Base Conversion Problem

Question

I'm trying to convert an integer to a string right now, and I'm having a problem.

I've gotten the code written and working for the most part, but it has a small flaw when carrying to the next place. It's hard to describe, so I'll give you an example. Using base 26 with a character set consisting of the lowercase alphabet:

0 = "a"
1 = "b"
2 = "c"

...

25 = "z"
26 = "ba" (This should equal "aa")

It seems to skip the character at the zero place in the character set in certain situations.

The thing that's confusing me is I see nothing wrong with my code. I've been working on this for too long now, and I still can't figure it out.

char* charset = (char*)"abcdefghijklmnopqrstuvwxyz";
int charsetLength = strlen(charset);

unsigned long long num = 5678; // Some random number, it doesn't matter
std::string key

do
{
    unsigned int remainder = (num % charsetLength);
    num /= charsetLength;

    key.insert(key.begin(), charset[remainder]);

} while(num);

I have a feeling the function is tripping up over the modulo returning a zero, but I've been working on this so long, I can't figure out how it's happening. Any suggestions are welcome.

EDIT: The fact that the generated string is little endian is irrelevant for my application.

Answer 1

If I understand correctly what you want (the numbering used by excel for columns, A, B, .. Z, AA, AB, ...) this is a based notation able to represent numbers starting from 1. The 26 digits have values 1, 2, ... 26 and the base is 26. So A has value 1, Z value 26, AA value 27... Computing this representation is very similar to the normal reprentation you just need to adjust for the offset of 1 instead of 0.

#include <string>
#include <iostream>
#include <climits>

std::string base26(unsigned long v)
{
    char const digits[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    size_t const base = sizeof(digits) - 1;
    char result[sizeof(unsigned long)*CHAR_BIT + 1];
    char* current = result + sizeof(result);
    *--current = '\0';

    while (v != 0) {
        v--;
        *--current = digits[v % base];
        v /= base;
    }
    return current;
}

// for testing
#include <cstdlib>

int main(int argc, char* argv[])
{
    for (int i = 1; i < argc; ++i) {
        unsigned long value = std::strtol(argv[i], 0, 0);
        std::cout << value << " = " << base26(value) << '\n';
    }
    return 0;
}

Running with 1 2 26 27 52 53 676 677 702 703 gives

1 = A
2 = B
26 = Z
27 = AA
52 = AZ
53 = BA
676 = YZ
677 = ZA
702 = ZZ
703 = AAA