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For-loop saying - ERROR Member reference base type 'int [13]' is not a structure or union

What is the problem with my for-loop? It is inside a helper function but the error "Member reference base type 'int [13]' is not a structure or union" is happening across my code. The 'int [13] changes to int[10] when I am using a 10 integer array, so I assume it is a problem there. Here are two examples:

        int newisbn13[13];
        newisbn13[0] = 9;
        newisbn13[1] = 7;
        newisbn13[2] = 8;
        for (int p = 3; p < newisbn13.length() - 1; p++)
            newisbn13[p] = isbn10[p-3];
ERROR: Member reference base type 'int [13]' is not a structure or union
Also:
int calc_check_digit_13(int input[], int size) {
    int sum = 0;
    for (int i = 0; i < input.length(); i++)
        int tempnum = 0;
        if (i % 2 == 0)
            tempnum = input[i];
        else if (i % 2 == 1)
            tempnum = input[i] * 3;
        sum = tempnum + sum;
etc. etc. etc.
ERROR: Member reference base type 'int *' is not a structure or union
What is causing this error throughout my code? Thank you for your help. 
newisbn13 is an array and in contrast to other languages like C# or Java it does not know its size.
You need to use sizeof(newisbn13) instead.
Or since c++17 you can use std::size(newisbn13).
However this will not work for calc_check_digit_13. Because input will decay to a pointer and neither sizeof nor std::size will work there. But probably the parameter size is what you want to use. 
for (int i = 0; i < size; i++) {...}
For your first block of code, you make a call to a non-existent member function of type int. In C++, int is a primitive type and has no member functions or member variables. 
For the second block, you're calling the same function but on a pointer to an array of ints, so the type is int * and not int[13], but its pretty much the exact same problem.
As churill pointed out, you can use sizeof(int[]) or std::size(int[]) to find the number of elements in the array. If you need a container for integers, I would recommend using std::vector<int> to manage your ints. This template class has tons of quality-of-life methods such as size() that can assist with what you might want to do.
        
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