Question

Assume one has an vector or array of N elements (N can be very large) containing the octal representation of a non negative integer. How do I get the decimal representation of the number from this array? The code has to be really fast.

EDIT: array A of N elements contains octal representation of a non-negative integer K, i.e. each element of A belongs to the interval [0; 7] (both ends included)

Example: A[0] = 2; A[1] = 6; A[2] = 3

Now a naive calculation would be 2*8pow0 + 6*8pow1 + 3*8pow2 = 2+ 48+ 192 = 242

I tried this but it does not seem to work for large inputs > 6K

//vector<int> A is the input
using namespace std;
vector<int>::iterator it = A.begin();

unsigned int k = 0;
unsigned int x = 0;
while(it < A.end()){
   x = x | (*it<<3*k);
   k++;
   it++;
}

I am also having problems converting a hexadecimal string to its decimal representation? Is this the correct way to do this in C++:
//Assume S to be your input string containing a hex representation
//like F23

std::stringstream ss;
ss << std::hex << S;
ss >> x;

Answer 1

This is what I came up with:

template<typename Iter>
int read_octal(Iter begin, Iter end)
{
    int x = 0;
    int f = 1;
    for (; begin != end; ++begin)
    {
        x += *begin * f;
        f *= 8;
    }
    return x;
}

int main()
{
    int test[] = {2, 6, 3};
    int result = read_octal(test + 0, test + 3);
    std::cout << result << '\n';
}

I tried this but it does not seem to work for large inputs > 6K

What exactly do you mean by 6k? An int usually has 32 bits, and an octal digit has 3 bits. Thus, you cannot have more than 10 elements in your range, otherwise x will overflow.

I am also having problems converting a hexadecimal string to its decimal representation?

Well, you could always write a function to parse a string in hex format yourself:

int parse_hex(const char* p)
{
    int x = 0;
    for (; *p; ++p)
    {
        x = x * 16 + digit_value(*p);
    }
    return x;
}

With the most portable version of digit_value being:

int digit_value(char c)
{
    switch (c)
    {
        case '0': return 0;
        case '1': return 1;
        case '2': return 2;
        case '3': return 3;
        case '4': return 4;
        case '5': return 5;
        case '6': return 6;
        case '7': return 7;
        case '8': return 8;
        case '9': return 9;
        case 'A': 
        case 'a': return 10;
        case 'B': 
        case 'b': return 11;
        case 'C': 
        case 'c': return 12;
        case 'D': 
        case 'd': return 13;
        case 'E': 
        case 'e': return 14;
        case 'F': 
        case 'f': return 15;
    }
}