Array indexing starting at a number not 0

Question

Is it possible to start an array at an index not zero...I.E. you have an array a[35], of 35 elements, now I want to index at say starting 100, so the numbers would be a[100], a[

Answer 1

C++ provides quite a bit more than C in this respect. You can overload operator[] to do the subtraction, and if you want report an error (e.g., throw an exception) if the subscript is out of range.

As a minimal demo, consider the following:

#include <iostream>
#include <stdexcept>

template <class T, int lower, int upper>
class array {
    T data[upper-lower];
public:
    T &operator[](int index) { 
        if (index < lower || index >= upper)
            throw std::range_error("Index out of range");
        return data[index-lower]; 
    }
    T *begin() { return data; }
    T *end() { return data + (upper-lower); }
};

int main() {
    array<int, -3, 5> data;

    for (int i=-3; i<5; i++)
        data[i] = i;

    for (auto const &i : data) 
        std::cout << i << "\t";
    std::cout << "\n";
}

Answer 2

strictly speaking, this solution does not loet you define an array starting at an index different from 0, but you may declare your memory this way:

typedef union
{
    unsigned char all[15000];
    struct
    {
        unsigned char sram[10000];
        unsigned char bram[5000];
    };
} memory;

this does convey the intent that the memory is contiguous, and that it is split in 2 parts. note that you should beware of the alignment of bram and sram, a #pragma pack(1) may be necessary.

Answer 3

No — as in you can't modify the lower bound in declaration like VB6.

Yes — as in you can do tricks like

int a[35];
int* actual_a = a-100;
printf("%d", actual_a[101]);
...

because x[a] is equivalent to *(x+a). This is highly unrecommended.

Answer 4

Not in C. You have to do the arithmetic yourself. There are probably bizarre work-arounds that work most of the time, like making a new pointer that is BRAM-11000 and using that instead.

Answer 5

Pointers and arrays are very similar in C, so you could easilly do something like

element SRAM_MEM[10000];
element BRAM_MEM[5000];

element* SRAM = SRAM_MEM;
element* BRAM = BRAM_MEM-10000;

BRAM[10001] = 0; // sets the first element of BRAM_MEM

Answer 6

The simplest way to do this:

you have:

int *array = memory ; // starts with 0;
array-= 1000 ; // now array[1000] is 0

In c++ just create class with operator[]