Converting Decimal to Octal Using Bitwise Operators

Question

The below function is intended to convert its parameter, an integer, from decimal to octal.

std::string dec_to_oct(int num) {
    std::string output;
    for(int i=10; i>=0; --i) {
        output += std::to_string( (num >> i*3) & 0b111 );
    }
    return output;
}

It works for any positive input, however, for num = -1 it returns 77777777777, when it should return 37777777777, so the first digit needs to be a 3 instead of a 7. Why is this happening? The function appears to be incorrect for all negative input. How can I adjust the algorithm so that it returns correctly for negative numbers?

Note: this is a CS assignment so I'd appreciate hints/tips.

Answer 1

This is because the arithmetic shift preserves the sign of the number. To overcome this, cast the input integer to the equivalent unsigned type first.

(((unsigned int)num) >> 3*i) & 7

---

Going further, you can make the function templated and cast the pointer to the input to uint8_t*, using sizeof to calculate the number of octal digits (as suggested by DanielH). However that will be a bit more involved as the bits for a certain digit may stretch over two bytes.

Answer 2

Copy paste of documentation

ios_base& oct (ios_base& str);

Use octal base Sets the basefield format flag for the str stream to oct.

Example

// modify basefield
#include <iostream>     // std::cout, std::dec, std::hex, std::oct

int main () {
  int n = 70;
  std::cout << std::dec << n << '\n';
  std::cout << std::hex << n << '\n';
  std::cout << std::oct << n << '\n';
  return 0;
}

Output:

70
46
106

So bottom line you are reinventing the wheel.