = total + *p;
p++;
}

return total / n;
}

float deviation(float data[], int n) {
float data_average = average(data, n);
float total;
float *p = data;

while (p < (data + n)) {
total += (data[p] - data_average) * (data[p + 1] - data_average);
}

return total / 2;
}

`p` is a pointer to a float. That's why you're getting the error.

float *p, total;

...

total += (datos[p]-prom)*(datos[p+1]-prom);


You can only use `ints` as array indices in C.

Array subscripts must be an integer, an `int` type. You can't use any other type as array subscript. `p` is declared as `float *p`, i.e, a pointer to `float`. You can't use a pointer as array indices.

You can use an int:

int p = 0;
...
while (p<n) {
// rest is the same

or, you can use `p` as a pointer directly, which is, I suppose, what you intended.

while(p<datos+n) {
total += (*p-prom)*(*(p+1)-prom);

Note, however, that you never increment `p`, so the loop will never end. Also, you never initialise `total`, so the result will be a random garbage value.

C11dr 6.5.2.1 **Array subscripting** ... "One of the expressions shall have type ‘‘pointer to complete object _type_’’, the other expression shall have integer type, and the result has type ‘‘_type_’’." ...

With `[]`, you get to do:

// pointer_type[integer_type]
float data[];
int n;
float total;
total = data[n];

// or the unconventional equivalent
// integer_type[pointer_type]
total = n[data]; // But let's leave that for another post

// Instead the OP did
// pointer_type[pointer_type]
// and received error: "Array subscript is not an integer"
float *p;
total = data[p];

The usage of `float` is not the issue here, but the usage of a pointer.

An integer type includes types `int`, `unsigned`, `size_t`, `long`, etc.

---

I think the OP wanted the following (or something like this)

float deviation(float data[], int n) {
if (i <= 0) return 0;
float data_average = average(data, n);
float total = 0.0; // don't forget to set to 0.0
float *p = data;

while (p < (data + n)) {
total += (*p - data_average) * (*p - data_average);
p++;
}
return sqrt(total / n); // div by 0 averted
}