"Array subscript is not an integer" in C program

Question

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;
}

The error message seems clear enough. An array subscript has to be an integer. In datos[p], p is a pointer, not an integer. You can't do that. (Well, you can do silly things like 5[array], but you're not doing that here; datos is also a pointer.) — Keith Thompson
– Keith Thompson, Commented Nov 27, 2013 at 18:52
@KeithThompson, I've never seen the 5[array] syntax. I tried it and it seems the same as array[5], but where/why would one use something like that? Thanks. — Charlie Burns
– Charlie Burns, Commented Nov 27, 2013 at 19:06
@CharlieBurns; array[5] is equivalent to *(array + 5) and same for 5[array which is equal to *(5 + array). That's why array[5] = 5[array]. where/why would one use something like that?: I think it is used only for obfuscation :) — haccks
– haccks, Commented Nov 27, 2013 at 19:09
added [code] int *q = dos; [/code]and changed this [code] total += (datos [p]-avg) * (data [p +1]-avg);} so {total + = (data [q]-prom ) * (data [q +1]-avg);} but the error continues. Specifically the error is shown in this line {total + = (data [p]-avg) * (data [p +1]-avg);} — Matias Rodriguez
– Matias Rodriguez, Commented Nov 27, 2013 at 19:09
@CharlieBurns: Hopefully you would never use it. It works because x[y] is by definition equivalent to *(x+y), and addition is commutative (even for pointer+integer). See this question; the accepted answer is massively upvoted, but my answer goes into the history. — Keith Thompson
– Keith Thompson, Commented Nov 27, 2013 at 19:10

sandymatt · Accepted Answer · 2013-11-27 18:53:14Z

8

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.

answered Nov 27, 2013 at 18:53

sandymatt

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3 Comments

Matias Rodriguez Over a year ago

aded int *q = datos and changed total += (datos[p]-prom)*(datos[p+1]-prom); for total += (datos[q]-prom)*(datos[q+1]-prom); but the error still appears and specifically on the line total += (datos[q]-prom)*(datos[q+1]-prom);

sandymatt Over a year ago

q is not an int -- it is an int*. Once again, you can only have int values as array subscripts. If you want to use q as your index, you will need to dereference it first with *q.

Ady96 Over a year ago

What can I do, when I need do this: double array[1000000000]; for(double i = 0 ; i < n; i++) array[i] = i; So, int dont have number as 1000000000. How can I iterate throught this array?

haccks · Accepted Answer · 2013-11-27 19:04:33Z

3

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.

answered Nov 27, 2013 at 19:04

haccks

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1 Comment

Matias Rodriguez Over a year ago

aded int *q = datos and changed total += (datos[p]-prom)*(datos[p+1]-prom); for total += (datos[q]-prom)*(datos[q+1]-prom); but the error still appears and specifically on the line total += (datos[q]-prom)*(datos[q+1]-prom);

Mr Lister · Accepted Answer · 2013-11-27 19:49:05Z

2

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.

edited Nov 27, 2013 at 19:49

answered Nov 27, 2013 at 19:35

Mr Lister

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Comments

chux · Accepted Answer · 2013-11-27 21:33:28Z

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
}

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